Mantis Shrimp Have 12 Colour Receptors. They See Worse Than You.

Stomatopods — the punching, dazzlingly coloured crustaceans called mantis shrimp — have what is probably the most elaborate visual system on Earth. Their compound eyes carry 12 distinct photoreceptor classes, including ones tuned to ultraviolet and to the polarisation of light. Humans get by with three.

The intuition that more receptors means finer colour discrimination turns out to be wrong. In 2014, Hanne Thoen and Justin Marshall at the University of Queensland trained Haptosquilla trispinosa to associate specific wavelengths of light with food, then tested whether the shrimp could tell two close wavelengths apart.

A human can tell apart two greens that differ by about 1 to 4 nanometres. The shrimp could not reliably distinguish wavelengths less than 25 nanometres apart. Their colour discrimination was an order of magnitude coarser than ours.

The paper, published in Science, proposed that the eye is not doing the same job ours is. Mammals compare receptor outputs against one another, which is how three receptors yield millions of distinguishable shades. The mantis shrimp may instead read out each receptor channel independently — a kind of fast pattern match in the eye itself, with the colour identified at the periphery without further processing.

The upside is speed. Stomatopods strike prey in under three milliseconds, with appendages accelerating at 10,000 times gravity. A nervous system that classifies a target's colour in the retina rather than the brain may be exactly what a predator on that timescale needs.

The most exotic eye on Earth, in other words, is also a shortcut. It is built to recognise, not to compare.