Metopograpsus thukuhar is a very common grapsid in the Indo-Pacific mangroves but is found only occasionally in a non-mangrove environment. Field observations investigated its spatial and temporal strategies and clarified its predatory abilities. Gut-content analysis was used to assess its natural diet. Metopograpsus thukuhar was mainly active during low tide, although many crabs were seen at high tide moving on the mangrove roots above the water level. It lived largely among the roots of the seaward Rhizophora mucronata and concentrated its activity within a definite area of the root apparatus of a single tree, appearing to be faithful to one or two specific crevices. The diet of M. thukuhar was principally based on macroalgae; mangrove leaves were also present, but animal items were rare. However, direct field observations of the crab's predatory behavior indicate that this grapsid is an opportunistic feeder with a certain degree of behavioral plasticity.