Hydrozoa

(Hydroids)

Phylum Cnidaria

Class Hydrozoa

Number of families 114

Thumbnail description
Invertebrates with a body plan that is comprised of a medusa with velum, a muscular projection from the subumbrellar margin that partially closes the subumbrellar cavity, and polyps; life cycles always involve the presence of a planula larva

Evolution and systematics

Considered a superclass by some experts, this group has representatives since the Precambrian (velellids). Because of the rarity of skeletal structures, however, the fossil record is fragmentary. Recent interpretations of the medusary nodule (the structure that, from a polyp, gives rise to a medusa by a special type of budding) suggest that the Hydrozoa are triploblastic. The subumbrellar cavity of the hydromedusae, and the layer of striated muscle that lines it, are originated by a morphogenetic pattern very similar to schizocoely, with a third layer of tissue that is formed between ecto- and endo-derm and that becomes hollow. The subumbrellar cavity, therefore, is interpreted as a coelom that later becomes open, with the origin of the velar opening. Molecular evidence is now available, showing, for instance, that the genes coding for mesodermic structures in the Bilateria is also present in the Hydrozoa. The polyp stage, thus, is diploblastic, whereas the medusae are triploblastic. If this view is confirmed by further evidence, the transition from diploblasts to triploblasts occurs every time a medusa is budded from a polyp, and one of the great mysteries of metazoan evolution is solved.

Hydrozoa (characterized by medusae with direct development or produced by lateral budding from polyps) are comprised in the subphylum Medusozoa, including also the Scyphozoa (characterized by medusa production by strobilation from polyps), and the Cubozoa (characterized by medusa production by complete metamorphosis of the polyp).

The classes of the superclass Hydrozoa include Automedusa, with the subclasses Actinulidae (one order, two families), Narcomedusae (one order, three families), and Trachymedusae (one order, five families); and Hydroidomedusa, with the subclasses Anthomedusae (two orders, 49 families), Laingiomedusae (one order, one family), Leptomedusae (two orders, 34 families), Limnomedusae (one order, three families), Siphonophorae (three orders, 16 families), and Polypodiozoa (one order, one family).

Physical characteristics

The Automedusa class is represented only by medusae; there are no polyp stages. Development is usually direct, sexes are separated; each fertilized egg leads to a planula that develops into a single medusa, except in some Narcomedusae, in which parasitic stages issued from the egg may give rise to several medusae by asexual budding. Medusae are not formed through a medusary nodule: the subumbrellar cavity and velum are formed by folding and deepening of the oral embryonic ectoderm, being only analogous to the subumbrellar cavity and velum of the Hydroidomedusa. The primary marginal tentacles are always formed before the subumbrellar cavity and the gastro vascular system. The marginal tentacles are deprived of tentacular bulbs. Sensory organs are in the form of ecto-endodermal statocysts, with an endodermal axis, growing out from the circular canal, with sensory cells characterized by numerous kinocilium-lacking rootlets, surrounded by stereocilia, innervated by the upper nerve ring; statoliths are of endodermal origin. Asexual reproduction is present only in "actinula"-like larvae and adults of Narcomedusae. The Actinulidae are all members of the interstitial fauna; they look like actinula-larvae, and the statocysts are the most distinctive medusan feature of these highly specialized medusae. The Narcomedusae have a flattened exumbrella, with lobed margin, incised by deep grooves. Usually with no radial canals, gametes are carried on the wide manubrium. Their intermediate tentaculated post-embryonic stages are juvenile medusae

and are inappropriately called "actinulae," like the juvenile polyps of some Anthomedusae. The Trachymedusae have a bell-shaped umbrella, with circular and radial canals. Gametes ripen on the radial canals. The manubrium is often on a peduncle.

The Hydroidomedusa class is represented by a succession of three stages during indirect development. The planula is a ciliated motile gastrula; it typically develops into a benthic, modular, larval stage, polyp or hydroid (except in the Porpitidae, Margelopsis and Pelagohydra, where the hydroid is floating). Hydroids can be solitary, but generally form modular colonies by simple budding. Polyps can be specialized for different functions (defensive dactylozooids, reproductive gonozooids, nutritive gastrozooids, etc.). Polyps give rise, by asexual budding, to planktonic, free-swimming, and solitary hydromedusae, representing the sexual adult. The sense organs of pelagic hydroidomedusae, when present, are ocelli (Anthomedusae, some Leptomedusae), or statocysts (Leptomedusae, Limnomedusae); sometimes cordyli of unknown function are also present (Leptomedusae). The Siphonophores have no visible sense organs. Medusae are often reduced to sporosacs (fixed gonophores), so that hydroids, by paedomorphosis, secondarily become the sexual stages. The Hydroidomedusa may also form pelagic, highly polymorphic colonies (Siphonophores). Medusa budding occurs via a medusary nodule or entocodon, forming a coelomlike cavity, the subumbrellar cavity, lined by striated muscle cells; primary marginal tentacles always develop after the subumbrellar cavity and the gastro vascular system. Both embryonic and larval stages, the planula and the polyp, are typically diploblastic; the adult sexual stages, the hydromedusae, acquire a triploblastic kind of organization during embryonic development (medusary nodule formation). Hydroidomedusae are frequently seasonal; the hydroid stage may develop several types of resting stages (frustules, propagules, cysts, stolon system) to overcome unfavorable ecological conditions.

The hydroids of the subclass Anthomedusae do not have a protective perisarc sheath around the polyps and are said athecates; they are usually colonial (but the most famous hydroid,

Hydra, is a solitary paedomorphic anthomedusa). The colonies can be either monomorphic or polymorphic, while the structure of the tentacles is in two forms: filiform tentacles do not present particular aggregations of cnidocysts, whereas capitate tentacles have cnidocyst knobs. The medusae are typically bell shaped; their gonads (aggregates of gametes commonly referred to as gonads) are confined on manubrium, sometimes extending on the most proximal parts of the radial canals. Their marginal sense organs, if present, are ocelli; the marginal tentacles are peripheral, hollow, or solid, usually with tentacular bulbs; sexual reproduction occurs through a complex planula. The hydroids of the subclass Laingiomedusae are unknown. The medusae have an almost hemispherical umbrella, with lobed margin, divided by peronial grooves or similar structures. The radial canals are four; there is no typical circular canal but a solid core of endodermal cells around the umbrella margin. The tentacles are solid, inserted above the exumbrellar margin, on the exumbrella. The manubrium is simple, quadrangular, tubular, or conical; the mouth opening is simple, quadrangular to circular; gametes are in four masses on the manubrium or as epidermal lining of interradial pockets of the manubrium. The hydroids of the subclass Leptomedusae are thecate: all parts of the colonies are typically protected by a chitinous perisarcal structure. The hydranth is protected by a hydrotheca, the nematophore by a nematotheca, and the gonophore by a gonotheca. Rarely, hydranths are naked. The medusae are typically with hemispherical or flattened umbrella; the masses of gametes are confined to radial canals, exceptionally extending onto the proximal part of manubrium; when present, the marginal sense organs are ectodermal velar statocysts, rarely cordyli, occasionally adaxial ocelli. The marginal tentacles are peripheral and hollow (except in Obelia), with tentacular bulbs. Sexual reproduction occurs through a complex planula. The hydroids of the subclass Limnomedusae are very simple; solitary or colonial; small, sessile; with or without tentacles; often close to planula structure and budding planula-like structures or frustules; there are no perisarcal thecae, but cysts and stolons are covered by chitin. The medusae usually have gamete masses along the radial canals or, exceptionally, on the manubrium. The marginal tentacles are peripheral, hollow, without a true basal bulb; their base is usually with a parenchymatic endodermal core embedded in umbrellar mesoglea. The marginal sense organs are internal, enclosed ecto-endodermal statocysts that are embedded in the mesoglea near the ring canal or in the velum. Exceptionally, medusae can be reduced medusoids. Sexual reproduction leads to simple planulae, without embryonic glandular cells.

The subclass Siphonophorae comprises generally pelagic, free-swimming, or floating species, forming highly polymorphic

modular colonies of polypoid and medusoid zooids attached to a stem or stolon supported by a floating and swimming system or nectosome (pneumatophores and nectophores).

The Polypodiozoa class is represented by a single species, Polypodium hydriforme Ussow, 1885, the only known metazoan adapted to intracellular parasitism. Polypodium has a unique lifecycle, having a succession of a free-living stage and of an intracellular parasitic stage of some Acipenseridae and Polyodontidae eggs. The earliest parasitic stage known is a binucleate cell observed in previtellogenetic fish oocytes. Further parasitic development leads to a didermic stolonal structure, with inverted germ layers, growing at the expense of the egg's yolk and forming numerous inverted buds. Before becoming free at fish spawning, eversion takes place and the germ layers take their normal position (ectoderm out, endoderm inside). Once liberated, the stolon becomes fragmented into individual buds, each originating a free-creeping globular stage that can multiply by longitudinal fission. These stages can move and feed, having an oral mouth-cone and tentacles. Germ cells are endodermal; the females have two kinds of gonads, each with a gonoduct opening in the gastral cavity; the gonads of the males form gametophores carrying cnidocysts. The free-living stage presumably represents the sexual medusae, the parasitic stages being considered as polypoid.

