“…According to Fotheringham and Brunenmeister (1975), the grapsid Sesarma reticulatum, which had the higher Hc 02 affinity, carries on gas exchange in the (perhaps hypoxic) water filling a deep burrow, whereas S. cinereum, with its lower 02 affinity, is basically an air-breather with a larger gill surface area than its congener. The difference between Uca pugilator and U. pugnax, however, is fairly large despite similar branchial surface areas (Pearse, 1950) and geographic ranges. In this case burrow P02 may differentiate the sand (U. pugilator) and the mud (U. pugnax) fiddler crabs.…”