Why do brittle stars emit light? Behavioural and evolutionary approaches of bioluminescence

In this study, we investigated the functions of bioluminescence (the production of light by living organisms) in five brittle star species. Bioluminescence is a widespread phenomenon in the marine environment, and is especially abundant in the class Ophiuroidea. It is assumed that light in marine invertebrates mainly plays a role of defense against predation, and many mechanisms of defense have been proposed for brittle stars. We investigated the potential functions of startle effect (use of light to deter predator), use of light to advertise a predator that the prey is toxic (aposematic signal) and attraction of a secondary predator (function usually called "burglar-alarm effect"). Predatory experiments, involving one or two predators from different trophic levels allowed us determining benefits of the light emission for several brittle star species. We clearly demonstrated that the three functions cited behind are used in brittle stars. It is clear now that brittle stars use a wide variety of defensive mechanisms involving light