What are they?

Chaetognaths, also known as arrowhead or bristlehead worms,  are a highly abundant predatory zooplanktonic group. These long, narrow, transparent aquatic worms are common in all marine waters from polar to tropical and occur in a range of depths. Chaetognaths are often found in great numbers in the mid waters above 200 m , rock pools, and in certain ocean currents. They form their own phylum, and so are as different to other worms as they are to humans. Morphologically, they are very unique. However, they are not a particularly diverse group with only 130 known species. 20% of  all species are  benthic, meaning they live on the sea floor, and often attach to algae or rocks.  Deep-sea species exist that can be orange or bioluminescent. The two main species of Chaetognath found in Irish waters are Parasagitta setosa and Parasagitta elegans.

Ecological Importance

Chaetognaths are predatory and are nocturnal carnivorous feeders. They typically feed on prey such as copepods, small crustaceans, larval fish and turn cannibalistic as a last resort. Indeed, chaetognaths can inject the neurotoxin tetrodotoxin into their prey to induce paralysis, as well as hooks in their head to catch and hold prey. Chaetognaths are predated on by larger organisms like fish, molluscs, toothed whales, and skates. Additionally, they can also be host to mutualistic species or to parasitic or ectoparasitic species too. They can be infected by these species by feeding on infected prey and can also pass these species further up the food chain.

Life Characteristics

Cold water species have an average lifespan of 2 years while warm water species typically have a much shorter lifespan of 6 weeks typically. Chaetognaths are between 1 mm and 12cm in length and are holoplanktonic, meaning they spend their entire lifespan in the water column. Adult Chaetognaths are hermaphroditic but usually cross-fertilise as sperm matures before eggs, making self-fertilisation unlikely. Fertilisation is internal and then eggs are released into the water column. There is no larval stage in chaetognaths, instead they undergo direct development whereby no major changes or metamorphosis occurs. A zygote can grow into a hatchling within 48 hours, with no parental investment occurring. Breeding occurs twice a year from April to June and September to December.

By Cora Twomey