They differ, however, from all other Hydrozoa by unique features: bilateral symmetry; presence of gonoducts; aberrant gametogenesis; unique structure of cnidocil apparatus; inversion of germ layers during parasitic life; and the complete separation of epidermal and muscle cells.

The umbrella of Hydroido- and Automedusae generally measures between 0.02 in (0.5 mm) and 2 in (50 mm), but in numerous species the size may be greater, reaching 3.9–7.8 in (100–200 mm) (Aequorea) and even, exceptionally, 15.7 in (400 mm) of diameter (Rhacostoma atlanticum). The colonies of the Hydroidomedusae usually have a reduced size. Most of them do not exceed a few inches (centimeters to a few decimeters) (i.e., Cladocarpus lignosus is 27.5 in [70 cm]); the hydranths are usually very tenuous, not exceeding a few inches (millimeters), but there are exceptions (i.e., Hydrocoryne miurensis: 2.3 in [6 cm]; Corymorpha nutans: 4.7 in [12 cm]; Monocoryne gigantea: 15.7 in [40 cm]; Candelabrum penola: 33.4 in [85 cm]; and Branchiocerianthus imperator: more than 6.5 ft [2 m]). The smallest polyps are those of the Microhydrulidae (Limnomedusae): they are reduced to a spherical or irregular body, ranging from 20 to 480 µm. The longest siphonophore is Apolemia uvaria, with colonies reaching 98.4 ft (30 m). The medusae of the hydrozoa are usually diaphanous, as are their polyps. Colored species, however, are frequent. Pigments derive from the diet or are produced directly. The most common color is reddish, deriving from crustaceans; other colors for medusae can be green, white, or orange; whereas polyps can be, according to the species, reddish, pink, white, or blue.

The medusae have typical and easily recognizable body architecture; the main feature that distinguishes them from other medusae is the velum. For this reason, they have also been called Craspedotae (with velum), as opposed to the medusae of the Scypozoa and Cubozoa, which were called Acraspedae (without velum). The polyps are quite varied in architecture, ranging from coral-like colonies (Millepora and the stylasterids) to gigantic polyps resembling those of the Anthozoa (Branchiocerianthus) to microscopic polyps reduced to a simple ball of tissue (Microhydrula).

Distribution

The Hydrozoa are cosmopolitan, and can be found in all water masses of the world, both in marine and in fresh waters. The Hydrozoa are known since the beginning of the modern study of animals; many species are Linnean and were described in the eighteenth century.

Habitat

The medusae of the Hydrozoa and the siphonophores are mostly planktonic; they are seasonal in occurrence and can be present in swarms, transported by the currents. Some medusae and some siphonophores, however, can be benthic. The polyp stages are usually benthic and live attached to the bottom, even though some species can be planktonic, such as the well-known Velella velella.

The Hydrozoa occur in all aquatic habitats, from anchialine caves to deep-sea trenches, from lakes and ponds to rocky coasts and the interstices among sand grains. The polyp stages of many species live exclusively on certain types of substrate, usually other organisms such as fishes, tunicates, polychaetes, bryozoans, mollusks, crustaceans, sponges, cnidarians, algae, sea-grasses, etc., with which they have symbiotic relationships ranging from simple epibiosis to commensalisms, mutualism, and parasitism.

Hydrozoa are mostly carnivores, using their habitat to acquire favorable positions to catch their prey. Planktonic stages are transported by currents, but can also move within water masses, searching for food. The position of benthic forms is decided by the planula, at the moment of settlement. Colonies are positioned at locations that will ensure a supply of new, fresh water around the hydranths, enhancing the transport of potential prey.

Behavior

The medusae are sharply individual; they can be gathered by winds and currents to form extensive swarms, but it is not known if they have any kind of social interaction while being

in close contact. The colonies of Hydrozoa, especially polymorphic ones, have been compared to superorganisms because of the complexity of the functions that they perform through the different types of zooids. The zooids of a colony usually derive from a single planula, thus being interconnected members of a single clone. It is possible, however, that different colonies merge their tissues or that different planulae aggregate to form coalescent colonies. In these cases, different individuals can be in such close connection that they become a single individual, possibly one of the extreme forms of social organization.

Most Hydrozoa have separate sexes. Fertilization is usually internal, with no copulation. Males spawn in the water and the sperm actively swim toward the eggs that are still on the maternal body (either a medusa or a polyp colony). The Hydrozoa are the first animals in which sperm attractants have been demonstrated, with species-specific attraction of sperms by the eggs. For many medusan species, both males and females spawn in the water, where fertilization occurs. Also in this case, however, sperm attractants facilitate gamete encounter.

The members of the same colony perform coordinated behaviors that surely involve communication. In Thecocodium brieni, for instance, the dactylozooids catch the prey with their tentacles, while the gastrozooids perceive that a prey is available and stretch towards the dactylozooids, detaching the prey from their tentacles and ingesting it. This division of labor, involving great coordination, is frequent in polymorphic colonies.

Planktonic animals do not show particular territoriality, being, by definition, transported by currents. It is probable, however, that medusae actively prevent being in too close contact to avoid competitive interactions while foraging. Territoriality is very strong in sessile organisms, where competition for the occupation of space is very evident. The arrangement of dactylozooids on the edge of many colonies is related to the defense of the territory from overgrowth by nearby animals. The feeding polyps can eat the settling larvae of potentially competing species, thus preventing competition for space.

When unfed, both polyps and medusae are always in search for food, with species-specific activity patterns. When the coelenteron is full of food, tentacles are usually contracted and do not catch prey, showing some control of cnidocyst discharge. Many medusae perform daily migrations through the water column.

Feeding ecology and diet

Hydrozoa are usually thought to feed on planktonic crustaceans such as copepods. Under laboratory conditions, they mostly survive with a diet based on Artemia nauplii. The medusae are mostly voracious carnivores and, when feeding on fish eggs and larvae, can be considered as being at the apex of trophic chains. The polyps are more varied in their food preferences. Recent investigations showed that the few species studied thoroughly feed on a great variety of prey, ranging from gelatinous plankton for the medusae to phytoplankton for the polyps. Some species have symbiotic zooxanthellae and are functionally photosynthetic.

Some medusae can remain immobile in the water, with their tentacles outstretched across the water column, performing ambush predation, whereas others can move across the water to contact prey, performing cruising predation. Polyps can simply extend their tentacles to catch passing prey, but they can also use special sense organs to perceive approaching prey and grab it actively, or they can form currents by moving their tentacles to direct food particles toward the mouth. Symbiotic species are very specialized in their feeding behavior; the most extreme cases are Halocoryne epizoica that feed on bryozoan tentacles, and Polypodium hydriforme that feed on sturgeon eggs from the inside, being the only intracellular parasitic metazoan.

Hydrozoa use cnidocysts as the main organelles to catch their food. The superclass has the richest variety of cnidocyst types of the whole phylum, with a fine range of adaptations to catch from tiny prey like the single cells of phytoplankton to the crustaceans and larvae of the zooplankton to the animals that live in the sand and mud, such as nematodes. Many species have very restricted diets, being specialized for just one type of food.

Both polyps and medusae are mostly carnivorous, feeding on almost all animals of proper size. In no other metazoan group (with the possible exception of parasitic trematodes) is the lifecycle of such paramount importance in defining the properties of a given species, and this is valid also for the type of prey. Tiny medusae that feed on fish eggs and larvae, sometimes impairing the success of recruitment, can be the most voracious predators of fish. The propensity of feeding on almost all types of larvae (both for polyps and for medusae) includes the Hydrozoa within the predators of almost all Metazoa with an indirect lifecycle.

Reproductive biology

Nothing is known regarding the courtship and mating. The eggs are brought in masses on female medusae or in female gonophores. According to the species, the eggs can be small and in great numbers, or they can be large and few, even a single one per gonophore.

In the Automedusae, with no polyp stages, there is little or no asexual reproduction in larval medusae (known as actinulae in the Narcomedusae) so that each fertilization event leads to one or a few adult medusae. In the Hydroidomedusae, the widespread asexual reproduction of the polyp stage can be considered as a polyembryony or as a larval amplification, as it happens in some parasites such as trematodes. Each fertilization event, therefore, leads to a single planula that produces a polyp colony that, in its turn, will produce many adult medusae. Since the lifespan of polyp colonies can last many years, each planula can lead to the production of hundreds or thousands medusae.

