A bivalve cemented to rock under a coat of sponge, given away by the eyes along the mantle edge.
A thick-shelled bivalve cemented by its right valve to rock, dead coral, concrete or wreckage, with the shell carrying rows of spines that in a clean specimen are long and flattened and in a living one are usually broken, worn or buried under growth. The shell itself is orange, red, purple or white and is almost never visible. Sponge, coralline algae, ascidians and hydroids settle on it until the animal is a lump of encrusting growth with a wavy gape running across it, and finding one is a matter of recognising that gape.
The mantle is what identifies it in the water. Along the edge of the opening is a fringe of short tentacles and, set among them, a row of small dark eyes, and the valves shut when a shadow or a light passes over them. That closing is often the first sign that a piece of encrusted rock is an animal. Feeding is by filtration, and the two valves are held together by a ball-and-socket hinge that keeps them aligned, which is unusual among bivalves.
The other bivalves on this site are elsewhere and look nothing like it: the pen shell is buried point-down in soft sediment with only its gaping end showing, and the file clams wedge into crevices and show a fringe of long red tentacles. This one sits out on hard substrate in plain sight and is invisible for a different reason. Species are separated on the shell, which is overgrown in life, so records stay at the genus. Shells cleaned of their growth have been traded and worked as ornaments for thousands of years. Recorded from about 3 to 30 m.
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.