*3.2.4 Chaetognatha*

Other smaller marine organisms are able to acquire luminescence through predation, such as at least two species of chaetognaths. This phylum comprises of small, elongated worms that are between 2 and 120 mm in length [77]. Commonly known as "arrow worms" at least two species have been shown to be luminescent and can be found at depths greater than 700 m in marine systems ranging from tropical to polar regions [78]. Luminescence in all of these species is emitted as a blue cloud of light and may facilitate a role in stunning their prey to assist with their hunting strategies giving the lack of visible light that will attenuate down to these depths. Despite being from evolutionarily distinct lineages within the chaetognaths, luminescent species such as *Caecosagitta macrocephala* [79] and *Eukrohnia fowleri*, have a relatively uncommon trait among chaetognaths, in that they have an orangepigmented gut lining [80]. Digestive systems in semi-transparent organisms that are orange in colour, have the capacity to mask any luminescence produced by ingested prey [78].

This provides strong evidence that some species will predate on luminescent organisms such as copepods in order to provide a dietary source of coelenterazine for their luminescent reaction as shown in a number of other marine organisms [12, 48]. Once absorbed, coelenterazine would be able to be passed through to their luminescent organs that harbour the light reaction, which tend to be found on the lateral and dorsal fins as well as along the sides of the body of these species [78].