The so-called planula larva is nothing more than a gastrula, thus being more an embryo than a larva. Since gametes are shed before or soon after fertilization, embryonic development takes place outside the maternal body. Planulae can be solid (stereogastrula) or hollow (coeloblastula); usually the species with medusae in the lifecycle have hollow planulae that live part of their life in the water column, swimming with cilia or flagella to reach the settling sites. The species with suppressed medusae usually produce solid planulae that fall to the bottom and settle near the parent colony. Adults, by definition, are the sexually reproductive morphs in a lifecycle; if a medusa is present, it is the adult in the lifecycle. The polyp stage, in this framework, is a specialized (and perennial) larva that produces a great number of adults throughout its long life. In many species, however, the medusa stage can be reduced or even suppressed, so that the larvae, by paedomorphosis, become the sexually mature morphs of the lifecycle. Almost half of the species of Hydroidomedusae have suppressed or reduced medusae; the group is, therefore, the most paedomorphic of the whole animal kingdom.

Some medusae (e.g., Eleutheria) have special brood pouches where they safeguard developing medusae. Some hydroids have gonothecae with apical brood chambers that retain the planulae for a certain period.

Many species are sharply seasonal, being active only in narrow periods of time. The medusae can be present for a few weeks or months, completely disappearing from the water column and being represented by the correspondent polyps in the benthos. Polyp colonies can regress to resting hydrorhizae for long periods, reactivating at the onset of favorable conditions. Planulae can become encysted and remain dormant just as the resting hydrorhizae, being covered by a chitinous sheath.

Conservation status

No species are listed by the IUCN Red List. For most Hydrozoa, the distribution and the abundance are not known and only the few remaining specialists know about their presence. Many species are endemic simply because they are not searched for and only the areas of activity of specialists are covered. The only groups mentioned in regional or national red lists are the calcified, coral-like Milleporidae and Stylasteridae, which are also listed by CITES. Inclusion of these taxa on conservation lists is linked to the assessment of species value according to appearance.

Milleporidae and Stylasteridae have been subjected to trade, as are some hydroids (known as "white weeds" in the North Sea). Their decline is linked to habitat degradation.

Significance to humans

The Hydrozoa are mostly inconspicuous, both in the polyp and in the medusa stage, and are generally overlooked. The famous treatise by Tremblay, describing transplants in Hydra, inspired the fantastic novel Frankenstein by Mary Shelley. The illustrations of some monographs, especially those on medusae by Haeckel, are renowned for their beauty. The modern music composer, Frank Zappa, wrote a song on a hydromedusa that has been named after him: Phialella zappai.

"White weeds" (the colonies of hydroids of the genera Hydrallmania and Sertularia) had been used as decoration before the sharp reduction of their populations. Some Hydrozoa are used as laboratory animals for experimental biology; Hydra is the most popular one, but others include Aequorea victoria (for the production of the labeling enzyme aequorein), Hydractinia spp., Laomedea spp., and Tubularia spp.

Hydroids are important members of fouling communities, inhibiting the functioning of power plants by clogging their pipes and reducing the velocity of ships by settling on their hulls. Some species have been reported as pests in aquaculture, feeding on the larvae of the reared species or on their food. Polypodium hydriforme is a threat to the production of caviar, being a parasite of sturgeon eggs.

Some species of medusae (e.g., Gonionemus) can inflict severe stings on humans, as do some hydroid colonies such as the species of Millepora (fire corals) and some aglaopheniids. When present in swarms, even small medusae like those of Clytia can inflict slight stings on swimmers. The most important threat to human activities is the predation of some medusae (e.g., Aequorea victoria) and floating hydroids (e.g., Clytia gracilis) on the eggs and larvae of commercially exploited fish. This kind of predation can reduce the success of fish recruitment, reducing the yield of fisheries.

Species accounts

List of Species

No common name

Halammohydra schulzei

order

Actinulida

family

Halammohydridae

taxonomy

Halammohydra schulzei Remane, 1927, North Sea.

other common names

None known.

physical characteristics

Body as a long gastric tube (manubrium) with a terminal mouth, with a small aboral cone, separated from manubrium by a neck, bearing an adhesive organ; aboral nerve ring; one aboral whorl of 28 (sometimes up to 32) amphicoronate solid tentacles with a large basal thickening, alternating with 12 ecto-endodermic statocysts; gonochoric; generally two opposite gonads; without brood pouch. Body 0.027–0.031 in (0.7–0.8 mm) high.

distribution

Cosmopolitan. (Specific distribution map not available.)

habitat

Interstices of marine sand.

behavior

Eretant in the marine sand.

feeding ecology and diet

Nothing is known.

reproductive biology

Dioecious, sex cells released in the sea, reproduction through a special actinulid larvae or halhydrula.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Corymorpha nutans

order

Capitata

family

Corymorphidae

taxonomy

Corymorpha nutans M. Sars, 1835, Norway.

other common names

None known.

physical characteristics

Hydroid: large, solitary, hydrocaulus subcylindrical, with short sensory papillae at base and numerous long, anchoring filaments; hydrocaulus with parenchymatic endoderm with numerous longitudinal anastomizing peripheral canals; hydrocaulus protected by a transparent membranous tube; hydranth flaskshaped to vasiform with about 20–80 oral filiform tentacles in several irregular whorls, and 20–32 aboral considerably longer filiform tentacles; hydranth and distal part of hydrocaulus bend downward; medusa buds in dense clusters on about 15–20 branching peduncles just above aboral tentacles.

Medusa: umbrella up to 0.23 in (6 mm) high (including apical process), cylindrical, with a high, pointed apical process and a long, narrow umbilical canal; mesoglea thick; velum wide; manubrium large, cylindrical, on short gastric peduncle, about two-thirds of the length of subumbrella, in full extension reaching slightly beyond exumbrellar margin; mouth simple, tube like, armed with cnidocysts; four radial canals and circular canal fairly broad; gonads completely surrounding manubrium, but living peduncle and mouth free; one single, long, marginal tentacle, moniliform; three perradial non-tentacular bulbs smaller than the tentacular one.

distribution

Atlantic, the Mediterranean, and Black Sea. (Specific distribution map not available.)

habitat

The hydroid lives at moderate depth with its basal part embedded in soft bottoms. The medusae are planktonic in coastal waters.

behavior

Nothing is known.

feeding ecology and diet

The hydroid, with its typical posture, is oriented with the mouth near to the bottom and it probably captures epibenthic vagile animals. The medusae feed on planktonic animals.

reproductive biology

Both hydroids and medusae are sharply seasonal, occurring in correspondence with spring plankton blooms. Hydroids are not produced immediately, fertilization and the sexual reproduction originate an encysted embryo that spends the unfavorable season in the sediments, to become a polyp at the onset of the following favorable season.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Fire coral

Millepora alcicornis

order

Capitata

family

Milleporidae

taxonomy

Millepora alcicornis Linnaeus, 1758, West Indies.

other common names

English: Sea ginger; French: Corail de feu; German: Feuerkorallen.

physical characteristics

Hydroid: colony forms massive, calcareous exoskeletons (coenosteum), forming horn-shaped, upright branches or plates; coenosteum with an internal complex network of coenosarcal tubes, covered externally by a thin ectodermal layer, coenosteal surface perforated by pores; margins of pores not protruding from surface of coenosteum; large gastropores surrounded by smaller dactylopores, forming indistinct cyclosystems; no gastrostyles or dactylostyles; polyps polymorphic; gastrozooids relatively short and stout, with an oral whorl of 4–6 short capitate tentacles, arising from gastropores; dactylozooids long, slender, mouth less, with scattered capitate tentacles, arising from dactylopores; cnidome with macrobasic mastigophores; gonophores arising from coenosarc within ampullae embedded in the coenosteum. Color variable, usually yellow-brown because of algal symbionts.

Medusa: short-lived, free-swimming eumedusoids with exumbrellar cnidocyst patches, narrow velum, four radial and circular canals, gonads occupying the place of an indistinct manubrium; without tentacles, without sense organs (this description applies to the eumedusoid of M. complanata, the only Millepora eumedusoid known in detail).

distribution

Tropical Pacific, Indian Ocean, Red Sea, and Caribbean. (Specific distribution map not available.)

habitat

Common to abundant in shallow waters, or in inner ledges and outer reefs, usually not exceeding 98.4 ft (30 m), correlated with dependence of the colonies on their symbiotic unicellular algae or zooxanthellae that need light for their processes of assimilation.

behavior

Nothing is known.

feeding ecology and diet

Feed upon planktonic animals, mostly crustaceans.

reproductive biology

Sexual reproduction by free-swimming eumedusoids, usually ripe from April–July.

conservation status

Not listed by the IUCN.

significance to humans

Millepora are severe stingers and can cause violent, uncomfortable burning, even in dried state.

