Habituation to a danger stimulus in two semiterrestrial crabs: ontogenic,ecological and opioid modulation correlates

Previous research on the habituation of the crab Chasmagnathus granulatus to an iterated visual danger stimulus disclosed two remarkable features of this mnemonic process: a) the ready acquisition and long-lasting nature of the habituated response, and b) the likely endogenous opioid modulation of the shortterm habituation, whose potential adaptive value was studied here. Both the habituation ability and the opioid modulation of such ability were analysed in Chasmagnathus granulatus and Pachygrapsus marmoratus, two closely related grapsid species that diverge widely in ecology. Results indicate that Chasmagnathus has a much greater capacity to habituate than Pachygrapsus, showing a more pronounced short-term habituation which seems to some extent mediated by an endogenous opioid mechanism, and a faster acquisition of a greater longterm habituation partially mediated by memorial representation of the contextual cues. A correlation between these differences and those related with the ecological pressure was discussed. The capacity of acquiring longterm habituation in crabs of the same species but different age was also assessed, showing that old animals clearly exhibit lower retention of the habituated response than young crabs. The possible correlation between this age-dependent change and an ontogenetic shift of habitat was discussed.