Invertebrates
Peacock Mantis Shrimp
Odontodactylus scyllarus · also called harlequin mantis shrimp, painted mantis shrimp
The peacock mantis shrimp is a small, gaudy crustacean that holds two of the most extreme records in biology at once: the fastest strike of any animal, and the most complicated eyes ever found. It is neither a mantis nor a shrimp, but a stomatopod, an ancient lineage of predatory crustaceans.
Overview
Peacock mantis shrimp live on Indo-Pacific coral reefs, where they dig U-shaped burrows in rubble and sand or occupy crevices, guarding the entrance and darting out to attack passing prey. They are boldly coloured — green body, orange arms, spotted leopard-print carapace, blue-and-red legs — and are among the larger and most conspicuous of the roughly 450 stomatopod species. They are solitary and aggressively territorial.
The punch
A smasher mantis shrimp’s second pair of legs are modified into heavy, folded clubs. Muscles slowly compress a saddle-shaped spring in the arm while a latch holds the club cocked; releasing the latch unloads the spring all at once. The club swings out in under three milliseconds at speeds around 23 m/s, faster than a .22 calibre bullet leaves some pistols.
The strike is so fast that it triggers cavitation — the water vaporises in the club’s wake and the resulting bubbles implode with enough energy to produce heat, light, and a second concussive blow. A peacock mantis shrimp uses this to shatter the shells of snails and crabs, and it will readily crack the glass of an aquarium. The clubs themselves are built from a specialised layered composite that resists the repeated impacts, a structure now studied by materials engineers.
The eyes
Each eye sits on a mobile stalk and moves independently, scanning in every direction. A horizontal band of six specialised rows across the middle of the eye carries the receptors: 12 to 16 distinct photoreceptor types (compared with three in humans), including several tuned to ultraviolet, plus unique cells that detect circularly polarised light, which no other animal is known to see.
Counter-intuitively, tests show mantis shrimp are relatively poor at telling similar colours apart. The favoured explanation is that they trade fine discrimination for speed, recognising a colour “signature” instantly in the eye without slow brain computation — an advantage for an animal that lives or dies by split-second decisions.
Reproduction
Peacock mantis shrimp pair up to breed, and in some populations a pair may share a burrow for years, with the female tending the eggs while the male hunts for both. She carries the egg mass under her body or keeps it in the burrow, cleaning and aerating it until the larvae hatch and join the plankton.
Mantis shrimp and people
Odontodactylus scyllarus is listed as Least Concern. It is caught for food in parts of Asia and collected for the marine aquarium trade, where it is notorious among hobbyists as a hitchhiker in “live rock” that then preys on tank inhabitants and, occasionally, cracks the tank. Its punch and its eyes have both become significant subjects of engineering and vision research.
Peacock Mantis Shrimp: common questions
How hard does a mantis shrimp hit?
A peacock mantis shrimp's club accelerates at up to 10,000 g and strikes at around 23 metres per second, delivering a peak force of more than 1,500 newtons — thousands of times its body weight. That is enough to smash the shell of a snail or crab, crack aquarium glass, and sever a finger. It is the fastest self-powered strike of any animal, driven by a spring-and-latch mechanism in the arm rather than by muscle alone.
What is cavitation and why does it matter here?
The club moves so fast that the water behind it cannot fill the space, forming vapour-filled bubbles that instantly collapse with a flash of light, a snap of sound, and temperatures briefly approaching that of the sun's surface. The result is that the prey is hit twice — once by the club, and a fraction of a millisecond later by the shockwave from the imploding bubble. Even a punch that slightly misses can stun or kill.
Do mantis shrimp really have the best eyes in the world?
They have the most complex known, but "best" is misleading. A human eye has three colour receptor types; the peacock mantis shrimp has 12 to 16, plus receptors for ultraviolet and for both linear and circular polarisation — the only animals known to detect the latter. Surprisingly, experiments suggest they are not especially good at fine colour discrimination. The current thinking is that their eyes do colour analysis in hardware, quickly and cheaply, rather than by brain processing, which suits a fast-moving ambush hunter.
Are there two kinds of mantis shrimp?
Broadly, yes. "Smashers" like the peacock mantis shrimp have club-shaped raptorial arms for hammering open hard shells. "Spearers" have arms lined with barbed spines that they use to impale soft-bodied prey such as fish and worms. Both types are ambush predators that live in burrows or reef crevices.