The blue-green slit in a living coral head is a scallop. It settles on a Porites colony as a larva and the coral grows around it, leaving a slot through which the animal filters water, so that an adult is permanently embedded in a living rock that keeps growing over it.
The arrangement is mutual. The scallop gets a fortress no predator can open. The coral gets a defender: when a crown-of-thorns starfish climbs onto a colony, the scallop claps its shell shut hard enough to fire a jet of water at it, repeatedly, and colonies carrying scallops suffer measurably less damage from starfish than colonies without them.
Along the mantle edge is a row of small blue eyes. A scallop's eye works on a mirror rather than a lens - a concave layer of guanine crystals that focuses light back onto the retina, the same principle as a reflecting telescope - and it is good enough to detect a shadow moving overhead, which is all the animal needs.
A hand passing above one will close it. Wait a minute and it opens again.
- a narrow slit in a living coral head, edged in bright blue-green mantle
- row of tiny blue eyes along the mantle edge
- closes instantly when a shadow passes
- shell up to 6 cm, almost entirely buried
