General Information

Chaetognaths, or arrow worms, are a phylum of approximately 100 species whose bodies are transparent, and range in size from just 2 millimetres to 120 millimetres. Chaetognaths gain their name from their distinctive torpedo, or arrow shaped bodies. They are abundant marine organisms that are present in the planktonic environment, and are predators of copepods, crustaceans, larval fish, and even other chaetognaths, as well as been an important prey for larger organisms. Chaetognaths sense their prey using mechanoreception, using ciliary fences along the body and fins that detect water movements; once a prey has been identified they spring forward and catch them with their grasping spines, which are located on their head. Additionally, some species are known to contain a neurotoxin that assists to subdue the prey, before consuming them whole.

Chaetognaths are hermaphrodites, meaning that they contain both male and female sex organs. These organs are separated within the chaetognaths body, with sperm located in the tail section, and eggs in the trunk section. Most chaetognaths are internal fertilizers who release their fertilized eggs into the ocean, whilst some attach fertilized eggs to the substrata, although there is one species that broods the eggs internally.  

The phylogeny of the 100+ species is constantly under review, as there is considerable debate regarding their taxonomy, as well as their origin and evolutionary development. Historically, chaetognaths have been considered as deuterstomes based on their embryonic development,  recent findings however have suggested that they are in fact protostomes, and may be closely related to ecdysozoans (organisms who moult).
 
This website provides information on Chaetognatha as a whole. This is because, whilst on Heron Island, the resources required to identify specimens to genus or species were not available. Additionally, multiple species were believed to have been captured, which made the identification process difficult. However, it is believed that at least one of the species captured was Sagitta tasmanica.