arrow worm

(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

Resources

Bone, Q., H. Kapp, and A. C. Pierrot-Bults, eds. The Biology of Chaetognaths. Oxford, New York, Tokyo: Oxford University Press, 1991.

Conway Morris, S. Fossil Priapulid Worms. London: Palaeontological Association, 1977; distributed by B.H. Blackwell.

Schram, F. R. "Pseudocoelomates and a nemertine from the Illinois Pennsylvanian." Journal of Palaeontology 47 (1973): 985–989.

[Article by: Annelies C. Pierrot-Bults, PhD]

A phylum of abundant planktonic arrow-worms. Their bodies are tubular and transparent, and divided into three portions: head, trunk, and tail. The head possesses one or two rows of minute teeth anterior to the mouth and usually 7–10 larger chaetae, or seizing jaws, on each side of the head. One or two pairs of lateral fins and a caudal fin are present.

Nine genera and about 42 species are recognized by some specialists. Most species belong to the genus Sagitta, which can be recognized by the presence of two pairs of teeth and two pairs of lateral fins.

Chaetognaths are cosmopolitan forms which live not only at the surface but also at great depths; however, no one species is found in all latitudes and at all depths. One of the Arctic species, Eukrohnia hamata, may extend to the Antarctic by way of deep water across the tropics. A few species are neritic and are not found normally beyond the continental shelf. Their food consists principally of copepods and other small planktonic crustaceans; however, they are very predacious and will even eat small fish larvae and other chaetognaths on rare occasions.

Studies have shown them to be useful as indicator organisms. Certain species appear to be associated with characteristic types or masses of water, and when this water is displaced into an adjacent water mass, the chaetognaths may be used as temporary evidence for such displacement.

Arrow worms Fossil range: Cambrian-Recent
Spadella cephaloptera
Scientific classification
Kingdom:	Animalia
(unranked):	Bilateria
Phylum:	Chaetognatha Leuckart, 1854
Classes
Archisagittoidea Sagittoidea

Chaetognatha, meaning hair-jaws, is a phylum of predatory marine worms that are a major component of plankton worldwide. About 20% of the known species are benthic and can attach to algae or rocks. They are found in all marine waters from surface tropical waters and shallow tide pools to the deep sea and polar regions. Most chaetognaths are transparent and are torpedo shaped. Some deep-sea species are orange. They range in size from 2 mm to 12 cm. The common term for the phylum is Arrow Worms. There are more than 120 modern species assigned to over 20 genera. Despite the limited diversity of species, the number of individuals is staggering.^[1]

Chaetognaths are transparent or translucent and are covered by a cuticle. They have fins and a pair of hooked, chitinous, grasping spines on each side of their heads that are used in hunting. The spines are covered with a hood when swimming. They have a distinct head, trunk and tail. All species are hermaphroditic, carrying both eggs and sperm. Some species are known to use the neurotoxin tetrodotoxin to subdue prey.^[2]

They have some developmental similarities to nematodes. Although they have a mouth with one or two rows of tiny teeth, compound eyes, and a nervous system, they have no respiratory, circulatory, or excremental systems. Materials are moved about the body cavity by cilia. Waste materials are simply excreted through the skin and anus. Chaetognaths swim in short bursts using a dorso-ventral undulating body motion, where their tail fin assists with propulsion and the body fins for stabilization and steering.^[3] At least one species of chaetognath, Caecosagitta macrocephala, has bioluminescent organs on its lateral fins.^[4] The arrrowworm rhabdomeres are derived from microtubules, which in turn form conical bodies, which contain granules and thread structures. The cone body is derived from a cilium, and the microtubules of the rhabdomeres are 20 nm long and 50 nm wide. ^[5]

Contents 1 Classification 2 Fossil record 3 References 4 External links

Classification

Chaetognaths are traditionally classed as deuterostomes by embryologists. Lynn Margulis and K. V. Schwartz place chaetognaths in the deuterostomes in their Five Kingdom classification.^[6] Molecular phylogenists, however, consider them to be protostomes. Thomas Cavalier-Smith places them in the protostomes in his Six Kingdom classification.^[7] The similarities between chaetognaths and nematodes mentioned above may support the protostome thesis - in fact, chaetognaths are sometimes regarded as a basal ecdysozoan or lophotrochozoan.^[8] Chaetognatha appears close to the base of the protostome tree in most studies of their molecular phylogeny.^[9] This may explain their deuterostome embryonic characters. If chaetognaths branched off from the protostomes before they evolved their distinctive protostome embryonic characters, they may have retained deuterostome characters inherited from early bilaterian ancestors. Thus chaetognaths may be a useful model for the ancestral bilaterian.^[10]

