Chaetognatha (Arrow Worms)

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Chaetognatha

(Arrow worms)

Phylum Chaetognatha

Number of families 6

Thumbnail description
Long, narrow worms that have transparent bodies and that are known for the hooks that they use to catch prey

Evolution and systematics

The phylum Chaetognatha is thought to be an old group with a long and separate path of development. It is not clear what the nearest relatives are, but evidence provided by developmental and molecular studies show that this group may be an early split-off of the metazoan phyla. It is a paradox that there are so few species, because the group has been so successful in increasing its number of individuals and maintaining widespread distribution. It is a soft-bodied group and several researchers (Owre and Bayer, 1962, and Conway-Morris, 1977) have raised doubts concerning fossils of certain species (e.g., Amiskwia sagittiformis) that have been recorded. A. sagittiformis has raised particular doubts because it has no substantive structures that are usually present in a chaetognath, such as hooks or teeth; more over it has a gut that runs to the end of the tail unlike any chaetognath.

Two orders are usually recognized, Phragmophora and Aphragmophora. The order Phragmophora contains three families (Heterokrohniidae [three genera], Spadellidae [four genera], Eukrohniidae [one genus]), and the order Aphragmophora also contains three families (Sagittidae [one genus], Pterosagittidae [one genus], Krohnittidae [one genus]). The genus Sagitta (Sagittidae) is split into 11 genera by some authors, but because of the uncertain nature of some of the groupings and the need for further molecular evidence to determine the true nature of these genera, the name Sagitta will be used in this chapter. There are about 100 species under discussion but there may be as many as 200 valid species, many of which may be benthic.

Physical characteristics

The animals are bilaterally symmetric with a head, trunk, and tail. They are transparent: the internal organs can clearly be seen in living specimens and in specimens that have been well preserved in formalin. The body cavity is filled with fluid that is surrounded by muscles and tough membrane; a multilayered epidermis covers the outer layer of the body. The head has a complex musculature that supports the grasping spines or hooks that are the most obvious and recognizable features of this animal. There also is a mouth, one or two rows of teeth, and eyes. The eyes have photoreceptive cells, and most species also have spots of pigment in their eyes.

The nervous system consists of six ganglia that are present in the head, a superficial cerebral ganglion, and two lateral ganglia at either side of the esophagus. The cerebral ganglion is connected to a ganglion present on the ventral side of the trunk; paired nerves extend from the ventral ganglion toward the tail. Very little is known about the functional roles of the different parts of the nervous system.

The animals detect the movement of prey with ciliary tufts present on the body. The body wall is folded in the neck region, which forms a hood that can be folded over the head to allow the animals to swim more smoothly. In some species there is a layer of loose tissue present in the collarette region of the neck. Chaetognatha have tail fins and one or two pairs of fins on the sides of the body; all fins are supported by fin rays.

The alimentary structure consists of the mouth, esophagus, gut (which can have paired diverticula), and anus. The anus is located just before the septum that separates the trunk and the tail. The ovaries are situated at the posterior end of the trunk in an opening present on both sides of the intestine, just before the septum. The tail contains the testes. Ripe sperm is stored in sperm packages within the seminal vesicles that project from both sides of the tail.

The warm-water species are generally smaller than the cold-water species, with adults between 0.12–5.91 in (3–150 mm) in size. The largest species, Sagitta gazellae, lives in Antarctic waters and reaches a size of 2.76 in (70 mm). The benthic species are the smallest arrow worms, with Spadella cephaloptera reaching maturity at 0.12 in (3 mm).

Distribution

Large numbers of Chaetognatha live in every region of the ocean. About half of the species are planktonic. Neritic (inhabiting shallow waters near coastlines) and open-ocean species are in the pelagic realm, the latter being confined to the waters above the continental shelf, which is about 656 ft (200 m) deep. Oceanic species are widely distributed, but a very common distribution pattern is tropical-subtropical from 40°N–40°S in all three oceans; the species Pterosagitta draco follows this line of distribution. More tropical species, such as Sagitta regularis, are confined to roughly 30°N–30°S and are usually seen only in the Indo-Pacific Ocean. Distributions are much more restricted for neritic species, such as Sagitta setosa, because of the topography and variability of the locations. The width of a shelf, outflow of rivers, or local upwellings can have a tremendous impact on neritic distribution.

