Simple and complex burrow morphology in two Macrophthalmus species on the intertidal mudflats of Barr Al Hikman, Sultanate of Oman

Burrowing Ocypodoidea crabs are an abundant component of many tropical and temperate coastal areas and central to the ecosystem functioning, for instance because they recycle nutrients, are important food for many shorebirds and alter the sediment by their burrowing behaviour. The burrow morphology of these crabs may differ between and within species, often correlated with differences in habitat preferences, crab morphology and life-history traits. Here we studied the burrow morphology and complexity of Macrophthalmus sulcatus and Macrophthalmus depressus, by means of casts (n = 7 and 10 respectively) and burrow excavations (n = 17 and 16 respectively) at the pristine intertidal mudflats of Barr Al Hikman in the Sultanate of Oman. We found that M. sulcatus construct simple burrows that were in all but one case inhabited by a single crab. By contrast, all burrows of M. depressus were complex with multiple entrances and many (deep-reaching) branches. There was a strong relation between M. sulcatus carapace width and burrow entrance size, indicating that the simple burrows are adapted to, and made by the occupant. There was no relation between M. depressus carapace width and burrow entrance size, and in six burrows more than one crab was encountered, suggesting that the complex burrows are not made by, and adapted to a single occupant. The complex burrows were found close to the shore whereas the simple burrows were found at the intermediate tidal zone. We speculate that the striking differences in burrow morphology may be explained by difference in habitat selection of the studied crabs, which most importantly relates to differences in sediment structure and tidal height. Also, crab morphology and life-history traits including the social life of the studied crabs could account for the observed difference in burrow morphology.