Echiura (Echiurans)

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Echiura

(Echiurans)

Phylum Echiura

Number of families 5

Thumbnail description
Worm-like invertebrates with non-segmented, bilaterally symmetrical bodies

Evolution and systematics

As the echiurans' body has no large hard parts, fossils of these animals are rare. There are two fossils for this group: a fossil from Illinois, United States, dated from the Late Carboniferous and another fossil from Namibia, dated from the Late Cambrian.

A free-swimming trochophore larva is present in echiurans, sipunculans, mollusks, and annelidans, which suggests a phyllogenetic relationship. Echiurans and annelids have many features in common. The most important difference between them is the absence of segmentation in Echiura. Some authors consider echiurans to be properly placed within the phylum Annelida, though other studies have shown that echiurans and pogonophorans have a close affinity and both may be closer to mollusks than to annelidans or sipunculans.

The phylum Echiura encompasses about 160 species and 40 genera, divided into two classes: Echiuridea, with three orders and four families (Echiuridae, Bonellidae, Ikedidae, and Urechidae), and Sactosomatidea, with a single family, Sactosomatidae, with one species, S. vitreum.

Physical characteristics

Echiurans, also known as spoon worms, have a body divided in two distinct regions: a sausage-shaped saccular, nonsegmented trunk and a ribbon-like proboscis at the anterior end. The length of the trunk may range from 0.39 in (1 cm) up to >19.6 in (>50 cm) and may be gray, dark green, reddish brown, rose, or red. It may be thick or thin, smoothed or roughened by glandular or sensory papillae. Internally, layers of muscles are responsible for peristaltic movements of the trunk. A pair of chitinous golden-brown chaetae usually occurs ventrally on the anterior part of the trunk. Some echiurans have one or two rings of chaetae around the anus.

The proboscis may be short or long, scoop- or ribbon-like, and flattened or fleshy and spatulate. It is generally white, rose, green, or brown. The distal end may be truncate or bifid. It is muscular, mobile, and highly extensible and contractile. It is able to extend 10 times its body length and can reach 3.2–6.5 ft (1–2 m). The ventral surface of the proboscis is ciliated, which helps in the feeding process. The mouth is located ventrally at the base of the proboscis and the anus is at the posterior extremity of the trunk.

Distribution

Echiurans are mainly marine, but some species live in brackish waters. The majority of spoon-worms are found in intertidal and shallow waters, but there are also species living at depths of 32,800 ft (10,000 m).

Habitat

Echiurans usually live in a U-shaped burrow with both ends of the burrow open. They are found mainly in soft benthic substrata such as sand, mud, or rubble, occupying burrows excavated by themselves or by other animals. Some species live in rock galleries excavated by boring invertebrates, whereas others live in empty shells, sand-dollar tests, coral or rock crevices, inside dead corals, or under stones. In general, some commensals are present inside the burrow, including polychaetes, crabs, mollusks, and fishes. The burrow provides

a protected, ventilated home, and remains of food discarded by the spoon-worm may be eaten by the commensals.

Behavior

Echiurans are slow but not sedentary, and animals without a proboscis can swim. One of the most important movements is the peristalsis of the trunk, which allows the animal to move slowly over the surface and construct burrows in the sand or mud. The movements by the peristalsis forces water through the tube, permitting the animal to obtain a supply of oxygen. In general, the burrow is kept clean and free from debris and fecal matter.

Feeding ecology and diet

The food of echiurans consists of dead organic matter and microorganisms that live on the substratum. Echiurans may be detritus feeders. They extend the proboscis out of the burrow onto the surface of the sediment. The tip of the ventrally ciliated surface collects particles, and glands produce mucus to adhere to these particles. Movements of the cilia conduct particles and mucus to the mouth. The proboscis is then extended in a new direction, and the procedure is repeated. A few species are filter feeders. The innkeeper worm, Urechis caupo, constructs a mucous net placed near the opening of the burrow. Peristaltic movements of the trunk draw water through the burrow, and particles and small organisms are trapped in the net. Ultimately, the worm eats both the net and the food.

Reproductive biology

Echiurans reproduce strictly by sexual means. Sexual di-morphism is pronounced only in the family Bonellidae, in which the male is much smaller than the female. Sexes are separate, and sperm and eggs are usually liberated at the same time in seawater where fertilization occurs, except in Bonellidae, in which individuals undergo internal fertilization.

Free-swimming and feeding trochophore larvae develop 22 hours to four days following fertilization. The larvae may drift in the plankton for up to three months, and during metamorphosis, it increases in length. The larva settles on the sub-stratum and begins life as an adult.

Conservation status

No species of Echiura are listed by the IUCN.

Significance to humans

Some species of echiurans are commonly used as laboratory animals for physiological, embryological, and biochemical studies. The substance bonellin has been studied because of its antibiotic properties.