No common name

Paracoryne huvei

order

Capitata

family

Paracorynidae

taxonomy

Paracoryne huvei Picard, 1957, Mediterranean coast of France.

other common names

None known.

physical characteristics

Stolonal colonies with encrusting hydrorhizae composed of naked coenosarc, with gastrozooids, dactylozooids, and gonozooids; gastrozooids with 12–26 tentacles irregularly distributed in four verticils at distal half; dactylozooids large, without tentacles or mouth; gonozooids without tentacles or mouth; gonophores as fixed sporosacs (cryptomedusoids), male and female in different colonies; male gonophores in number of 2–10 per gonozooid, ovoids, with a characteristic apical prolongation; female gonophores 2–4 per gonozooid, bigger and more spherical than males; young gonophores with an apical prolongation (as in the males), which is absent and replaced by an orifice at maturity.

distribution

Western Mediterranean. (Specific distribution map not available.)

habitat

The colonies live on rocks and mussels, from the intertidal to the very shallow subtidal. They form very evident pink patches on bare rocks.

behavior

Nothing is known.

feeding ecology and diet

Nothing is known.

reproductive biology

The dioecious colonies are present in the winter and disappear in the summer. After sexual reproduction, the planulae encyst and remain dormant until the onset of the following favorable season.

conservation status

In spite of the great tradition of biodiversity exploration in the Mediterranean, this very distinctive species has been discovered in relatively recent times. It became very frequent and abundant in the 1970s and 1980s, but is now extremely rare again. The resting stages surely play an important part in these patterns of abundance.

significance to humans

None known.

No common name

Polyorchis penicillatus

order

Capitata

family

Polyorchidae

taxonomy

Melicertum penicillatum Eschscholtz, 1829, California, United States.

other common names

None known.

physical characteristics

Hydroid: unknown.

Medusa: bell up to 2.3 in (60 mm) high, with a gastric peduncle, up to 160 marginal hollow tentacles in a simple row along exumbrellar margin; tentacular bulbs tubular, adnate, with bright red ocellus on short spur; four radial canals with numerous, short, blind lateral diverticula; ring canal with centripetal diverticula; manubrium prismatic, pendulous, sausage shaped; four crenulated oral lips with distinct cnidocyst row; gonads pendulous and long, hanging from peduncular manubrium pouches.

distribution

Pacific Ocean, from the coasts of the North American continent to Hawaii. (Specific distribution map not available.)

habitat

Bays and gulfs.

behavior

It is a benthic medusa, but it can periodically come to the surface and sink back to the bottom.

feeding ecology and diet

Foraging takes place mostly on the bottom of bays, where the medusae feed on crustaceans, mainly copepods.

reproductive biology

The medusae are present all year round and are mostly ripe. In spite of this widespread occurrence of the medusae, the polyp stage remains unknown.

conservation status

Concern has been expressed about the impact of coastal development on this animal. Because of its preference for bays, it is very sensitive to coastal pollution.

significance to humans

It is used in aquarium displays for educational purposes.

By the wind sailor

Velella velella

order

Capitata

family

Porpitidae

taxonomy

Medusa velella Linnaeus, 1758, Mediterranean Sea.

other common names

None known.

physical characteristics

Hydroid: the colony is a flattened oval to slightly S-shaped float, with a triangular sail and concentric air chambers; up to 1.5 in (40 mm) long and 0.78 in (20 mm) wide, higher in the center than at the edges. Float and sail are supported by chitin covered by mantle tissue; margin of float is soft and flexible. The center of colony underside is a single large gastrozooid encircled by a ring of medusa-producing gastro-gonozooids and a peripheral band of dactylozooids. Central-feeding zooid oval, with an elongated hypostome, without tentacles or medusa buds. Gastro-gonozooids are spindle shaped, with a swollen mouth region, lacking tentacles, but with warts of cnidocyst clusters concentrated in distal half; on proximal half of hydranth, numerous medusa buds growing in groups from short blastostyles. Dactylozooids are mouth less, long, and tapering, oval in cross section, with cnidocysts concentrated in two lateral bands on the narrow sides. The float is deeply blue when alive, medusa buds yellow-olive from symbiotic algae.

Medusa: with four exumbrellar cnidocyst rows, four radial canals; two pairs of opposite, perradial tentacles, a short adaxial one and a long abaxial one, each with a large terminal cnidocyst cluster; two perradial marginal bulbs without tentacles; manubrium conical with quadrate base; mouth tubular; gonads irregularly arranged perradially and interradially.

distribution

Atlantic and Indo-Pacific. (Specific distribution map not available.)

habitat

The hydroid floats, with the sail out of the water and the zooids hanging down in the hyponeuston. The medusae are planktonic, in shallow water and possibly in deep water as well, where the young colonies have been recorded, before climbing to the surface.

behavior

Nothing is known.

feeding ecology and diet

The hydroid colony feeds on surface plankton, from crustaceans to appendicularians, and especially on fish eggs.

reproductive biology

Floating hydroids can occur in immense offshore swarms, sometimes stranding along shorelines. The colonies liberate rarely observed young medusae that sink toward the bottom. Development of new colonies occurs in deep waters.

conservation status

Not listed by the IUCN.

significance to humans

The massive strandings can entrap sand and protect the shore from erosion. Feeding on fish eggs makes Velella a potential competitor for man in the use of fish resources.

No common name

Aequorea victoria

order

Conica

family

Aequoreidae

taxonomy

Mesonema victoria Murbach and Shearer, 1902, British Columbia, Canada.

other common names

None known.

physical characteristics

Hydroid: small colonies, unbranched, with creeping hydrorhiza; pedicels twice as long as hydrotheca. Hydrotheca thin, with an operculum made by many flaps not articulated to hydrothecal rim that converge and close the theca when the hydranth is contracted. Hydranth has 20 tentacles, with an intertentacular membrane at base; gonotheca of thin perisarc, originating right below the theca; a single medusa bud.

Medusae are small at liberation, with two tentacles and four radial canals. They can grow up to 4.7 in (120 mm) wide and 1.5 in (40 mm) high. Umbrella saucer shaped to hemispherical, a thick hemispherical projection of mesoglea protrudes into the manubrium; lips of manubrium are fringed and can close the manubrium completely on the sides of the mesogleal projection. Tentacles can be up to 150, lip lobes are usually half the number of tentacles. Each tentacular bulb has an abaxial pore connecting the circular canal to the outside.

distribution

Northeastern Pacific. (Specific distribution map not available.)

habitat

The hydroid grows on mussel shells. The medusae are planktonic, in coastal and open waters.

behavior

The function of the hemisphercal mesogleal projection into the manubrium is to allow the closed mouth to rotate around it, thus pushing the contents of the stomach into the radial canals. The pores in the circular canal are used to eject undigested materials and are more anal pores than excretory pores (as they are commonly called).

feeding ecology and diet

Unknown for the hydroid, whereas the medusae have been reported to feed on fish larvae and on gelatinous plankton.

reproductive biology

The hydroid is tiny and rarely observed, as are the newly released medusae. Medusa production, however, is very intense, since the species can be present in swarms. The swarms produce great quantities of planulae to support future blooms through the hydroid generation.

conservation status

The massive use of this species to extract the bioluminescent enzyme aequorein might have some influence on population viability.

significance to humans

By feeding on larvae, it might have a negative impact on fish recruitment; but it has a great role in experimental biology because of its bioluminescent enzyme.

No common name

Aglaophenia pluma

order

Conica

family

Aglaopheniidae

taxonomy

Sertularia pluma Linnaeus, 1758, North Sea, United Kingdom.

other common names

None known.

physical characteristics

The colonies resemble a feather, with a straight stem (hydrocaulus) and two series of alternate branches (hydrocladia). Hydrocaulus monosiphonic up to 5.9 in (150 mm), brown, unbranched or dichotomously branched; basal part athecate, followed by one or two prosegments; remainder internodes, each with three nematothecae and a pseudonematotheca; nodes oblique; hydrocladia alternate, with whitish cormidia separated by transverse nodes. Hydrotheca deep, rim with nine cusps of varied length; intrathecal adcauline septum usually well developed, median nematothecae two-thirds adnate, not reaching the margin of the hydrothecae; lateral nematothecae reaching the rim of the hydrothecae; aperture of nematothecae gutter shaped. Male and female corbulae white, with free costa; male close, with slit-like openings between the costae, female with fused costae and smaller slits. Hydranth small, transparent, cylindrical; hypostome held at level of hydrothecal rim, rounded and low; 10 tentacles emerging at hydrothecal rim.

distribution

Usually considered cosmopolitan, but many other species of the same genus have probably been identified as this nominal species. (Specific distribution map not available.)

habitat

It grows on algae, in shallow rocky bottoms.

behavior

Nothing is known.

feeding ecology and diet

The shape of the colonies is suited to intercept food particles suspended in the water column. The feeding hydranths are small and in great number; they have been observed to beat their tentacles to create a current towards the mouth. This might be an adaptation for active filter feeding of small food items such as phytoplankton.

reproductive biology

Colonies dioecious, almost continuously fertile.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Zappa's jellyfish

Phialella zappai

order

Conica

family

Phialellidae

taxonomy

Phialella zappai Boero, 1987, Bodega Bay, California, United States.

other common names

None known.

physical characteristics

Hydroid: colony simple, unbranched; hydranth very extensile, with about 14 tentacles, alternately held upward and downward. Oral part of the hydranth is globular, separated from the rest of the body. Hydrotheca cylindrical, elongated, with an operculum of about seven cusps separated from the hydrothecal wall by a thin line, not always evident. Diaphragm present. Pedicel is as long as the hydrotheca or a little shorter, annulated throughout. Gonothecae on the stolon, arising from short, annulated pedicels, with wavy or smooth walls, tapering below and truncate above, with one or two medusa buds.

