Why would a diver be excited when finding a half-eaten sea star? Even better, what if this sea star was the notorious coral-devouring crown-of-thorns?
There are a number of marine creatures that prey upon echinoderms, less so specifically sea stars, but there is one particularly diminutive denizen that sports a voracious, some may say merciless, appetite for this pentamerous prey: the rare harlequin shrimp, Hymenocera picta. It not only vies for the honour of most glamorous crustacean on the reef, but is equally respected by underwater naturalists for its startling success as a sea-star predator.
Working in pairs, harlequin shrimp have been observed subduing and devouring prey more than 100 times their size – even the gigantic crown-of-thorns sea star, which has very few natural enemies.
Now, there’s a good reason to get excited!
Taxonomically, harlequin shrimp are referred to as a monotypic genus, which means they are sufficiently distinct from other shrimp as to be placed in a genetic grouping on their own.
This fascinating animal occurs throughout the Indian and Pacific Oceans and comes in two colour forms: where the Indian and western Pacific Oceans morph, they show reddish brown spots, while the central and eastern Pacific harlequin shrimp sports deep pink, purple or blue spots over the carapace.
The dramatic, paddle-like claws and ornate sensory antennae, combined with multiple prominent and eye-catching spots, give this snappy shrimp the overall impression of an elaborate flower … or possibly of a noxious sea slug, which may serve to protect it against pesky predators. This form of mimicry, in which a harmless animal imitates an animal known for its toxicity, is known as Batesian mimicry, and is common practice on coral reefs.
Harlequin shrimp are often encountered in lifelong, monogamous breeding pairs and may set up house, and territory, for a number of months in one location – it’s then that divers are given the otherwise scarce opportunity to observe this species. The female lays between 100 and 5000 eggs per season, which she cares for until they hatch.
Together, this husband and wife team scours the reef for unsuspecting sea stars. Upon locating one, they begin snipping at the sticky tube feet that are designed to keep the sea star attached to the substrate, allowing it to move around and, hopefully, to evade predators. But, one by one, these tube feet are cut and the sea star eventually loses its grip. This is the turning point in its fate, for now it can be flipped over, rendering it utterly helpless and in the claws of the hungry harlequins. In some cases, the sea star will engage its last-resort survival tactic of jettisoning one of its arms, which it can later regrow. In most instances, though, the entire body is dragged back to the shrimp’s lair and slowly chewed to death by the happy couple.
It is purported the pair may even feed the sea star, thus keeping it alive during the weeks it takes to finish the meal. Ruthless …
So when you next find that sea star with only four arms, you may pause to wonder just where that missing limb may be.