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 up-wellings 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 fow-leri, 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.

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