The behavioural repertoire of the Scopimerinae crab, Dotilla fenestrata, is the result of a juxtaposition of rigid and plastic patterns, as an adaptation to the intertidal habitat. On the one hand, the activity of D. fenestrata can be classified in a number of stereotyped behavioural patterns, the occurrence of which may change throughout the low water or on the basis of the crabs' relative size, according to an expected schedule. For example, digging is more pronounced in larger individuals and mostly occurs when the crab emerges or just before it seals itself inside the burrow at the flooding tide. In contrast, maintenance of the burrow is a constant behaviour. Feeding rate, estimated from the number of pseudofaecal pellets emitted, is constant both throughout low water, with the exception of noon when it decreases, and with the feeding crab's size. The behavioural patterns that assure water uptake and temperature regulation (sponging and visits inside the burrow) are more frequently performed around noon. Agonistic patterns are mostly performed by larger individuals; the rate of aggression by resident crabs, against neighbours but mostly against smaller passers-by, increases with the flooding tide. On the other hand, for a surface deposit-feeder like D. fenestrata, plastic behaviour is mainly caused by the unpredictable distribution of food over the emerged platform. The result is a number of adaptive flexible behaviours, like: (1) the presence within the same population of both burrow-centred crabs and wanderers; (2) the construction by the former subpopulation of two types of burrows (feeding-trench burrows and igloos); and (3) significant differences in the behavioural budgets of the inhabitants of the two burrows. The ability to ‘switch’ modes of activity allows responses to both the predictable and unpredictable elements of intertidal environments, the mangrove swamp included.