Melatonin in octopus (Octopus vulgaris): tissue distribution, daily changes and relation with serotonin and its acid metabolite.

Information regarding melatonin production in molluscs is very limited. In this study the presence and daily fluctuations of melatonin levels were investigated in hemolymph, retina and nervous system-related structures in the cephalopod Octopus vulgaris. Adult animals were maintained in captivity under natural photoperiod and killed at different times in a regular daily cycle.