The largest concentration of individuals is found in the epipelagic layer shallower than 656 ft (200 m). Arrow worms that live in surface waters, such as Sagitta enflata, are often transparent, which helps them avoid predators such as fishes. There are fewer individuals but more species found in the mesopelagic layer from 656–3,280 ft (200–1,000 m). The guts of species in the mesopelagic layers, such as Eukrohnia fowleri, are very often yellow or red in color, which they obtain by consuming prey of the same colors. Species from deeper waters, such as Sagitta planctonis, are more muscular and less transparent.

The mid-water species usually perform diurnal vertical migration. They swim to the surface at night to feed and sink to deeper layers during the day. There also is an ontogenic vertical distribution: juveniles live higher up in the water column than the adults. This results in more species being present in the upper layers at night than during the day. In general, the very deep-living species are thought to be globally distributed. However, the high cost of sampling these waters has prevented extensive surveys. As a result, very little is known about the deep bathypelagic and benthopelagic layers.

Neritic species are adapted to more variation in environmental conditions. They show more restricted areas of distribution than the oceanic species. Because they can tolerate a wider range of environmental conditions, it has been possible to keep specimens for a limited amount of time in an aquarium to study feeding and swimming behavior. However, scientists have so far been unable to keep specimens alive in an aquarium setting for more than one generation.

Benthic species live attached to objects on the substrate such as sea grass or rocks. The distributions of most benthic species are very restricted; some species have only been found in the area of their type locality. Spadella cephaloptera is presumed to be globally distributed, but it may be a species complex.

Habitat

Chaetognaths are strictly marine and can live in every part of the ocean. The benthic Spadellas are very small and adhere to rocks or other objects by using special adhesive papillae, but they are also able to swim short distances.

Behavior

The pelagic species position themselves in water in a slightly oblique position. If their side bristles detect movement, a quick sweep of their tail allows them to swiftly dart off in the direction of prey. They are able to swim across short distances very quickly. Their ability to swim is probably dependent on their body muscles, but the precise manner of how it is done is still not known. The fins are thin, transparent structures supported by fin rays and do not contain muscle fiber. Fins enlarge body width and enhance the buoyancy of the animal. In some species the fins contain gelatinous material. Fins do not play an active role in swimming.

Feeding ecology and diet

Chaetognaths are carnivores and swim very actively in order to catch prey. They detect their prey by sensing movement with the ciliar tufts on their body. The chaetognath uses its hooks to grab prey, which is usually a small copepod whose nicely rounded shape makes it easy for the chaetognath to swallow. However, arrow worms can feed on anything that is of a certain size, including fish larvae, other chaetognath species, and even phytoplankton. The function of the teeth is not known. Arrow worms appear to immobilize their prey after capture. Tetrodotoxin, which is probably synthesized by the bacterium Vibrio algynoliticus, has been isolated from the head of several chaetognath species. How chaetognaths acquire this bacterium is still unknown, but tetrodoxin can cause immobilization. However, direct observations of feeding chaetognaths are very scarce, and it is not known how many species of chaetognaths may have or use the venom.

Digestion is rapid, and it is very rare that arrow worms are found with full guts. It is estimated that chaetognaths consume between two and 50 prey in a day, the latter number coming from laboratory experiments to establish the worms' food saturation point. Chaetognaths play an important part in the food chain. Estimates have shown that they comprise 30% of the biomass of copepods. Chaetognaths are mainly eaten by fishes, but they are also prey for larger carnivorous animals. Chaetognaths do acquire parasites from prey but not very frequently; the most common parasites ingested by chaetognaths are trematodes, nematodes, and cestodes (helminths), although the latter occurs very infrequently. There is no host-specifc parasite known in chaetognaths, which is remarkable for such an old group.

Reproductive biology

Arrow worms are hermaphrodites; male reproductive organs are in the tail, and female organs are in the trunk. They are protandric, meaning that the sperm develops first and the eggs follow later. The ovaries are present in the lower part of the trunk. Openings just before the septum divide the trunk and tail on either side of the gut. Sperm is deposited on the body and swims toward the openings, which they enter; the openings are also used for egg laying. The maximum length of mature ovaries varies with the species. The ovaries can be very long. In species such as Pterosagitta draco, when the ovary is full of eggs it can extend all the way to the neck. However, in Sagitta enflata, the ovaries are very short.

