R. desmaresti has a dorso-ventrally flattened body and may reach a size of. The carapace is small and is only fused to the first two segments of the thorax. R. demsaresti is distinguished from the other British species, Platysquilla eusebia, by the number of spines on the last segment of the raptorial claw; R. desmaresti has five spines, while P. eusebia has a dozen or more. It lives in burrows below from the subtidal zone down to depths of around.