Smashing mantis shrimp

To about 10 cm and usually well short of it, olive-green to brown, sometimes with a reddish or bluish cast, with the raptorial appendages folded beneath the front of the body and ending in a swollen calcified heel rather than a barbed spear. That heel makes it a smasher. The carapace and the tail plate carry low ridges. Colour and markings vary a good deal, and species-level identification within the gonodactylid mantis shrimps rests on the ridges of the tail plate and the inner face of the raptorial limb, neither of which a normal field photograph shows, so a green smasher in rubble is often recorded no further than the family. The separation from the peacock mantis shrimp is easy and is as much about address as appearance: peacocks are larger, vividly coloured, and excavate their own burrows in coarse sand, while this animal occupies a ready-made cavity in hard substrate and does not dig.

It lives in holes in coral rubble and dead coral heads, in empty bivalve and gastropod shells, and in debris and wreckage, and it defends the cavity. The club is used to break the shells of snails, hermit crabs and small bivalves, and to fight other mantis shrimps over cavities, which are the limiting resource. Those fights are largely ritualised: the defender curls its armoured tail plate over the entrance and takes the blow on it, so contests are usually settled by an exchange of strikes without injury. Females brood the egg mass inside the cavity, holding it with the mouthparts and turning it continuously.

It is one of the animals most often seen looking out of a hole in rubble on a muck site, and loose debris turned over by a current frequently has one in it. It has also been carried outside its range: an introduced population in Hawaii has displaced the native mantis shrimp Pseudosquilla ciliata over much of the shallow reef there, which is unusual for a stomatopod.