Peacock mantis shrimp

Between 3 and 18 cm, olive to emerald green with orange antennal scales, red raptorial appendages, leopard spotting across the front of the carapace and blue stalked eyes; males are the more vivid. Structurally the thing to look at is the raptorial appendage, which ends in a swollen calcified club rather than the barbed spear of the spearing mantis shrimps — that division between smashers and spearers is the primary split in the group and it is visible in a decent photograph. It excavates its own U-shaped burrow in coarse sand, rubble and gravel at the base of reef, between about 3 and 40 m.

Two things about this animal are famous and both are usually reported wrongly. The strike has been measured at 14 to 23 metres per second with accelerations around ten thousand times gravity, over about 2.7 milliseconds; the comparison to a rifle bullet is out by more than an order of magnitude. What is genuinely remarkable is that it hits twice — impact forces of 400 to 1,500 newtons, followed by a separate collapse of the cavitation bubble the strike creates, adding another 170 to 500. And the eyes: twelve classes of colour photoreceptor are real, and the conclusion usually drawn from them is not. When the discrimination was tested behaviourally, the animals could only tell wavelengths apart when they differed by fifty to a hundred nanometres, roughly ten times worse than a human, a bird or a goldfish. The proposal is that the eye scans and reads a twelve-channel pattern rather than comparing hues — recognition rather than discrimination, speed traded against resolution. Two caveats worth carrying: the species tested was a different mantis shrimp, and the researchers’ own later review calls the explanation hypothetical. The eyes do also see both linear and circular polarised light, which no other animal is known to do.