The open-ocean species are adapted to a large space and as such have not been cultured. This makes it difficult to study behavior and life cycles. For example, knowing whether they cross-fertilize or fertilize themselves would be important in studying genetic variation, but scientists have not yet determined the method of fertilization for these species. What is known about the reproductive behavior of chaetognaths comes from observations of Spadella cephaloptera. This species has been held in aquaria for observation studies. The sperm package of this species is deposited on the body of another individual of the same species. The sperm moves into the ovaries and is stored there to fertilize the eggs. The fertilized eggs are then released into the sea water. In deep-water species such as Eukrohnia fowleri, brood sacs hang from the ovary openings.

Arrow worms do not have larval stages. Small chaetognaths hatch from the eggs and then continue to grow. The animals require a temperature that is high enough to allow eggs to develop. At higher temperatures, individuals mature in a shorter period of time than they do in cool water. If the temperature is too low, they use energy in order to grow and reach a greater length. For example, the temperature of the water in which it grows greatly affects the size of Sagitta tasmanica; mature individuals range from 0.31 to 0.67 in (8–17 mm) in size. When the animals are transported to water masses that are too cold for them to reproduce, they grow to be giant in size, but they never reach a mature state and are referred to as sterile expatriates. In cold water like the Canadian Arctic, they may take two years to mature, while in tropical waters their complete life span may be as little as six weeks.

Conservation status

No Chaetognatha species is listed by the IUCN Red List. Planktonic species have huge populations with very large distributions and as such, are not expected to die out. There is

no other animal on the planet with such a large distribution area, and species worldwide are not threatened. However, climate changes have been known to cause distribution areas to shift and local populations of species are able to appear and disappear. For example, we can expect populations of warm-water species in the North Atlantic to begin moving northwards if ocean temperatures continue to increase, and it is possible that the arctic-subarctic cold-water species Sagitta elegans could disappear from the North Sea, which is its southernmost area of distribution.

Significance to humans

There is no direct significance between humans and chaetognaths. However, chaetognataha are an important part of the marine food chain, and they are used as indicators to determine the level of water from the Atlantic Ocean moving into the North Sea. Arrow worms may play a role in the oceanic distribution of tetradotoxin-producing bacteria.

Species accounts

List of Species

Pterosagitta draco
Sagitta bipunctata
Sagitta enflata
Sagitta planctonis
Sagitta setosa
Eukrohnia fowleri
Spadella cephaloptera

No common name

Pterosagitta draco

order

Aphragmophora

family

Pterosagittidae

taxonomy

Sagitta draco (Krohn, 1853), Mediterranean Sea off Messina.

other common names

None known.

physical characteristics

Individuals have 8–10 hooks, 6–10 anterior teeth, and 8–18 posterior teeth. Maximum adult body length is 0.43 in (11 mm), and the relative tail length is 38–45% of total body size. The body is firm, broad, and opaque. One pair of fins is present on the tail segment; fins are completely rayed and round. Head is fairly large. There is a large, broad collarette along the trunk and part of the tail. There are no gut diverticula. Eyes are small with T-shaped pigment spots. Seminal vesicles are present on the head and trunk, close to tail fin, and touch lateral fins. Ovaries are long, reaching the neck region; ova are large.

distribution

Epipelagic, occurring between 40°N and 40°S in all three oceans.

habitat

Lives in the pelagic realm of tropical and subtropical oceans, at depths of 3–85 ft (10–300 m).

behavior

Shows no evidence of diurnal vertical distribution. Rapid darting movements are made over short distances to catch prey.

feeding ecology and diet

Prefers small copepods. Feeds at night after moving to superficial layers.

reproductive biology

Reproduction is dependent on water temperature and occurs several times a year. Hermaphroditic, and sperm is stored in the oviducts waiting for the eggs to mature. Fertilized eggs are released in sea water. There is no larval stage.

conservation status

Not listed by IUCN.

significance to humans

None known.

No common name

Sagitta bipunctata

order

Aphragmophora

family

Sagittidae

taxonomy

Sagitta bipunctata Quoy and Gaimard, 1827, Strait of Gibraltar.

other common names

None known.

physical characteristics

Individuals have 8–10 hooks, 5–8 anterior teeth, and 8–16 posterior teeth. Maximum adult body length is 0.75 in (19 mm), with a relative tail length of 22–29% of total body size. The body is firm and opaque. There are two pairs of lateral fins and no fin bridge. Anterior fins are medium in length, rounded, and completely rayed. Posterior fins are medium in length, completely rayed, and triangular in shape. This species does not have a collarette, nor does it have gut diverticula. Eyes are small, with T-shaped pigment spots. Seminal vesicles are present in the head and trunk and touch the tail fin, although they are well separated from posterior fins. Ovaries are medium in length and reach the region of the ventral ganglion; ova are large.

