Author: Jan Scholten
Chaetognatha is a phylum of predatory marine worms that are a major component of plankton worldwide. About 20% of the known species are benthic and can attach to algae or rocks. They are found in all marine waters from surface tropical waters and shallow tide pools to the deep sea and polar regions. Most chaetognaths are transparent and are torpedo shaped. Some deep-sea species are orange. They range in size from 2 mm to 12 cm. The common term for the phylum is Arrow Worms. There are more than 120 modern species assigned to over 20 genera. Despite the limited diversity of species, the number of individuals is staggering.
Chaetognaths are transparent or translucent and are covered by a cuticle. They have fins and a pair of hooked, chitinous, grasping spines on each side of their heads that are used in hunting. The spines are covered with a hood when swimming. They have a distinct head, trunk and tail. All species are hermaphroditic, carrying both eggs and sperm. Some species are known to use the neurotoxin tetrodotoxin to subdue prey.
They have some developmental similarities to nematodes. Although they have a mouth with one or two rows of tiny teeth, compound eyes, and a nervous system, they have no respiratory, circulatory, or excremental systems. Materials are moved about the body cavity by cilia. Waste materials are simply excreted through the skin and anus. Chaetognaths swim in short bursts using a dorso-ventral undulating body motion, where their tail fin assists with propulsion and the body fins for stabilization and steering. At least one species of chaetognath, Caecosagitta macrocephala, has bioluminescent organs on its lateral fins.