Invertebrates
Harlequin Shrimp
Hymenocera picta · also called harlequin shrimp, clown shrimp
The harlequin shrimp is a small, spectacularly patterned reef shrimp with a single, unusual obsession: it eats starfish, and nothing else. A bonded pair tracks a sea star by scent, prises it off the reef, flips it helplessly onto its back, and then feeds on it slowly — sometimes for days, sometimes while the star is still alive.
Overview
Harlequin shrimp live on coral reefs across the Indo-Pacific and into the eastern Pacific, at low density. They are only 3–5 cm long, cream-white with large rounded blotches of pink, purple, or blue, and have flattened, petal-like claws and broad paddle antennae.
A one-food diet
The harlequin shrimp is an extreme specialist. It detects sea stars by smell, follows the trail, and cannot switch to other prey. It snips off the tube feet and soft tissue with its modified claws, working inward from an arm tip, and will tackle stars many times its own size — including the coral-killing crown-of-thorns.
Farming the victim
Because a sea star is a huge meal that takes days to eat, the shrimp keeps it fresh: it starts at an arm tip, leaves the disc intact, and may flip, move, or dampen the star so the remaining tissue does not rot before the shrimp finishes.
A pair for life
Harlequin shrimp form long-term monogamous pairs that share a small territory, hunt as a team, and coordinate their moulting and breeding. The male guards the female when she is soft and defenceless after a moult.
Status
The harlequin shrimp has not been assessed. It occurs naturally at low density across a wide range and is collected for the aquarium trade, though its awkward diet limits demand.
Harlequin Shrimp: common questions
Do harlequin shrimp really eat only starfish?
Yes, sea stars are effectively their entire diet, which makes them one of the more extreme dietary specialists among reef animals. A harlequin shrimp locates a sea star by smell, climbs onto it, and uses its modified claws to snip and lever off the tube feet and then the softer tissue. It will take a range of sea star species, from small ones up to those far larger than itself, including the destructive crown-of-thorns. It does not eat algae, detritus, fish, or other invertebrates, so in an aquarium it must be supplied with live or fresh sea stars, which is the main reason it is a specialist keeper's animal rather than a common one.
Why does the harlequin shrimp keep its prey alive?
It is a way of preserving a large meal. A sea star is far bigger than a shrimp and takes days to consume, and dead flesh would quickly decay in warm water. By eating slowly, starting at an arm tip and leaving the central disc and other arms intact, and sometimes by flipping the star, moving it to shelter, or keeping it moist, the shrimp keeps the remaining tissue fresh and even allows the star to keep regenerating a little, effectively farming it. Many sea stars can survive the loss of arms, so the individual being eaten may live for some time before the shrimp reaches the vital parts.
Are harlequin shrimp monogamous?
They form durable pair bonds. A male and female live together on a small territory, recognise each other, and coordinate their activities, including hunting as a team and timing their moult and reproductive cycles. After a female moults she is soft and cannot defend herself or mate for a short period, and the male stays close and guards her. Pairs generally remain together for long periods, and a shrimp that loses its partner will seek a new one. This combination of teamwork and long-term partnership is unusual among shrimp and is part of what makes the species popular despite its awkward diet.
Can harlequin shrimp help control crown-of-thorns starfish?
On a small scale, potentially. Crown-of-thorns starfish periodically undergo population explosions that strip large areas of living coral, and managers are always looking for ways to reduce them. Harlequin shrimp readily attack and kill crown-of-thorns, and trials have placed shrimp on affected reef patches to see whether they can suppress local numbers. The effect is necessarily limited, because the shrimp are small, slow, occur naturally at low density, and can only deal with one star at a time, so they are at best a supplement to other control methods rather than a solution to a full-scale outbreak.