Crabs that breathe air with their legs-Scopimera and Dotilla

Abstract

Sand-bubbler crabs, Scopimera and Dotilla (Crustacea, Brachyura, Ocypodidae, Scopimerinae), are small (carapace width 1 cm) round-bodied crabs which occur in vast numbers throughout the Indo-Pacific on suitable tropical and sub-tropical sheltered sandy beaches. At high tide they are found in burrows that contain a trapped pocket of air in which the crabs are able to breathe until low tide. During low tide they become active and emerge from their burrows to feed on surface detritus and stranded plankton^1–3. One of the most striking, and unique, features of these crabs is the presence of large membranous disks (‘tympana’) on the meral segments of the legs (Fig. 1) and sometimes (in Dotilla) on the thoracic sterna³. Although in 1893 Aurivillius suggested that the membranes might be used for hearing⁴, their function has remained unknown. Here I present evidence which suggests that, rather than being auditory organs, the tympana are primarily respiratory structures used in aerial gas exchange. As the generic name Scopimera means "thighs with windows in them"⁵, I have coined the term ‘gas windows’ for these membranes.