Arrow worms (Phylum Chaetognatha)
Voracious nocturnal predators of plankton-rich seas, arrow worms have puzzled taxonomists for over 150 years. To this day it remains controversial where arrow worms sit in the animal family tree, as they resemble various unrelated invertebrate groups in some ways, and are completely unique in others. They are sometimes considered one of the most taxonomically isolated groups amongst animals.
Arrow worms are transparent or translucent, grow up to 12 cm in length, and have a torpedo- or dart-shaped body with one or two pairs of lateral fins and a caudal fin on the tail. The hooded head culminates in a mouth flanked with sickle-shaped grasping spines that give the group their scientific name (Chaetognatha means 'bristle-jaws'). At small scale, they are effective predators and bear some resemblance to the alien in the movies of the same name.
Arrow worms hunt actively with the aid of compound eyes and ensnare small, planktonic animals including larval fishes using their mouth spines. Some use a potent neurotoxin called tetrodotoxin, similar to that used by blue-ringed octopuses, to subdue their prey. As with blue-ringed octopus and other users of tetrodotoxin, the toxin itself is actually produced by symbiotic bacteria.
A typical arrow worm, in the genus Spadella. The head (with the grasping bristles retracted within a hood for swimming) is at right, the tail at left. Two testes can be seen near the tail and two ovaries near the midpoint in the transparent body. Source: https://commons.wikimedia.org/wiki/File:Chaetotilted.png
Arrow worms are hermaphrodites, each individual having a pair of both testes and ovaries. During mating, a sperm-containing spermatophore is placed on the neck region of each partner. Sperm released from the spermatophore swim along the midline before entering a small pore near the tail, where they fertilise the eggs. Hatchlings are like miniature versions of the adults.
Arrow worms are considered to have high ecological impact because they act as links between smaller and larger animals in the food web. As well, different types of arrow worms prefer different environmental conditions, making them useful as biological indicators: the abundance or absence of particular arrow worm species gives biologists clues as to the temperature and salinity of the ocean where they are found.
A Sagitta arrow worm with bristles in position for grasping prey. Image by Zatelmar CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2777766
Despite their voracious predatory behaviour and use of neurotoxins, arrow worms don't have it all their own way. Recent studies have shown that some become infected by giant viruses (sometimes called giruses). These are so large that early microscopic studies on arrow worms completely missed the viruses by misinterpreting them as bristles of the arrow worm or infecting bacteria: one giant virus found in arrow worms is several times larger than moderate-sized bacterial cells.
Around 120 species of arrow worms have been described worldwide, 19 of which are known from Australia. Despite their abundance and high ecological relevance, vanishingly little study has been carried out on Australian arrow worms since the 1940s. It is very likely that many species have yet to be discovered.
Thanks to Robin Hare from the University of Western Australia for help preparing this profile