distribution

Epipelagic, occurring between 40°N and 40°S in all three oceans.

habitat

A pelagic, oceanic species that lives in tropical and subtropical regions at depths of 0–328 ft (0–100 m).

behavior

One of the few species in the northwest Atlantic that lives in the upper 164 ft (50 m) of the water column. Shows evidence of diurnal vertical distribution, with population numbers being most abundant at depths of 0–33 ft (0–10 m) at night and 33–82 ft (10–25m) in the daytime. Rapid darting movements are made over short distances to catch prey.

feeding ecology and diet

Prefers small copepods.

reproductive biology

Reproduces one or more times a year depending on water temperature. Hermaphroditic, and sperm is stored in the oviducts waiting for the eggs to mature. Fertilized eggs are released in the sea water. There is no larval stage.

conservation status

Not listed by the IUCN.

significance to humans

The first chaetognath species ever described.

No common name

Sagitta enflata

order

Aphragmophora

family

Sagittidae

taxonomy

Sagitta enflata Grassi, 1881, Bay of Naples.

other common names

None known.

physical characteristics

Individuals have 8–10 hooks, 4–10 anterior teeth, and 4–15 posterior teeth. Maximum adult body length is 0.98 in (25 mm), and the relative tail length is quite short at 14–17% of total body size. The body is flaccid and transparent. There are two pairs of lateral fins and no fin bridge. Anterior fins are very short, partially rayed, and round. Posterior fins are short, partially rayed, and round. There is no collarette, nor are there gut diverticula. Eyes are small, with star-shaped pigment spots. Seminal vesicles are round, touch the tail fin, and are well separated from the posterior fins. Ovaries are small, reaching to the middle of the post fins; ova are large.

distribution

Epipelagic and cosmopolitan, occurring from 40°N to 40°S.

habitat

Oceanic and pelagic species that lives in tropical and subtropical waters at depths of 0–985 ft (0–300 m). Reported to tolerate different degrees of salinity and may occur in inshore waters.

behavior

Shows evidence of diurnal vertical distribution. Juveniles occur at higher levels of the water column than do the adults. Rapid darting movements are made over short distances to catch prey.

feeding ecology and diet

Prefers small copepods. Feeds at night after moving to superficial layers.

reproductive biology

Reproduces several times a year, but the number of times is dependent on water temperature. Hermaphroditic, and sperm is stored in the oviducts waiting for eggs to mature. Fertilized eggs are released in the sea water. There is no larval stage.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Sagitta planctonis

order

Aphragmophora

family

Sagittidae

taxonomy

Sagitta planctonis Steinhaus, 1896, south Equatorial Current, Atlantic Ocean.

other common names

None known.

physical characteristics

Individuals have 8–11 hooks, 6–9 anterior teeth, and 10–14 posterior teeth. Maximum adult body length is 1.46 in (37 mm), and relative tail length is 19–21% of total body size. The body is large, firm, and opaque. There are two pairs of lateral fins and a fin bridge. Anterior fins are long, angular, partially rayed, and reach the middle of the ventral ganglion. Posterior fins are long, angular, and partially rayed. There is a large collarette

and gut diverticula. Eyes are small with T-shaped pigment spots. Seminal vesicles are conical and touch posterior fins. Ovaries are very broad and long, reaching the neck region; ova are fairly small

distribution

Shallow-mesopelagic and cosmopolitan, occurring between 40°N and 40°S.

habitat

Occurs in the pelagic realm of tropical and subtropical regions of the ocean, at depths of 328–1,640 ft (100–500 m).

behavior

Shows evidence of diurnal vertical distribution. Juveniles occur at higher levels in the water column than the adults. Rapid darting movements are made over short distances to catch prey.

feeding ecology and diet

Prefers small copepods. Feeds at night after moving to superficial layers.

reproductive biology

Reproduces once or twice a year, depending on water temperature. Hermaphroditic, and sperm is stored in the oviducts waiting for the eggs to mature. Fertilized eggs are released in sea water. There is no larval stage.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Sagitta setosa

order

Aphragmophora

family

Sagittidae

taxonomy

Sagitta setosa Müller, 1847, North Sea.

other common names

None known.