Medusa: newly released medusae sub-spherical, about 0.002 in (0.6 mm) in diameter, with four tentacles, four interradial tentacular bulbs deprived of tentacles, and four radial canals with medial darker areas. Eight statocysts, with 1–3 statoliths, are on the inner edge of the ring canal, supported by a cushion of cells. Manubrium is short (one third of the bell cavity), with four short lips. Tentacular nematocysts are in clusters, giving a moniliform appearance to the tentacles. Perradial tentacular bulbs are almost round or triangular, interradial tentacular bulbs much smaller, but evident. The medusae grow rapidly, reaching 0.11 in (3 mm) in diameter in 10 days, dome shaped with four well-developed inter-radial tentacles, and eight developing adradial tentacles. At this age, the eggs are clearly visible. Manubrium cruciform, lips more evident and starting to bend upward. Tentacular bulbs still round, tending to elongate. Tentacles moniliform. Development continues with an increase in size and number of tentacles (36 the highest number observed). Adult specimens dome shaped, with gonads almost in the middle of the radial canals. Manubrium cruciform, with folded lips bending upward, with four gastric pouches; four black spots may be present at its base. Tentacular bulbs triangular, but still rounded. Tentacles evidently moniliform.

distribution

Endemic to the west coast of the United States (California). (Specific distribution map not available.)

habitat

The hydroid grows on mussel shells. The medusa is in coastal plankton.

behavior

Nothing is known.

feeding ecology and diet

Under laboratory conditions, both polyps and medusae feed on Artemia nauplii; in the wild they probably feed on small crustaceans.

reproductive biology

Each gonotheca produces one or two medusae that, under laboratory conditions, continue to develop also after reaching sexual maturity. Some specimens become mature 10 days after liberation. They can develop a new gonad after the first spawning.

conservation status

Not listed by the IUCN.

significance to humans

This species was named after the modern music composer Frank Zappa (1940–1993), who, in exchange, wrote a song about it ("Lonesome Cowboy Nando").

Portuguese man of war

Physalia physalis

order

Cystonectae

family

Physaliidae

taxonomy

Holothuria physalis Linné, 1758, Atlantic Ocean.

other common names

English: Blue bottle; French: Galère, frégate, vaisseau de guerre hollandais, vaisseau de guerre portugais.

physical characteristics

The only pleustonic siphonophore. Colonies consist of a large, purplish blue horizontal pneumatophore that floats on the sea surface, reaching 11.8 in (30 cm) in length in the largest specimens; pneumatophore carries the polyps, which form cormidia at the oral end; at its top, an erectile, longitudinal crest or sail that may be left- or right-handed, all drifting at the mercy of the winds. Pneumatophore asymmetrical; two forms, each the mirror image of the other. Each cormidium consisting of a gastrozooid associated with a tentacle and a gonodendron; however, unlike other siphonophores, tentacle separating from the basigaster during the later stages of development. The tentacles may attain several meters in length; a continuous formation of new cormidia. As the cormidia mature, new gastrozooids gradually lose their tentacles and becoming palpons. Small medusoid gonophores develop at the bases of the terminal palpons. Functional gonophores of a given colony are of a single sex only.

distribution

Widely distributed in tropical and subtropical regions in the three great oceans and in the Mediterranean. (Specific distribution map not available.)

habitat

Only pleustonic siphonophore can occur in great numbers (navies); frequently blown ashore by strong winds.

behavior

The effect of the wind on the sail is to move the left-handed specimens to the right of the wind direction and vice versa for the right-handed ones.

feeding ecology and diet

Feed on small planktonic organisms or small fishes.

reproductive biology

Dioecious, release their sexual cells in the sea, development through siphonula larvae.

conservation status

Not listed by the IUCN.

significance to humans

The tentacles can stretch out to many feet (meters) below the float and can inflict powerful and very painful stings on swimmers who become entangled in their tentacles.

Immortal jellyfish

Turritopsis nutricula

order

Filifera

family

Clavidae

taxonomy

Turritopsis nutricula McCrady, 1859, South Carolina, United States.

other common names

None known.

physical characteristics

Hydroid: small colonies stolonal, monosiphonic (simple stem), larger ones erect, irregularly branched and increasing in diameter from base to apex, polysiphonic (compound stem); branches basally adnate to hydrocaulus or other branches, then curving away at an acute angle and becoming free; hydrocaulus and hydrocladia covered by a firm bi-layered perisarc, mostly encrusted with detritus and algae, terminating below hydranth base; hydranths terminal, naked, elongated, fusiform, with 12–38 filiform tentacles scattered over distal three quarters of column, proximal ones shorter than distal; hypostome elongated conical; medusa buds arising mostly one by one from short pedicels below hydranths, pear shaped, enclosed in perisarc.

Medusa: umbrella 0.15–0.43 in (4–11 mm) high, bell shaped to piriform, higher than wide, mesoglea thicker at apex; manubrium large, cross-shaped in transverse section, red; four radial canals passing through the four compact vacuolated endodermal masses situated above digestive part of manubrium; four-lipped mouth with a continuous row of sessile cnidocyst clusters along margin; 80–120 closely spaced marginal tentacles; gonads interradial, mature females often with developing embryos and planulae; with adaxial ocelli.

distribution

Atlantic, Indo-Pacific, and the Mediterranean. (Specific distribution map not available.)

habitat

The hydroids live in shallow, highly oxygenated water, often under overhangs. The medusae are members of the coastal plankton.

behavior

Nothing is known.

feeding ecology and diet

In the laboratory, both hydroid and medusae can survive with a diet of Artemia nauplii; the diet in the wild is unknown.

reproductive biology

Medusae are produced by the hydroid in summer months and can survive in the laboratory for one month. A peculiarity of this species is the possibility of ontogeny reversal under laboratory conditions: the medusae, in fact, can rearrange their tissues and go back to the polyp stage if subjected to sub-lethal stress and also at the end of their lifespan, after spawning. For this reason, this medusa has been called "the immortal jellyfish" by the media.

conservation status

Not listed by the IUCN.

significance to humans

The possibility for this species to perform ontogeny reversal in the laboratory offers a unique opportunity to study the genetic control of aging and the mechanisms of rejuvenation.

No common name

Eudendrium glomeratum

order

Filifera

family

Eudendriidae

taxonomy

Eudendrium glomeratum Picard, 1951, Banjuls-sur-Mer, Mediterranean Sea.

other common names

None known.

physical characteristics

Colonies up to 11.8 in (30 cm) high, composed of compound and branched hydrocauli; older perisarc brown and thick; thin and yellowish to transparent in younger regions. Polyp is urn shaped, hypostome peduncled, tentacles filiform, 24–28 in one whorl. Gonophores are fixed. Female mature gonophore with unbranched spadix; male gonophore provided with one or two chambers; mature balstostyle with either a normal number of tentacles or a reduced number of partly atrophied tentacles. Diagnostic feature is holotrichous macrobasic euryteles (24 10–28 11µm), concentrated in several groups characteristically conspicuous at the basal half of the polyp, with long butt (four times length of capsule), spirally coiled around the main axis of the cnidocyst.

distribution

Atlantic, Indo-Pacific; mostly in temperate waters. (Specific distribution map not available.)

habitat

Rocky cliffs, from the surface to 328 ft (100 m) and below. It can be very abundant under favorable conditions, forming a facies.

behavior

The hydranths use the peduncled hypostome as a sphincter dividing the coelenteron from the outside. They stretch their tentacles in the water to catch their prey.

feeding ecology and diet

Some species of the genus Eudendrium have been reported to feed on the plankton surrounding their colonies. They can also ingest the mucus that they produce and possibly digest organic particles and organisms trapped into it, acting as mucous filter feeders.

reproductive biology

Fertilization occurs while eggs are still on the gonophores; the planulae can remain attached to the mother colony by mucous threads to be able to settle in its vicinities. Planulae can aggregate and form chimerical hydroid colonies.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Hydractinia echinata

order

Filifera

family

Hydractiniidae

taxonomy

Alcyonium echinatum Fleming, 1828, (Scotland, United Kingdom).

other common names

None known.

physical characteristics

Hydrorhiza about 0.11 in (3 mm) thick, with numerous blunt conical chitinous spines with jagged edges, encrusting on gastropod shells but also other solid substrata; white to pale pink, giving rise to different kinds of polyps: gastrozooids slender, widening upwards, about eight tentacles in one whorl, hypostome conical; gonozooids shorter than gastrozooids, with few tentacles and a ring of gonophores; dactylozooids long, slender.

