Evidence of seasonal variation in bioluminescence of Amphipholis squamata (Ophiuroidea, Echinodermata): effects of environmental factors.

The bioluminescence of Amphipholis squamata was assessed from freshly collected individuals for 16 successive months, and from individuals maintained in the laboratory under various experimental conditions of salinity, temperature and photoperiodic regime. Field investigations showed that bioluminescence intensity and kinetics varied seasonally, with the light produced being brighter and faster in winter and summer. The seasonal variation was not correlated with changes of ambient salinity. However, it was correlated with changes in temperature, the luminescence being brighter and faster in coldest and warmest seasons, and with the changes of photoperiod, the luminescence being brighter and faster in seasons with shortest and longest day length. Laboratory investigations also demonstrated that luminescence was not affected by salinity conditions. Conversely, luminescence was affected by temperature, the light production being brighter and faster in warmer conditions (in agreement with field observations) and dimmer and slower in colder conditions (in disagreement with field observations). Light production was also affected by photoperiod since experimental changes of natural light:dark regime caused the bioluminescence to decrease. Considering that photoperiod guides the biology of A. squamata and that reproduction takes place during coldest months in the species, an endogenous factor of neurophysiological nature linked to the ophiuroid reproductive cycle is proposed to induce the luminescence to peak in winter. This was confirmed by the fact that seasonal variation of luminescence was different between adult and juveniles, the latter showing no winter peak of luminescence. It is suggested that the luminescence normally associated with defense could also be part of an intraspecific visual signal related to individuals aggregating for reproduction during winter.