Fossil record

Due to their soft bodies, chaetognaths fossilize poorly. Even so, several fossil chaetognath species have been described.^[11] Chaetognaths appear to have originated in the Cambrian Period. Complete body fossils have been formally described from the Lower Cambrian Maotianshan shales of Yunnan, China (Eognathacantha ercainella Chen & Huang^[12] and Protosagitta spinosa Hu^[13]) and the Middle Cambrian Burgess Shale of British Columbia (Oesia disjuncta Walcott^[14]). A more recent chaetognath, Paucijaculum samamithion Schram, has been described from the Mazon Creek biota from the Pennsylvanian of Illinois. Chaetognaths were thought possibly to be related to some of the animals grouped with the conodonts. The conodonts themselves, however, are thought to be related to the vertebrates. It is now thought that protoconodont elements (e.g., Protohertzina anabarica Missarzhevsky, 1973), are probably grasping spines of chaetognaths rather than teeth of conodonts. Previously chaetognaths in the Early Cambrian were only suspected from these protoconodont elements, but the more recent discoveries of body fossils have confirmed their presence then. ^[15]

References

1. ^ Bone, Q. et al. (1991). The biology of chaetognaths. London: Oxford University Press. 
2. ^ Thuesen, E.V. (1991), "The tetrodotoxin venom of chaetognaths", in Bone, Q. et al., The biology of chaetognaths, London: Oxford University Press, pp. 55–60 
3. ^ Jordan, C.E. (1992). "A model of rapid-start swimming at intermediate reynolds number: Undulatory locomotion in the chaetognath Sagitta elegans". Journal of Experimental Biology 163: 119–137. 
4. ^ Haddock, S.H.D. & J.F. Case (2004). "A bioluminescent chaetognath". Nature 367: 225–226. doi:10.1038/367225a0. 
5. ^ "Photoreception."Encyclopædia Britannica from Encyclopædia Britannica 2006 Ultimate Reference Suite DVD . 2009.
6. ^ Systema Naturae 2000 Taxon: Phylum Chaetognatha per Margulis and Schwartz (select Margulis & Schwartz in 'Classification by') - last retrieved November 25, 2006
7. ^ Systema Naturae 2000 Taxon: Phylum Chaetognatha per Cavalier-Smith (select Cavalier-Smith in 'Classification by') - last retrieved November 25, 2006
8. ^ Matus, D.Q. et al. (2006). "Broad taxon and gene sampling indicate that chaetognaths are protostomes". Current Biology 16: R575–R576. doi:10.1016/j.cub.2006.07.017. 
9. ^ Marletaz, F. et al. (2006). "Chaetognath phylogenomics: a protostome with deuterostome-like development". Current Biology 16: R577–R578. doi:10.1016/j.cub.2006.07.016. 
10. ^ Papillon, Daniel et al. (2004). "Identification of chaetognaths as protostomes is supported by the analysis of their mitochondrial genome". Molecular Biology & Evolution 21 (11): 2122–2129. doi:10.1093/molbev/msh229. PMID 15306659. 
11. ^ Vannier, J., M. Steiner, E. Renvoise, S.-X. Hu, and J.-P. Casanova. 2007. Early Cambrian origin of modern food webs: evidence from predator arrow worms. Proceedings of the Royal Society B 274:627–633.
12. ^ Chen, J.-Y., and D.-Y. Huang. 2002. A possible Lower Cambrian chaetognath (arrow worm). Science 298:187.
13. ^ Hu, S.-X. 2005. Taphonomy and palaeoecology of the Early Cambrian Chengjiang Biota from Eastern Yunnan, China. Berliner Paläobiologische Abhandlungen 7:1–197.
14. ^ Szaniawski, H. 2005. Cambrian chaetognaths recognized in Burgess Shale fossils. Acta Palaeontologica Polonica 50:1-8.
15. ^ Szaniawski, H. 2002. New evidence for the protoconodont origin of chaetognaths. Acta Palaeontologica Polonica 47:405-419.

External links

• Image of Pseudosagitta gazellae with a krill in its gut from the Tasmanian Aquaculture and Fisheries Institute
• Chaetognatha of the World - last retrieved December 13, 2006
• List of valid chaetognath species
• Hints for identifying chaetognaths

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