It lives on the crown-of-thorns, the animal that eats reefs and carries venomous spines that put divers in hospital, and it lives on the underside of it, walking among the tube feet.
And it feeds on its host. Not on scraps the host drops: on the sea star itself, grazing the mucus off its surface and taking bites of the tube feet. Fifty of them have been counted on a single star. The host cannot reach them, gets nothing out of the arrangement, and carries them anyway.
What it gets in return is a bodyguard that no fish will touch. Almost nothing on the reef attacks a crown-of-thorns, so a centimeter-long shrimp living on one is about as safe as a small crustacean can be.
It matches whatever it is riding - blue on a blue sea star, brick red with a pale stripe on a crown-of-thorns - and it finds hosts by smell. Take one off its star and it will track down another of the same species rather than settle for the first one it meets.
- colored to match the sea star it is on - blue on a blue sea star, red on a crown-of-thorns
- pale stripe down the middle of the back
- usually on the underside, in among the tube feet
- often several on one host, occasionally dozens
- up to 1.5 cm