Medusae reduced to fixed gonophores, sexes generally on different gonozooids; male gonophore yellow to white, ovoid; female gonophore pink and spherical.

distribution

Atlantic, Arctic, and the Mediterranean. (Specific distribution map not available.)

habitat

Hydroids grow on gastropod shells inhabited by hermit crabs, on soft bottoms.

behavior

Under laboratory conditions, the colonies can be aggressive with the formation of hyperplasic stolons that sting other organisms while competing for the substrate.

feeding ecology and diet

Gastrozooids feed on benthic organisms (nematodes, crustaceans, etc.) while being transported by the hermit crab. They also can feed on the larvae of the crab.

reproductive biology

Eggs and sperms are released in the water, where fertilization occurs and planulae are formed. Gamete production is almost continuous, but at a slower pace in winter time.

conservation status

Not listed by the IUCN.

significance to humans

This species, and in general many Hydractiniids, is easy to rear under laboratory conditions and is widely used for experimental biology as a laboratory animal.

No common name

Hydrichthys mirus

order

Filifera

family

Pandeidae

taxonomy

Hydrichthys mirus Fewkes, 1887, New England.

other common names

None known.

physical characteristics

Hydroid: hydrorhiza as a thin encrusting plate on the skin of fish host. Polyps of two types: gastrozooids are elongate, tubular, with distal mouth, and no tentacles; nearly all with a small bud at base; mouth with an armature of some microbasic euryteles and desmonemes; no nematocysts in other regions of colony, apart from medusa buds; gonozooids very contractile, with up to three lateral branches bearing clusters of medusa buds at different stages of development; no sign of perisarc. Medusa buds on gonozooids, hydrorhiza, and possibly also on hydranths; exumbrellar nematocysts present; well-developed buds stiff, with two big tentacular bulbs and no apparent sign of tentacles.

Medusa: umbrella height 0.15 in (4 mm), dome shaped, with apical projection; jelly soft. Manubrium with well-developed folds in stomach wall, bearing the gametes; manubrial folds nearest to mouth much developed, almost covering lips. Above these main folds, two or three additional folds sometimes developed. Gametes almost completely covering interradial portions of stomach.

distribution

Indo-Pacific; very rare. (Specific distribution map not available.)

habitat

The hydroid lives symbiotically with fish; the medusa is planktonic, possibly with a tendency to forage near the bottom.

behavior

The gastrozooids can bend to touch the surface of the fish. They have a distal annular contractile ring resembling a sucker, but feeding on fish was not observed. Under laboratory conditions, the hydroid do not take Artemia nauplii. After the death of the fish, colonies can detach and become free living. It is therefore possible that they can survive in the plankton if the host dies. In the laboratory, the medusae tend to stay on the bottom of rearing jars, with their long tentacles completely extended, attached to the bottom of the finger bowl, stretching them for a long distance while swimming parallel with the bottom. Tentacles are then contracted and the bell is drawn quickly backwards. Prey is captured in lips during this fast backward movement. Medusae possibly have a benthic feeding habit. After feeding, they swim vigorously for some hours.

feeding ecology and diet

The hydroid most probably feeds on the supporting fish, using the mouth as a sucker. The medusa feeds on crustaceans.

reproductive biology

Nothing is known.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Distichopora violacea

order

Filifera

family

Stylasteridae

taxonomy

Millepora violacea Pallas, 1766, Pacific Ocean.

other common names

None known.

physical characteristics

Hydroid: colony violet-blue, small, and flabellate; branches compressed and granular, with low longitudinal ridges, ending with two flattened lobes, usually whitish, growing in opposite directions; gastro- and dactylopores very long, extending for a great distance down the center of the lobes; pore rows on lateral edges of lobes and main stem; dactylopores in equal number on both sides of pore rows; gastrostyles ridges bearing tall, slender, often fused, spines; no ring palisade; ampullae in groups, opening to surface by irregularly shaped pores.

distribution

Indo-Pacific. (Specific distribution map not available.)

habitat

Coral formations in shallow water.

behavior

Nothing is known.

feeding ecology and diet

Nothing is known.

reproductive biology

Nothing is known.

conservation status

Like almost all stylasterids, it is protected by international treaties.

significance to humans

None known.

No common name

Laingia jaumotti

order

Laingiomedusae

family

Laingiidae

taxonomy

Laingia jaumotti Bouillon, 1978, Laing Island, Bismarck Sea.

other common names

None known.

physical characteristics

Hydroid: unknown.

Medusa: umbrella lobed, divided by peronial grooves or similar structures, with no exumbrellar cnidocyst tracks; four radial canals; no typical circular canal, but a solid core of endodermal cells around umbrellar margin; tentacles solid, bent shortly after their point of origin, inserted on the exumbrellar surface above bell margin; marginal bulbs largely displaced towards exumbrella, forming peronial-like structures; manubrium simple, quadrangular, tubular, or conical; mouth opening quadrangular to circular; gonads on manubrium in four interradial pouches; no sense organs.

distribution

Bismarck Sea, Papua New Guinea. (Specific distribution map not available.)

habitat

Superficial layers of the sea.

behavior

Nothing is known.

feeding ecology and diet

Nothing is known.

reproductive biology

Nothing is known.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Craspedacusta sowerbyi

order

Limnomedusa

family

Olindiidae

taxonomy

Craspedacusta sowerbyi Lankester, 1880, found on a water-lily in a tank at Regent's Park, London.

other common names

None known.

physical characteristics

Hydroid: freshwater, solitary or forming small colonies of 2–4, rarely 7 polyps; hydranths without tentacles, cylindrical, with apical mouth (hypostome) surrounded by cnidocysts forming a spherical capitulum under which the polyp is slightly tapering, forming a distinct neck; basal portion of hydranths with periderm covering, attaching colony to substrate; medusa buds lateral, on the middle or lower part of body column, often becoming terminal by hydranth reduction; asexual reproduction by frustules, transversal division and resting stages (cysts).

Medusa: umbrella 0.39–0.78 in (10–20 mm) wide, slightly flatter than a hemisphere; mesoglea fairly thick; with well-developed, marginal cnidocyst ring; velum broad and well developed; manubrium large, upper portion conical with broad square base, tapering downwards to cross shaped distal region; mouth with four simple or slightly folded lips, extending beyond umbrella margin; four straight radial canals and circular canal broad and massive; four large, smooth, triangular pouchlike gonads, with rounded comers, hanging down into subumbrellar cavity from points of junction of radial canals with manubrium; with 200–400 or more hollow marginal tentacles, in several series situated at different levels on umbrella margin; oldest four perradial marginal tentacles being largest and highest; bases of marginal tentacles adherent to exumbrella; surface of marginal tentacles covered with evenly distributed papillae, each with 3–10 cnidocysts; 100–200 or more statocysts, usually about half number of marginal tentacles; statocysts situated in velum, forming centripetal tubes with basal enlargements near umbrella margin.

Numerous species of Craspedacusta have been described, mainly from China; it is not excluded that they represent anything more than variations of a single species.

distribution

Cosmopolitan in freshwaters and sometimes in brackish waters of temperate and tropical areas. (Specific distribution map not available.)

habitat

Freshwater surfaces and calm rivers, often found in tanks and aquaria.

behavior

Hydroids live on water plants; the medusae are active swimmers, living usually near surface.

feeding ecology and diet

Feed on small freshwater planktonic organisms, mainly protozoa, rotifers, crustaceans, worms, and fish larvae.

reproductive biology

Dioecious, sex cells released in freshwater; the planula gives rise to polyp colonies.

conservation status

Not listed by the IUCN.

significance to humans

The medusae may damage and injure fishes in fish farms.

No common name

Olindias phosphorica

order

Limnomedusa

family

Olindiidae

taxonomy

Oceania phosphorica Delle Chiaje, 1841, Mediterranean Sea.

other common names

None known.

physical characteristics

Hydroid: the polyps have not been yet found in field. Weill (1936) described from laboratory observations a small solitary hydranth without tentacles enclosed in a cylindrical or irregularly curved hydrothecae covering more than half its length, and much longer than the polyp itself; mouth distal surrounded by large cnidocysts.

