Firefly squid modulate photophore expression in relation to their social environment

Abstract Firefly squid ( Watasenia scintillans ) possess distinct blue and green bioluminescent photophores. Owing to their possession of three visually active pigments (a highly uncommon trait in the deep sea, where monochromatic visual systems dominate), it has been postulated that firefly squid may utilise two distinct photophore colours for private, intraspecific communication. In this study, firefly squid were exposed to a range of social treatments and their photophore responses recorded. We found that the expression of green photophores is stronger in social settings compared to solitary settings. Each individual’s spatial distribution of active photophores could not be used to distinguish it from others, suggesting it is unlikely that photophores facilitate individual recognition in this species. Finally, individuals adjusted their own expression of blue and green photophores in response to animations of the photophores of their perceived social neighbours. Populations of simulated firefly squid following these interaction rules were found to increase their ratio of green to blue photophores in synchrony, suggesting green photophores may serve a role in coordinating a synchronous group activity, such as rising to the surface to spawn. Future work disentangling the hue and brightness information provided by blue and green photophores respectively in this species could provide crucial insight into whether this species’ trichromatic opsin system does indeed translate into behavioural evidence for colour vision.