Arrow worms or Chaetognathids are also known as ‘Glass worms’ and ‘Tigers of the zooplankton’. There are over 120 species of Arrow worms most of which are found swimming in large numbers in the plankton. Some species are found attached to algae or other surfaces. They are found in a wide range of marine environments from the open ocean and polar waters to tide pools, marine caves and coastal lagoons. In some deep-sea environments they are the most abundant type of animal present.
All species are predators, feeding on copepods and other small crustaceans, larval fish and other arrow worms. They are eaten by fishes, squids and some sea birds.
Arrow worms are small marine invertebrates (from 2 to 120 millimeters in length). Their name comes from their transparent or translucent soft bodies which are long and thin, with side and tail fins. They have spines made from chitin around their mouth which can be retracted under a hood. Their Greek name Chaetognatha means “bristle-jaw”.
Their bodies consist of a head, trunk and tail. The head is well-developed and armed with 1 to 2 rows of teeth, two small pigmented eyes, other sense organs and two rows of grasping chitinous spines. A protective strong hood (part of their body wall) can be folded down over the bristles and teeth when they are not feeding. Their teeth contain neurotoxins to help them kill their prey.
They do not have circulatory, respiratory, or gas exchange systems and a simple nervous system. Materials are moved about the body cavity by cilia. Waste materials are excreted through the skin and anus. The mouth opens into a muscular pharynx which contains glands to lubricate the passage of food. From here, a straight intestine runs the length of the trunk to an anus just in front of the tail.
Some species of Chaetognathids use bioluminescence to escape from predators by making a cloud of light as a diversionary display.
All species are hermaphrodites, meaning they possess both male and female sex organs, the ovaries, are just behind the mid-body mark while the testis are in the tail. During mating, each individual places a spermatophore (a spermatophore is a capsule created by males of various animal species which contains sperm) onto the neck of its partner. The sperm rapidly escape from the spermatophore and swim along the midline of the animal until they reach a pair of small pores just in front of the tail. These pores connect to the oviducts, into which the developed eggs have already passed from the ovaries, and it is here that fertilisation takes place.
The eggs are planktonic, or attached to nearby algae, and hatch into miniature versions of the adult, without a well-defined larval stage.