Influence of light regimes on phyllosomal growth and timing of moulting in Thenus orientalis (Lund) (Decapoda : Scyllaridae)

Newly hatched phyllosomas of Thenus orientalis (Lund) were successfully reared under conditions of natural light (D/L), continuous dark (24D) or continuous light (24L). Survival, duration of intermoult periods, moult increment in total body length, and timing of moulting through larval stages were monitored. Survival and growth under 24L was lower than that under D/L and 24D; this may be a result of decreased phyllosomal feeding activity under 24L, caused by a photopositive reaction drawing the phyllosomas away from food at the bottom. There were no differences in phyllosomal development under D/L and 24D. When phyllosomas were reared under D/L, moulting was synchronized and occurred around dawn. Moulting of those reared under 24D or 24L was not synchronized and occurred irregularly. These results indicate that, in phyllosomas, light regime influences the endogenous rhythmic function responsible for the regulation of moulting time. When phyllosomas reared under D/L metamorphosed to the nisto stage, synchronized moulting switched from dawn to after dusk. Furthermore, nistos and juveniles moulted only nocturnally, with apparent synchronicity. This suggests that the switch from planktonic to benthic lifestyle, with consequent different predation pressures, necessitates a change in the timing of moulting.