Every leg ends in a paddle, so it swims well and buries itself backwards in seconds.
A small sand crab with a nearly circular carapace to about 5 cm across, cream to pale greenish and covered in a fine dense pattern of purple-red dots gathered into rosettes, with a single sharp spine projecting from each side. What separates the genus from the swimming crabs is that all four pairs of walking legs, not only the last, end in flattened paddles, so the animal digs and swims with its whole body. Matuta planipes is closely similar and is told from it on the arrangement of granules and spot pattern rather than on anything visible in a photograph, and the two are best treated together underwater. Portunid swimmers such as Portunus and Thalamita carry paddles only on the rearmost pair of legs and have a much wider, more angular carapace.
Lives on shallow open sand, often in the top few metres and sometimes in water barely deep enough to cover it, and spends much of the day buried with only the eyes and the antennal filaments clear of the surface. It swims in short strong bursts when flushed and buries backwards, working down out of sight in a few seconds. Predatory and scavenging, taking small molluscs, crustaceans and carrion. Recorded to about 15 m but overwhelmingly a shallow animal. On a night dive the buried crabs are found as small paired eye reflections on otherwise blank sand, and a shallow safety stop over sand is often where they turn up rather than the dive itself.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.
