Bioluminescence of Tomopteridae species (Annelida) : multidisciplinary approach

While the majority of bioluminescent pelagic organisms emit blue light, some transparent planktonic annelids (Tomopteridae) have been found to emit yellow light from parapodial glands (rosette and hyaline). Since long wavelengths have a weak propagation coefficient and appear to be imperceptible to most midwater species, this bioluminescence is commonly thought to act as a private communication channel. This work aims to explore this functional hypothesis through a multidisciplinary approach of bioluminescence in Tomopteris helgolandica as main model species and in four related species: T. carpenteri, T. pacifica, T. planktonis and T. septentrionalis. The structural homology between the rosette of the tail-bearing species and the hyaline glands of the tail-less species is evidenced, suggesting that both gland types evolved from a common light-emitting structure and differentiated along a functional and migrational axis extending from endocrine secretion close to the coelomic ramus to exocrine secretion close to the lateral margin of the pinna. In T. helgolandica, bioluminescence is triggered by the activation of nicotinic cholinergic receptors and a Ca2+-dependent mechanism involving L-type volgate-gated channels. Despite significant differences in their light emission patterns, cholinergic control is also suggested in T. planktonis. Indeed, an unexpected diversity in emission patterns is highlighted including yellow and blue, intra- and extraglandular displays. Consequently, the ecological advantage provided by this bioluminescence could be more diverse than originally suspected. Although the first behavioural approach of light perception by T. helgolandica tends to support the hypothesis of intraspecific communication, the possibility of a defensive use of bioluminescence should remain an open question.