Multifunctional behaviour in a sandy shore crab enhances performance in extreme intertidal environments

Soft sediment shores in the tropics are highly dynamic environments, where behavioural patterns of organisms are constrained by tidal conditions, and environmental temperatures during an organisms' activity periods can exceed their thermal tolerance levels. In such extreme habitats, behavioural responses to environmental changes are key to survival, driving differential performance. We investigated sponging behaviour (water uptake from sediments) of the deposit-feeding crab, Scopimera intermedia, on tropical sandy shores to determine its thermoregulatory function. The thermal physiology of the crabs and their habitat conditions were quantified by measuring thermal performance curves and recording environmental temperatures during the crabs' activity periods. Environmental temperatures were combined with experimental data to investigate the role of sponging on the thermal performances of the crabs by simulating field body temperatures. Sponging rate was strongly and positively correlated with feeding rate, as sponging replenishes water for flotation feeding. Sponging, however, also reduced body temperatures on average by 1.3 degrees C. Simulated populations of crabs which were unable to sponge had more variable body temperatures, which exceeded the critical thermal maximum of the crabs (39 degrees C) nearly 2000 times more often than crabs able to sponge. Sponging is, therefore, a multifunctional behavioural trait important for both feeding and thermoregulation. The evolution of such multifunctional traits is likely to be a widespread, but overlooked phenomenon in intertidal species, as maintaining a functional body temperature is energetically costly in habitats where environmental conditions fluctuate strongly such as on tropical shores.