'Igloo' construction by the ocypodid crab, Dotilia myctiroides (Milne- Edwards) (Crustacea; Brachyura): The role of an air chamber when burrowing in a saturated sandy substratum

Dotilla myctiroides, an inhabitant of tropical sandy shores, builds a unique burrow structure in semi-fluid, unstable sand, in addition to a vertical burrow constructed in well-drained and firm sand. The crab rotates in the sand, forming a circular wall of sand pellets around it and subsequently roofs the wall with pellets. The resulting burrow structure, termed an 'igloo', encloses a small quantity of air together with the crab itself. In this study, the function of the igloo was examined and compared to that of the vertical burrow structure. In the air-filled chamber beneath the sand surface, which results from the construction of the igloo, the crab continued to scoop up the sand from the lower surface of the chamber, and attached the scooped pellets to the ceiling of the chamber. Consequently, the crab, together with the chamber, continually moved deeper into the sand until it was below the water level. In contrast, 40% of the crabs artificially buried in sand without a surrounding air chamber remained where they had been buried 12 h later. This indicated that the air chamber greatly facilitates vertical movement of the crab. The making of the igloo is considered to be an adaptation that enables the construction of an air chamber in semi-fluid sand, where a vertical burrow cannot be constructed, because the sand is not sufficiently firm to prevent it from collapsing. The descent of the crab into the sand through the construction of an igloo may prevent an encounter with dangers such as predators arriving with the incoming tide, a role played by the construction of a vertical burrow when the sandy substratum is firm and well-drained.