Chaetognatha

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Chaetognatha

(kētôgnăth`ə), phylum of predominantly pelagic marine animals commonly known as arrowworms. Arrowworms have slender, transparent bodies, usually under 1 in. (2.5 cm) long. Lateral and caudal fins propel the animal in sudden darting movements. The well-developed head bears eyes and other sense organs, grasping spines used in the capture of prey, and rows of teeth flanking the mouth. A protective hood can be folded down over the bristles and teeth. The digestive system includes a glandular pharynx, a straight intestine, and a short, muscular rectum. The nervous system centers in a bilobed, dorsal brain and several other nerve ganglia. Although widely distributed, arrowworms prefer warm, shallow seas and are particularly plentiful in the Indo-Pacific region. They are voracious predators; some feed on freshly hatched fish nearly as large as themselves. They are influential planktonic consumers when abundant.

Chaetognatha

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.

Chaetognatha

 

(arrowworms), a phylum of marine invertebrates having a transparent elongate body that measures 5 mm to 9 cm in length. There are lateral fins and a caudal fin. The head bears sickle-shaped bristles that serve to capture prey. The coelom is divided by transverse septa into a head, trunk, and tail. The digestive tract is straight. There are no circulatory or excretory systems. The nervous system consists of an epipharyngeal brain and a ventral ganglion that are connected by long longitudinal cords. Chaetognaths are hermaphrodites. They are predators that live amid marine plankton. There are six genera, embracing about 30 species. Chaetognaths are usually assigned to the group of deuterostome animals; sometimes the invertebrates are regarded as an independent taxonomic category higher than a phylum.

REFERENCES

Filatova, Z. A. “Klass shchetinkocheliustnykh.” In Rukovodstvo po zoologii, vol. 3, part 2. Moscow, 1951.
Beklemishev, V. N. Osnovy sravnitel’noi anatomii bespozvonochnykh, 3rd ed., vols. 1–2. Moscow, 1964.
Dogel’, V. A. Zoologiia bespozvonochnykh, 6th ed. Moscow, 1975.

A. V. IVANOV

Chaetognatha

[‚kē′täg·nə·thə]
(invertebrate zoology)
A phylum of abundant planktonic arrowworms.
References in periodicals archive ?
A structural study of these fossils is necessary to obtain a better knowledge and understanding of the early evolution of conodonts and chaetognaths.
2A1, D, F) suggest that they grew similarly to the grasping spines of chaetognaths, by basal accretion of thin laminae (Szaniawski 2002).
Structurally the fossils are most similar to the grasping apparatuses of Phakelodus which is an ancestor of chaetognaths.
New evidence for the protoconodont origin of chaetognaths.
Their diet comprised seven major prey groups (copepods, chaetognaths, amphipods, euphausiids, ostracods, unidentified fish, and unidentified gelatinous prey) and was dominated by copepods and chaetognaths.
anonychus comprised seven major prey groups and was dominated by copepods (N=70%, WM=85%, F=74%, %IRI=87%) and chaetognaths (N=24%, WM=11%, F=48%, %IRI=12%) (Table 2).
The diet of Berryteuthis anonychus collected in the northeast Pacific during May was dominated by calanoid copepods and chaetognaths.
Individuals that were captured in the water column (<50 mm TL) had pelagic taxa such as chaetognaths, hyperiid amphipods, calanoid copepods, and ostracods in their stomachs.
After settlement, from approximately 20 to 60 mm SL, they showed a wide-ranging diet that included shrimp, copepods, chaetognaths, and squid.
Open-habitat prey, such as chaetognaths, are known to be pelagic as well as benthic, as are sergestid shrimp, calanoid copepods, mysids, and stomatopods (Williams, 1968; Manning, 1969; Gosner, 1978; Stuck et al.
Pollock also commonly consumed small amounts of other prey, such as larvaceans, gastropods, and chaetognaths.
By winter, large copepods virtually disappeared from diets in some areas (Sobolevskii and Senchenko, 1996); chaetognaths and epibenthic prey such as mysids, shrimps, caprellid amphipods, and cumaceans were incorporated in the diet as vertical distributions of the fish changed and pelagic prey became scarce[12] (Merati and Brodeur, 1996; Sobolevskii and Senchenko, 1996; Brodeur et al.