Sand Artist on the Beach - Sand Bubbler Crab

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Have you ever wondered about the of small sand balls spread all over the beach during ebb tide on a fine day? If you look closer, you will find that by each of the sand ball piles there is a tiny hole. This is the burrow where a Sand Bubbler Crab (Scopimera spp.) lives—and, sure enough, the little balls are the work of the crab. Why would the Sand Bubbler Crab make so many sand balls?

The tiny Sand Bubbler Crab – with a carapace about 1 to 1.5 centimetres across – is often found on sandy beaches sheltered from strong wind and waves. It takes sharp eyes to spot the greyish orange crab, which blends so well with the environment. There are about five species of Sand Bubbler Crab in coastal China. The commonest species found in Hong Kong is Scopimera intermedia. It is rather difficult to distinguish the different Scopimera species – which sometimes requires the use of microscopes to identify the shapes of the gonopods in order to differentiate them.

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The Chinese name of Sand Bubbler Crab denotes the thin membrane (gas window) on the merus of its four pairs of walking legs. Merus is the section of the leg closest to the body (corresponding to the human thigh). In the past, biologists thought that the membrane is similar to an eardrum, and hence is hearing related. Later, in the 1980s, scientists carried out experiments with the use of electron microscopes and discovered that the membrane actually helps the crab to absorb oxygen from the atmosphere.

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Sand Bubbler

When the tide recedes, a large flat sandy beach is exposed. On fine days with scorching sun, creatures on the beach will easily be overheated and desiccated. Sand Bubbler Crab has a special structure to help it absorb water—stiff hairs grow towards the ground at the base of the first and second walking legs, which can absorb water from the surface through capillary action. In addition, local research has shown that Sand Bubbler Crab frequently holds its pincers up, to raise its body and lower its temperature.

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Sand Bubbler Crab is characterised by its feeding pattern. It picks up sand grains with its two pincers in turn, and scrapes off any organic matter on the sand grains with the pair of mouthparts. The indigestible sand grains are then rolled into little balls and discarded. As the crab sifts through the sand, a galaxy of sandy balls pile up on the beach, hence earning its English common name of “Sand Bubbler Crab”.