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Parasagitta elegans
Common name(s):
Elegant arrow worm
Description:
Transparent. With eye spots darker than body. Up to 4 cm in open ocean. Smaller (2 cm) in Puget Sound. Sagittid chaetognaths lack gut diverticula. P. elegans has three sets of lateral fins; anterior lateral (located on the midsection), posterior lateral and caudal. The chaetognaths are so named for their "grasping" or "raptorial" spines located on their head. These "bristles" aide in prey capture.
Misidentification:
P. elegans is the only chaetognath known from the southern Puget Sound region.
Distribution:
Boreal. Marine. Pelagic. Planktonic. Denser populations occur in circumpolar environments. These arrow-worms have been collected at depths to 200 meters. They are most abundant in late summer in the southern Puget Sound region.
Ecology:
Chaetognaths are thought to compose 5-15% of the world's zooplankton biomass. P. elegans is an important planktonic predator, primarily of larval or small crustaceans, with copepods comprising most of its diet. P. elegans is preyed upon by larger browsers and predators of pelagic plankton. These animals have also been observed to be cannibalistic.
Life history:
P. elegans populations generally have 1 - 4 generations per year with higher rates occuring in warmer climes. Chaetognaths exhibit direct development and life spans average around 200 days. Chaetognaths are hermaphroditic. The ovaries are located in the region between the posterior lateral fins with the testes arranged just posteriorly. During coupling, sperm is exchanged internally and fertilized eggs are released into the water. More research is needed concerning the mechanisms and paths of fertilization.
Prey capture and consumption:
In the laboratory, P. elegans consumes around .035 times its body weight daily. Arrow worms lie motionless waiting for prey to wander within striking distance. Their predatory response is thought to be triggered by motion. They dart quickly at their prey trapping it in their raptorial spines. It has been found that after capture, prey is subdued through envenomation. Tetrodotoxin is produced by symbiotic Vibrio bacteria.
Additional Resources:
Research by Erik V. Thuesen
Research on a bioluminescent chaetognath
The Biology of Chaetognaths, Q. Bone, H. Kapp, and A. Pierrot-Bults, Oxford University Press, 1991.
Invertebrates, Brusca and Brusca, Sinauer Associates Inc., 1990.
Last updated Wednesday, August 16, 2006, by Lisa Ferrier
