Sand Bubbler Crabs
A crab that eats, cleans and breathes with its legs, while creating spectacular patterns on sandy beaches.
Sand bubbler crabs occur in vast numbers on sheltered tropical and sub-tropical sandy beaches throughout the Indo-Pacific. At low tide they emerge from their burrows and feed on organic detritus and plankton stranded on the sand. They collect the sand with their claws, separating and eating the organic material with their mouthparts, essentially legs modified for feeding. The cleaned sand is sorted into balls or sand bubbles, that they kick behind them, creating large patterns around their burrows.
Sand bubbler crab amongst balls of sand cleaned for food, (of the genera Scopimera or Dotilla in the family Dotillidae), Koh Lipe, Thailand.
If spooked by potential danger or a predator during feeding, they rapidly return to their burrows, through the maze of sand balls. A stealthy approach is required to approach close enough to photograph them. Some species leave paths between the balls to expedite their retreat to safety. They begin the cleaning and feeding process as efficiently and safely as possible, close to their burrows and progressively feed outwards, in a radial pattern, further away from their burrow and safety. Within a couple of hours of low tide, entire beaches can be covered in these patterns.
During low tide, these patterns can cover vast areas of sheltered tropical sandy beaches. This the work of one crab, less than 1 cm in size, in a couple of hours.
During high tide, the crabs return their burrows that contain a trapped pocket of air in which the crabs are able to breathe. The sand balls they created disintegrate. As the tide recedes, they re-emerge and the process begins again, with organic matter freshly deposited by the retreating tide. They are important as nutrient cyclers and as an indicator species, abundance indicates a healthy habitat.
Patterns created by sand bubbler crabs on Noah Beach, Cape Tribulation, Australia. Each patch is the work of one crab. Photo by Rob and Stephanie Levy. Flickr. Creative Commons License
They possess tympana, large membranous disks on their legs through which they are able to breathe. In 1893 Aurivillius suggested that the membranes might be used for hearing. The generic name Scopimera means “thighs with windows in them”. David P. Maitland presented evidence in Letters to Nature, Crabs that breathe air with their legs-Scopimera and Dotilla, 1986, “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”, I have coined the term ‘gas windows’ for these membranes”.
Watch them in action in this clip from The Blue Planet.