VIMS Zooplankton Ecology - BATS Chaetognatha

Chaetognaths are a small phylum of over 100 species. This number will likely grow as new species remain, especially in benthic and bathyal habitats. Chaetognaths are considered to be one of the most taxonomically isolated groups in the Animal Kingdom. Since their discovery in 1778, many attempts have been made to align them to nearly every phyla, but none have been embraced by the scientific community. Darwin noted that chaetognaths were "remarkable for the obscurity of its affinities".

Chaetognaths are exclusively marine. Although small (2-120 mm long), these invertebrates are often abundant in coastal and oceanic plankton, with biomass estimates up to 30% of that of copepods. Chaetognaths are voracious carnivores and play an important role in marine food webs as a trophic link between smaller plankton and larger predators. While most studies focus on planktonic species, the growing literature on benthic species indicates that chaetognaths may also play an important role in the benthos. Certain species are associated with discrete temperature and salinity conditions and, thus, have become hydrological indicators of oceanic water currents.

The general shape of a chaetognath resembles an arrow; hence, their common name is "arrow worms". Chaetognaths exhibit bilateral symmetry and are typically transparent. The body is divided morphologically into three well-differentiated regions: head, trunk (intestine and ovaries), and tail (testes). All chaetognaths have large cephalic grasping spines, or hooks, for capturing prey, from which their name was derived (chaete = spine, gnathos = jaw). Chaetognaths are also defined by the presence of one or two pairs of lateral fins. Chaetognaths are hermaphroditic; they contain both male and female reproductive organs. Aside from the spatial separation of the gonads, self-fertilization is also restricted because the male and female organs mature at different times during the life cycle.