Put ‘em up!

Giovanna Fasanelli | VOLUME 30, ISSUE 2

With its stinging fists ready to strike, the pugilistic boxer crab has earned its name.

He may only qualify for the flyweight division, but he sure packs a stinging punch! This gorgeous orange, black, and white crustacean is affectionately known by divers as the ‘boxer’ or ‘pom-pom’ crab, though marine biologists refer to it as Lybia tessellata, hailing from the crustacean family Xanthidae.

Attractive appearance aside, this feisty marine denizen is cherished by underwater naturalists for its curious habit of brandishing a fistful of stinging anemone polyps in each claw. These polyps are laden with microscopic stinging cells known as cnidocytes that, when touched, fire immobilising, venom-filled barbs into the victim’s skin. A defensive tactic worthy of James Bond’s weapon specialist M!

When your maximum attainable size is a mere two-and-a-half centimetres, protecting yourself from enemies requires ingenuity – which this colourful crab has in spades.

Running the gauntlet through enemy territory, both handguns pointed menacingly ahead, the boxer crab moves between feeding grounds with weapons outstretched, ensuring all can see just what they’d be in for should they make any untoward moves. When threatened by a predator, this pugnacious reef fighter immediately begins waving its toxic white gloves around in an almost taunting fashion. Watch out for that sneaky upper-cut!

Should two well-matched boxers encounter one another, expect to witness spectacular outbursts of ritualistic glove-waving, at which point the less confident opponent may back away or face becoming embroiled in direct combat. Curiously, this does not involve their anemone-filled fists, but rather tackling one another with their hindmost legs. In fact, the claws, or chelae, are held to the rear, safely out of the way. Reasons for keeping their pet anemones away from the contact zone during disputes are not well understood, though it has been suggested that the cost of damaging or losing such valuable accessories may be considered too high.

One common anemone species utilised by the Lybia crabs is Triactis producta, which may also be found living a free and independent life on the seabed. It, too, engages in a symbiotic relationship of its own: by day, the tiny colony, which grows to a maximum size of 1cm at the base, relies upon the photosynthetic by-products of the zooxanthellae algae embedded within its tissues. This is the same type of algae that lives in association with many corals and clams. By night, the anemone extends specialised feeding tentacles into the water column to ensnare passing microscopic prey.

Boxer crabs have evolved specialised claws with which to delicately hold the basal column of the anemone colony. When the crab moults, it places its ‘boxing gloves’ down in a safe place so that they can be picked up again once the new exoskeleton has formed. Crabs that have lost one of their gloves have been witnessed tearing their remaining anemone colony apart which, thanks to asexual budding abilities of the anemone, stimulates each half to regenerate into complete colonies.

Sporting stinging polyp colonies as hands presents a feeding problem for the crab, as its chelae are unavailable for the procurement of food particles. But rather than being a handicap, this crafty crustacean instead uses the sticky polyps to mop up its meal of microscopic morsels, which it cleans off with its mouthparts.

These amazing creatures can be found in shallow tropical seas around the Indo-Pacific. Unbelievably, their bright carapaces blend in extremely well against sandy and gravel substrates, so keep an extra watchful eye out next time you’re snorkeling in the boxing ring!

Creature Features