Arrowworms (70 species) are carnivorous members of the plankton. They alternately dart and glide through the water, feeding on copepods and young fish. Typically arrowworms have a pair of eyes and 4-14 large spines for grasping prey. The head may be enclosed in a hood, probably to reduce resistance when swimming. Their development suggests they belong to the evolutionary line that led to the echinoderms and chordates. Of economic importance as predators of young fish, arrowworms are also used as indicator species of ocean and coastal waters. Sagitta elegans is typical of the phosphate-rich waters of the Atlantic Ocean. Sagitta setosa is typical of phosphate poor water in coastal areas.
Right: Assorted chaetognaths in a sample of plankton from the Discovery Expedition 1926.