Medusa: umbrella 0.78–1.5 in (40–60 mm) wide, almost hemispherical, mesoglea fairly thick; 11–19 centripetal canals per quadrant; 30–60 primary tentacles; usually two ecto-endodermal statocysts at base of each primary tentacle; 100–120 secondary tentacles; 100–170 marginal clubs.

distribution

Atlantic and the Mediterranean. (Specific distribution map not available.)

habitat

Lives near the bottom or at the surface near shores.

behavior

Lives usually fixed to algae or Posidonia.

feeding ecology and diet

Feeds on small planktonic organisms.

reproductive biology

Dioecious, the sex cells are released in the sea; the planula gives rise to a reduced polyp stage.

conservation status

Not listed by the IUCN.

significance to humans

The sting of the medusae may cause skin irritations.

No common name

Hydra vulgaris

order

Moerisiida

family

Hydridae

taxonomy

Hydra vulgaris Pallas, 1766, European freshwaters.

other common names

French: Hydre d'eau douce.

physical characteristics

Solitary freshwater hydroids; 0.47 in (12 mm) in height; with 7–12 hollow filiform tentacles, but often moniliform distally, in one whorl under hypostome; hermaphrodic species, eggs and sperm developed directly in ectoderm of polyps in wart-like protuberances, "testis" developing on upper part of hydranth, "ovaries" on lower part, with up to eight eggs enveloped; in a chitinous embryotheca when fecundated, embryotheca with long, thin spines; asexual reproduction by lateral buds, leading only to temporary colonies; lower part of hydranth with simple pedal disc and with central pore, no perisarc except on encysted embryos.

distribution

Cosmopolitan. (Specific distribution map not available.)

habitat

Freshwater surfaces, rivers, and ponds.

behavior

Fixed on freshwater plants, stones, empty freshwater shells, and worm larval tubes (Trichoptera); able to move with the help of the tentacles.

feeding ecology and diet

Feed on small freshwater planktonic organisms: primarily protozoa, rotifers, crustaceans, and worms.

reproductive biology

Protanderic hermaphoditic; female sex cells fertilized when fixed on mother and enveloped in a chitinous spiny embryotheca.

conservation status

Not listed by the IUCN.

significance to humans

Hydra is one of the most popular laboratory animals and has greatly contributed to the development of experimental biology.

No common name

Solmundella bitentaculata

order

Narcomedusa

family

Aeginidae

taxonomy

Charybdea bitentaculata Quoy and Gaimard, 1833.

other common names

None known.

physical characteristics

Umbrella up to 0.4 in (12 mm) wide, usually much smaller, rounded apex, keel-shaped along the axis leading to tentacles, apical mesoglea very thick, lateral walls thin; velum well developed; manubrium short, lenticular, with eight rectangular manubrial pouches with rounded edges; mouth circular, simple; two long, tapering, opposite tentacles issuing from umbrella above manubrium, near apex; gonads in subumbrellar wall, under manubrial pouches; four peronia in deep grooves; no peripheral system or otoporpae; 8–32 statocysts. Monotypic genus.

distribution

Widely distributed in all oceans and in the Mediterranean. (Specific distribution map not available.)

habitat

From surface to bathypelagic zone.

behavior

Often parasitized by larvae of another Narcomedusae, Cunina peregrine.

feeding ecology and diet

Small planktonic animals.

reproductive biology

Dioecious, sex cells released in sea water, direct development.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Obelia dichotoma

order

Proboscoida

family

Campanulariidae

taxonomy

Sertularia dichotoma Linnaeus, 1759, coast of southwest England.

other common names

None known.

physical characteristics

Hydroid: colonies extremely varied in size and shape. Stems erect, up to 13.7 in (35 cm) high, monosiphonic and branched, flexuous to straight, thickened in old colonies. Internodes have several annuli at their base. Hydrothecae alternate, lateral, borne on completely annulated pedicels at the upper part of the internodes. Hydrotheca bell shaped, usually not very deep, thin walled; rim to crenate, slightly flared; diaphragm transverse to oblique. Gonothecae usually inverted, conical on annulated pedicels, truncated at the distal end, with a short distal neck when mature, where the aperture is.

Medusa: umbrella 0.09–0.23 in (2.5–6 mm) wide, circular, flat; mesoglea very thin; without gastric peduncle; mouth with four simple lips; four radial canals; gonads sac like, hanging from middle to end of the radial canals; numerous short, stiff, solid, not extensile marginal tentacles; tentacles with short endodermal roots extending into bell mesoglea; eight statocysts situated on underside of basal bulbs of some marginal tentacles. The velum is absent.

distribution

Almost cosmopolitan, even though the identity of the many nominal species of Obelia is still matter of debate, as is their distribution. (Specific distribution map not available.)

habitat

The hydroid is very common on rocks and algae on shallow hard bottoms; the medusa is very common in coastal waters.

behavior

Both polyps and medusae can move their tentacles in a rhythmic way to create currents that bring food particles toward the mouth.

feeding ecology and diet

Both hydroids and medusae feed on plankton, usually crustaceans, but also possibly phytoplankton.

reproductive biology

Both medusae and polyps are quite constant in presence in their respective environments and reproduction occurs continuously.

conservation status

Not listed by the IUCN.

significance to humans

Obelia is traditionally depicted in all zoology textbooks as a paradigmatic hydrozoan, so that many generations of students have learned its name. Unfortunately, the medusae of Obelia, being deprived of velum, are far from being the archetype of the medusae of the Hydrozoa. Furthermore, the figures of Obelia medusae present in many textbooks depict a campanulate medusa, and not a flat one, as is the medusa of this genus.

No common name

Apolemia uvaria

order

Physonectae

family

Apolemiidae

taxonomy

Stephanomia uviformis Lesueur, 1811, Nice (France).

other common names

French: Stéphanomie pamproide, stèphanomie à, grains de raisins.

physical characteristics

Pneumatophore bulb shaped, widening near the apex. Extended colonies are up to 32.8–98.4 ft (10–30 m). Nectosome with up to 12 nectophores in two parallel rows on the stem; largest is 0.14 in (3.7 mm) high by 0.13 in (3.4 mm) wide by 0.16 in (4.2 mm) deep. Nectophore consisting of two wings looking like those of a butterfly, with a deep ventral furrow. Nectosac large. Lateral radial canals form S-shaped bends with short branches on the upper bend. Groups of five or six nectosomal tentacles issuing from the base of the nectophores near the pedicular canal, at the base of the muscular lamellae. Siphosome measuring up to several feet (meters) in length, composed of several cormidia. Each cormidium consists of a gastrozooid and about 50 palpons, both with thin filiform tentacles of a single type issuing from their bases. Palpons are very long and delicate. Opaque spots bearing cnidocysts on the outer surface cover bracts, like the nectophores. Only physonect siphonophore whose nectophores are separated from each other by a cluster of 5–6 nectosomal tentacles.

distribution

The Mediterranean and Atlantic Ocean. (Specific distribution map not available.)

habitat

Occurs in the top 328 ft (100 m) of water.

behavior

Epiplanktonic.

feeding ecology and diet

Feeds on other planktonic organisms such as crustaceans, polychaetes, mollusks, tunicates, and even small fishes.

reproductive biology

Dioecious, release their sexual cells in the sea, development through siphonula larvae.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Polypodium hydriforme

order

Polypodiozoa

family

Polypodiidae

taxonomy

Polypodium hydriforme Ussov, 1885, Black Sea.

other common names

None known.

physical characteristics

Polypodium lifecycle as a succession of a free-living stage and of a stage parasitizing the eggs of some Acipenseridae and Polyodontidae fishes. The earliest known stage is a binucleate cell, parasitizing previtellogenetic fish oocytes. Further development may last several years, leading to a convoluted didermic stolonal structure, with inverted germ layers, forming numerous inverted buds. Before fish spawning, eversion takes place and the germ layers take their normal position (ectoderm outside, endoderm inside). The stolon exits the egg and becomes fragmented into individual buds, each giving rise to a free-creeping globular stage that multiplies by longitudinal fission. Globular stages can move and feed, having an oral mouth-cone and either 24, 12, or six tentacles, according to season. Germ cells are endodermal. So-called females have two kinds of gonads, each with a gonoduct opening in the gastral cavity; so-called males deprived of gonoducts, their gonads forming gametophores carrying cnidocysts. It is not known how the parasites get into young previtellogenic fish oocytes. The free-living stages are presumably homologous to sexual medusae, the parasitic stages being considered as polypoid. By their stolonal parasitic budding stage and their cnidome, the Polypodiozoa seem to present some affinities with the Narcomedusae, to which they were previously associated. P. hydriforme was, until recently, the only known metazoan adapted to an intracellular parasitic life.

distribution

Freshwater basins of Russia, Romania, Iran, and North America. (Specific distribution map not available.)

habitat

Fish gonads.

behavior

Parasitic.

feeding ecology and diet

Parasitic.

reproductive biology

Early developmental stages intracellular parasites of the eggs of Acipenseridae and Polyodontidae fishes; free-living stages a small medusae.

conservation status

Not listed by the IUCN.

significance to humans

Polypodium parasitize and destroy the eggs of the fishes producing caviar and, consequently, has a great economical impact.

