Egg cowrie

The shell is smooth, heavy, glossy white and shaped like its name; the living animal covers it almost completely with a mantle of dense matte black, speckled with small raised pale spots and papillae. The contrast is the whole photograph, and it exists only while the animal is undisturbed — touched or lit hard from close range, the mantle withdraws into the shell aperture and the subject becomes a white lump.

It is not a true cowrie. Ovula ovum belongs to the Ovulidae, the group that also contains the small allied cowries found on sea fans and soft corals, and the family as a whole feeds on octocorals. This species works on the fleshy leather corals of the genus Sarcophyton and their relatives, rasping the tissue away and leaving pale grazed patches, and it is often found sitting on the damage it has caused. That association is the way to find one: search the leather corals rather than the sand.

Largest and most active at night; by day it tends to sit tucked under the folds of the coral or in an overhang nearby. The egg masses are laid on the coral itself, as flat pale patches the female may remain with.

For photography, approach with the focus light dimmed and off the animal, and take the first frame from further back than feels necessary. The mantle is the shot, the shell is what you get for rushing.