Mantis shrimp have up to 16 kinds of light receptor, but may not see 'more colours'.
Humans see colour with three kinds of receptor; some mantis shrimp have twelve for colour, plus more for ultraviolet and polarised light. That inspired claims that they see unimaginable colours. Tests suggest otherwise: they struggle to tell similar hues apart. Their eyes may be built for speed instead, like their punch.
Mantis shrimp are neither shrimp nor mantises but a separate order of crustaceans that split from their relatives around 400 million years ago, with more than 520 living species, mostly in warm, shallow seas. They live in burrows and are formidable predators. 'Spearers' impale soft prey such as fish with barbed limbs, while 'smashers' crack snails, crabs and oysters with a heavily mineralised club.
The smashers' strike is one of the fastest movements in nature. In the peacock mantis shrimp, the club accelerates at about 10,400 g and reaches 23 metres per second from a standing start. It moves so fast that it creates vapour bubbles in the water, which collapse with force of their own and even emit a flash of light, so the prey is hit twice. Even a miss can stun. The blow is powered by a spring-and-latch mechanism and is over quicker than a nerve signal can travel, so it cannot be steered once launched.
Their eyes are equally extraordinary. Each sits on a mobile stalk and is divided into three regions, giving depth perception within a single eye. A central band holds 12 colour receptor types, several of them for ultraviolet, plus receptors for polarised light; some species can detect circularly polarised light, which is not known in any other animal. Yet experiments found they cannot distinguish wavelengths less than about 25 nanometres apart. One explanation is that, rather than the brain comparing signals as ours does, the eye simply registers which receptor fires, like reading a barcode: cruder, but fast, which suits a territorial animal that fights often.
The story is not settled. A 2022 study found that one species could tell strongly and weakly saturated colours from grey, which a pure barcode system would struggle with, so they may use a mix of approaches. Mantis shrimp also glow with fluorescent signals during courtship, at wavelengths their eyes are tuned to, and in some species pairs stay together for up to 20 years. Engineers are studying their clubs and eyes as models for tough materials and new optics.
Source: Wikipedia — Mantis shrimp · Text summarised from Wikipedia (CC BY-SA 4.0)
