HABITUATION TO A DANGER STIMULUS IN 2 SEMITERRESTRIAL CRABS - ONTOGENIC, ECOLOGICAL AND OPIOID MODULATION CORRELATES

Previous research on the habituation of the crab Chasmagnathus granula tus to an iterated visual danger stimulus disclosed two remarkable fea tures of this mnemonic process: a) the ready acquisition and long-last ing nature of the habituated response, and b) the likely endogenous op ioid modulation of the short-term habituation, whose potential adaptiv e value was studied here. Both the habituation ability and the opioid modulation of such ability were analysed in Chasmagnathus granulatus a nd Pachygrapsus marmoratus, two closely related grapsid species that d iverge widely in ecology. Results indicate that Chasmagnathus has a mu ch greater capacity to habituate than Pachygrapsus, showing a more pro nounced short-term habituation which seems to some extent mediated by an endogenous opioid mechanism, and a faster acquisition of a greater long-term habituation partially mediated by memorial representation of the contextual cues. A correlation between these differences and thos e related with the ecological pressure was discussed. The capacity of acquiring long-term habituation in crabs of the same species but diffe rent age was also assessed, showing that old animals clearly exhibit l ower retention of the habituated response than young crabs. The possib le correlation between this age-dependent change and an ontogenetic sh ift of habitat was discussed.