physical characteristics

Individuals have 8–9 hooks, 6–8 anterior teeth, and 10–16 posterior teeth. Maximum adult body length is 0.55 in (14 mm), and the relative tail length is 16–25% of total body size. The body is small, narrow, and transparent. There are two pairs of lateral fins and no fin bridge. Anterior fins are relatively short, completely rayed, and round. Posterior fins are short, completely rayed, and round. The collarette is small or not present, and there are no gut diverticula. Eyes have star-shaped pigment spots. Seminal vesicles are present in the head and trunk, touch the tail fin, and are somewhat separated from the posterior fins. Ovaries are short and ova are small. Recent molecular evidence has revealed substantial genetic differences between populations from the North and Mediterranean seas.

distribution

Neritic, occurring in the North, Mediterranean, and Black seas.

habitat

Lives in shallow pelagic waters of the Mediterranean, Black, and North seas, especially where the continental shelf is relatively wide. Reported to tolerate different degrees of salinity and may inhabit inshore waters and estuaries. Occurs in the western part of the Baltic Sea as long as the salinity is not too low.

behavior

Nothing is known.

feeding ecology and diet

Prefers small copepods. Feeds at night after moving to superficial layers.

reproductive biology

Reproduces once a year, during the summer months with a peak in late August. Hermaphroditic, and sperm is stored in the oviducts waiting for the eggs to mature. Fertilized eggs are released in the sea water. There is no larval stage.

conservation status

Not listed by the IUCN.

significance to humans

Used as an indicator species to follow the movement of water masses.

No common name

Eukrohnia fowleri

order

Phragmphora

family

Eukrohniidae

taxonomy

Eukrohnia fowleri Ritter-Zahony, 1909, southern Indian Ocean.

other common names

None known.

physical characteristics

Individuals have 8–14 hooks and 2–31 posterior teeth. There are no anterior teeth. Maximum adult body length is 1.57 in (40 mm), and the relative tail length is 22–27% of total body size. The body is firm, broad, and opaque. Transversal muscles are present in the anterior part of the trunk. The neck is narrower than the region at the septum between trunk and tail segment. Pair of long, lateral fins are located on both the trunk and tail and are partially rayed. This species does not have a collarette, nor does it have gut diverticula. The head is small, with big oval eyes that have diamond-shaped pigment spots. Seminal vesicles are oval and rounded and do not touch the tail fin or the lateral fins. Ovaries are short and broad, and the ova are large.

distribution

Deep-mesopelagic, occurring between 1,640 and 4,920 ft (500–1,500 m). Circumglobal, from 70°N to 70°S in all three oceans.

habitat

Oceanic species that lives in the pelagic realm in all oceans except the Arctic and Antarctic.

behavior

Nothing is known.

feeding ecology and diet

Prefers small copepods.

reproductive biology

Reproduction is not seasonally determined. Growth and reproduction are very slow, and the fertilized eggs are brooded in a sac that hangs out of the ovaries.

conservation status

Not listed by the IUCN.

significance to humans

None known.

No common name

Spadella cephaloptera

order

Phragmphora

family

Spadelliidae

taxonomy

Sagitta cephaloptera Busch, 1851 Northeast Atlantic, Orkney Islands.

other common names

None known.

physical characteristics

Individuals have 7–10 hooks, 3–5 anterior teeth, and 2–4 posterior teeth. Maximum adult body length is 0.22 in (5.5 mm), and the relative tail length is 49–58% of total body size. The body is firm, broad, and opaque. Transversal muscles are present in the trunk. A pair of short lateral fins is located almost entirely on the tail segment and is connected to the tail fin by rays. The species has a collarette, small gut diverticula, and a prominent head. Eyes contain pigment spots. Seminal vesicles are oval or rounded, touching both the tail fin and lateral fins. Ovaries are short and broad. The ova are large and are sometimes loose in the trunk cavity. There are glands and adhesive papillae on the ventral side of the tail.

distribution

Neritic and benthic. Occurs between 0 and 82 ft (0–25 m), from about 40°N to 40°S.

habitat

Adheres to objects such as rocks or sea grass in benthic coastal areas. It can swim for short distances.

behavior

Lives in benthic coastal layers. Benthic species are the only chaetognatha that can be observed in aquariums for long periods of time. Observations about behavior are known from these benthic species.

feeding ecology and diet

Prefers small copepods.

reproductive biology

Two mature individuals with full seminal vesicles maneuver close to each other to mate. One will deposit a sperm package or spermatophore on the dorsal neck region of the other individual. Then the sperm moves toward the genital opening and is stored until eggs are mature. The eggs are fertilized internally and then released into the water.