The method rests on their passivity. Arrow worms swim in short darts but cannot hold position against a current, so a population effectively labels the water it is carried in. In the north-east Atlantic, Sagitta elegans has long been associated with cooler, more nutrient-rich Atlantic inflow and Sagitta setosa with warmer coastal water, and shifts in which species dominates a sampling station have been read as a change in circulation.
They are also serious predators. A chaetognath seizes copepods with grasping spines that snap shut from a hood, and many species inject tetrodotoxin, apparently produced by associated bacteria, into the prey. In the plankton food web they are among the main consumers of copepods.
Their classification has been harder than their ecology. Chaetognaths develop in a way superficially like deuterostomes, and were placed there for decades, but molecular phylogenetics now places them among the protostomes, near the base of that group. Their exact position remains one of the more stubborn unresolved questions in animal phylogeny.