No common name

Liriope tetraphylla

order

Trachymedusa

family

Geryoniidae

taxonomy

Geryonia teraphylla Chamisso and Eysenhardt, 1821, Indian Ocean.

other common names

None known.

physical characteristics

Umbrella 0.39–1.1 in (10–30 mm) wide, hemispherical, apex somewhat flattened; mesoglea thick, rigid; velum broad; manubrium small, on long, cylindrical gastric peduncle, longer than umbrellar diameter; mouth with four simple lips lined with cnidocysts; with normally four radial canals (sometimes more); 1–7 centripetal canals in each quadrant; with marginal cnidocyst ring; typically four long hollow perradial tentacles with cnidocyst rings and four small solid interradial tentacles with adaxial cnidocyst clusters; with gonads variable in shape and size, generally heart shaped, on either side of the middle of radial canals; eight statocysts. Extremely variable species.

distribution

Worldwide distribution, common in temperate regions of all oceans and the Mediterranean. (Specific distribution map not available.)

habitat

Chiefly near surface, sometimes in great shoals.

behavior

Bold and rapid medusa.

feeding ecology and diet

Very rapacious, feeds on other small planktonic animals and fishes sometimes three times its size.

reproductive biology

Dioecious, sex cells released in sea water, direct development.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Aglantha digitale

order

Trachymedusa

family

Rhopalonematidae

taxonomy

Medusa digitale O. F. Müller, 1776, North Atlantic.

other common names

French: Méduse doigtier.

physical characteristics

Umbrella cylindrical, 0.39–1.5 in (10–40 mm) high, about twice high than wide, with a small conical projection; lateral mesogleal walls thin, subumbrellar muscles strong; peduncle slender, long, conical, almost as long as subumbrellar cavity; manubrium small; mouth with four simple lips; eight long, sausage-shaped gonads, arising from the radial canals near the apex of the subumbrella and hanging freely in subumbrellar cavity; 80 or more solid marginal tentacles with a core of single endodermal chordal cells; eight free statocysts.

distribution

Worldwide distribution in all oceans and entering the Mediterranean. (Specific distribution map not available.)

habitat

From surface down to considerable depths, 1,970 ft (600 m).

behavior

Prolate medusae using the entire bell for jet propulsion; extremely active medusae.

feeding ecology and diet

Other small planktonic animals.

reproductive biology

Dioecious, sex cells released in sea water, direct development.

conservation status

Not listed by the IUCN.

significance to humans

None known.

Resources

Books

Boero, F., J. Bouillon, S. Piraino, and V. Schmid. "Asexual Reproduction in the Hydrozoa (Cnidaria)." In Reproductive Biology of Invertebrates. Volume XI, Progress in Asexual Reproduction, edited by R. N. Hughes. New Delhi: Oxford and IBH Publishing Co., 2002.

Bouillon, J., F. Boero, F. Cicogna, and P. F. S. Cornelius, eds. Modern Trends in the Systematics, Ecology and Evolution of Hydroids and Hydromedusae. Oxford: Clarendon Press, 1987.

Bouillon, J., C. Gravili, F. Pages, J. M. Gili, and F. Boero. An Introduction to Hydrozoa. In press.

Cornelius, P. F. S. "North-west European Thecate Hydroids and Their Medusae. Part 1. Laodiceidae to Haleciidae." In Synopses of the British Fauna (New Series), edited by R. S. K. Barnes and J. H. Crothers. The Linn. Soc. London and Est. Coas. Sci. Assoc., Field Studies Council. 50: i–vii, 1995.

——. "North-west European Thecate Hydroids and Their Medusae. Part 2. Sertulariidae to Campanulariidae." In Synopses of the British Fauna (New Series), edited by R. S. K. Barnes and J. H. Crothers. The Linn. Soc. London and Est. Coas. Sci. Assoc., Field Studies Council. 50: i–vii, 1995.

Periodicals

Boero, F. "The Ecology of Marine Hydroids and Effects of Environmental Factors: A Review." Marine Ecology—Pubblicazioni della Stazione Zoologica di Napoli I 5 (1984): 93–118.

Boero, F., J. Bouillon, and S. Piraino. "Classification and Phylogeny in the Hydroidomedusae (Hydrozoa, Cnidaria)." Scientia Marina 60 (1996): 17–33.

Boero, F., C. Gravili, P. Pagliara, S. Piraino, J. Bouillon, and V. Schmid. "The Cnidarian Premises of Metazoan Evolution: From Triploblasty, to Coelom Formation, to Metamery." Italian Journal of Zoology 65 (1998): 5–9.

Bouillon, J. "Classe des Hydrozoaires." Traité de Zoologie 3, no. 2 (1995): 29–416.

Bouillon, J., and F. Boero. "Phylogeny and Classification of Hydroidomedusae. The Hydrozoa: A New Classification in the Light of Old Knowledge." Thalassia Salentina 24 (2000): 1–46.

——. "Phylogeny and Classification of Hydroidomedusae. Synopsis of the Families and Genera of the Hydromedusae of the World, with a List of the Worldwide Species." Thalassia Salentina 24 (2000): 47–296.

Bouillon, J., F. Boero, F. Cicogna, J. M. Gili, and R. G. Hughes, eds. "Aspects of Hydrozoan Biology." Scientia Marina 56, no. 2–3 (1992): 99–284.

Bouillon, J., D. Medel, F. Pages, J. M. Gili, F. Boero, and C. Gravili. "Fauna of the Mediterranean Hydrozoa." Scientia Marina In press.

Carré, C., and D. Carré. "Ordre des Siphonophores." In Traité de Zoologie, edited by P. P. Grassé, and D. Doumenc, vol. 3, no. 2 (1995): 523–596.

Gili, J. M., and R. G. Hughes. "The Ecology of Marine Benthic Hydroids." Oceanography and Marine Biology: An Annual Review 33 (1995): 351–426.

Kirkpatrick, P. A., and P. R. Pugh. "Siphonophores and Velellids." Synopsis of the British Fauna (New Series) no. 29 (1984): 154.

Miglietta, M. P., et al. "Approaches to the Ethology of Hydroids and Medusae (Cnidaria, Hydrozoa)." Scientia Marina 64, Suppl. 1 (2000): 63–71.

Mills, C. E., F. Boero, A. Migotto, and J. M. Gili. "Trends in Hydrozoan Biology—IV." Scientia Marina 64, Supl. 1 (2000): 1–284.

Piraino, S., F. Boero, B. Aeschbach, and V. Schmid. "Reversing the Life Cycle: Medusae Transforming into Polyps and Cell Transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa)." Biological Bulletin 190 (1996): 302–312.

Piraino, S., F. Boero, J. Bouillon, P. F. S. Cornelius, and J. M. Gili, eds. "Advances in Hydrozoan Biology." Scientia Marina 60, no. 1 (1996): 1–243.

Other

"Cheating Death: The Immortal Life Cycle of Turritopsis." [July 10, 2003]. <http://zygote.swarthmore.edu/intro6.html>.

"Hydrozoa." Cnidaria Home Page, University of California, Irvine. [July 10, 2003]. <http://www.ucihs.uci.edu/biochem/steele/hydrozoa.html>.

"Hydrozoa Nematocysts." September 15, 1995 [July 10, 2003]. <http://www.pitt.edu/AFShome/s/s/sshostak/public/html/cnidocyst_database/hydr.html>.

"Hydrozoan Society: Dedicated to the Study of Hydrozoan Biology." [July 10, 2003]. <http://www.ucmp.berkeley.edu/agc/HS/>.

"Hydrozoan Taxonomy." [July 10, 2003]. <http://www.biology.duke.edu/hydrodb/databases.html>.

Mills, C. E. "Hydromedusae." June 10, 2003 [July 10, 2003]. <http://faculty.washington.edu/cemills/Hydromedusae.html>.

Peard, Terry. "Freshwater Jellyfish." January 13, 2003 [July 10, 2003]. <http://nsm1.nsm.iup.edu/tpeard/jellyfish.html>.

Raskoff, Kevin A. "Midwater Medusae." January 2003 [July 10, 2003]. <http://www.mbari.org/kraskoff/medusae2.htm>.

Schuchert, Peter. "Hydrozoa." June 2003 [July 10, 2003]. <http://www.geocities.com/peterschuchert/Hydrozoa.htm>.

Ferdinando Boero

Jean Bouillon