Harlequin shrimp

About 5 cm, the female slightly larger. Nothing else looks like it: the second pair of chelipeds is enormously expanded into flat petals, the antennules are flattened into little leaf-like sails, and the cream body carries large flat blotches — pink to purple edged in yellow, or browner and edged in blue depending on where in the Indo-Pacific you are. Those two colour forms were long treated as separate species, picta and elegans, and WoRMS now sinks the second into the first on the grounds that both morphs turn up in Japanese specimens; no genetic test of the two has been published, and the aquarium trade still separates them. The only real confusion is with the tiger shrimp, which shares the flattened-cheliped body plan but is banded rather than blotched, spiny rather than smooth, and about a third the length.

A specialist predator on sea stars — Linckia, Fromia, Nardoa and crown-of-thorns among them — and almost always found as a male-female pair working together. In the one detailed published observation, the smaller male pierced the tube feet at the arm margins with the small chelae of the first legs, the female joined him, and after thirty to sixty minutes of combined effort the star was turned over; they then fed on the tube feet and gonad. The much-repeated account of a pair keeping a star alive for days, feeding on it slowly and even feeding it to prolong the meal, is not supported by anything published. It is mechanically plausible, since a sea star survives a great deal of tissue loss, and it is what aquarists describe — but it should be read as widely reported rather than established. Shallow, mostly under 20 m, and common across the Indo-Pacific.