Species accounts

List of Species

Green bonellia
Innkeeper worm

Green bonellia

Bonellia viridis

order

Bonellioinea

family

Bonellidae

taxonomy

Bonellia viridis Rolando, 1821, Naples, Italy.

other common names

English: Weenie worm; French: Bonnellie, bonnelie verte; Spanish: Gusano marino verde; Danish: Igelwurm.

physical characteristics

Female's trunk ovoid to sausage shaped, and about 5.9 in (15 cm) long. Proboscis long and bifurcate at the end. When completely extended it can reach 4.9 ft (1.5 m). Trunk and proboscis pale to dark green, caused by presence of a dermal pigment, bonellin. One pair of ventral chaetae. Males 0.039–0.11 in (1–3 mm) long with a ciliated and planariform-like body, without pigment, proboscis, month, anus, or blood vascular system. Male's body occupied mainly by reproductive structures, and the male is often found living within the nephridia (genital sac) of the female.

distribution

Northeastern Atlantic Ocean, Mediterranean, Red Sea, and Indopacific.

habitat

Female does not make its own burrow, but inhabits burrows excavated in gravelly bottoms or burrows with multiple exits in rocky substrata or in clefts in rocks. It can be found in depths from 33 to 328 ft (10–100 m). A number of commensals inhabit the burrow. Male lives in the genital sac or on the body of the large female.

behavior

Contraction of proboscis stem permits animal to move, and its bifurcate end has powerful cilia that help in locomotion and feeding. Moves back and forth inside burrow, as well as out of it. Contractions of the female's body wall renew the oxygen supply in the burrow.

feeding ecology and diet

Detritus feeder; food consists of detritus, small animals, organic material located at the root of vegetation and patches of sand between small rocks. Extends the proboscis from the burrow and grazes on the surrounding substratum with its bifid terminal lobes. Cilia on the ventral side of the proboscis move small particles and muscles pick up the larger ones. Cilia and muscles transfer the bolus mixed with mucus to the mouth. All metabolic needs of male are supplied by exchange with the female's body, indicating male has a parasitic mode of life.

reproductive biology

Sexes are separate and fertilization occurs in the genital sac, where male often lives. Larva is free swimming. Bonellin has an important role in the determination of sex; it is considered

to be a masculinizing factor. If the larva settles on ocean floor, it develops into a 3.9-in (10-cm) long female. If the larva settles on a female's body (particularly its proboscis), it develops into a 0.039–0.078-in (1–2-mm) long adult male in 1–2 weeks. Male lives as a parasite and produces a ready supply of sperm.

conservation status

Not listed by the IUCN.

significance to humans

Specimens are commonly used as laboratory animals. The pigment bonellin has been studied as an important antibiotic. The substance has powerful and lethal effects on a number of organisms.

Innkeeper worm

Urechis caupo

order

Xenopneusta

family

Urechidae

taxonomy

Urechis caupo Fisher and MacGinitie, 1928, Elkhorn Slough, Monterey Bay, California, United States.

other common names

English: Fat innkeeper worm.

physical characteristics

Sausage-shaped trunk, and proboscis short and scoop-like. Pink body reaches length of 19.6 in (50 cm) and the cuticle of trunk is rugose. One pair of ventral anterior chaetae and one circle of stronger chaetae around the anus.

distribution

Pacific Ocean, west coast of United States (California).

habitat

Lives in U-shaped burrow in sand or sandy mud in intertidal or subtidal regions. Can be found in estuarine areas. Known as innkeeper worm because its burrow is occupied by several commensals.

behavior

Able to construct its own tube. The proboscis starts a hole in the substratum and excavation process is continued until the burrow becomes a U-shaped tunnel with two exits. Both anterior ventral and posterior anal chetae are used during construction and maintenance of the tube. Can move over a smooth surface and spend the day obtaining food, cleaning the burrow, resting, and making respiratory movements with their tube. Can swim.

feeding ecology and diet

Produces a funnel-shaped mucous net, with its distal end attached to the internal wall of the burrow. By peristaltic movements, the trunk pumps seawater through the burrow and particles, and organisms larger than 0.00004 (1 µm) are trapped in net, which is detached from the body when loaded with food; animal eats net with food.

reproductive biology

Spawning occurs in early summer. All individuals spawn at the same time, ensuring fertilization, which occurs externally; sperm and eggs are liberated in the seawater where the fertilization occurs. A free-swimming and feeding trochophore larva develops after fertilization.

conservation status

Not listed by the IUCN.

significance to humans

Specimens are commonly used as laboratory animals and may also be used as bait.

Resources

Books

Beesley, P. L., G. J. B. Ross, and C. J. Glasby, eds. Polychaetes and Allies: The Southern Synthesis. Fauna of Australia. Vol. 4A, Polychaeta, Myzostomida, Pogonophora, Echiura, Sipuncula. Melbourne, Australia: CSIRO Publishing, 2000.

Grassé, P. P. Traitè de Zoologie. Vol. 5. Paris: Masson et Cie, 1959.

Stephen, A. C., and S. J. Edmonds. The Phyla Sipuncula and Echiura. London: Trustees of the British Museum (Natural History), 1972.

Periodicals

Agius, L. "Larval Settlement in the Echiuran Worm Bonellia viridis: Settlement on Both the Adult Proboscis and Body Trunk." Marine Biology 53 (2002): 125–129.

Fisher, W. K., and G. E. MacGinitie. "The Natural History of an Echiuroid Worm." Annals and Magazine of Natural History 10 (1928): 204–213.

Jaccarini, V., L. Agius, P. J. Schembri, and M. Rizzo. "Sex Determination and Larval Sexual Interaction in Bonellia viridis Rolando (Echiura)." Journal of Experimental Marine Biology and Ecology 66 (1983): 25–40.

Nishikawa, T. "Comments on the Taxonomic Status of Ikeda taenioides (Ikeda, 1904) with Some Amendments in the Classification of the Phylum Echiura." Zoological Science 19 (2002): 1175–1180.