A Champion Fighter: The Mosaic Boxer Crab (Lybia tesselata)

Author: Markus Roth

If you love to observe the unique behavior of certain species, you will be ecstatic to watch a boxer crab use its symbiotic anemones to defend itself and gather food.

There has been one critter on top of my wishlist since I took up underwater photography: the mosaic boxer crab (Lybia tesselata). Mosaic boxer crabs are perhaps best known for holding sea anemones with their pincers, creating the impression that they’re wearing gloves or holding pom poms. The anemones' stinging tentacles deter predators, and in turn, the anemones can move about and are provided with food courtesy of their host.

Boxer Crab Anatomy

Boxer crabs are tiny, with many measuring less than an inch across the carapace. They live in coral reefs anywhere in the warm waters of the Indo-Pacific, and this range includes the waters near Mozambique, the Cook Islands, the Philippines, Indonesia, and Samoa. They are also found around Hawaii, Japan, Thailand, and other areas where the water temperature averages about 24°C (75°F) throughout the Pacific and Indian Ocean. L. tesselata are rarely seen, emerging only in dim lighting conditions to feed, and only when they are certain it is safe.

They have two antennae, five pairs of legs, and two chelipeds (or pincers) on the front two legs. A shell primarily made out of chitin, a natural polymer material, covers the crab's body. The chitin forms a thick, protective coat around the carapace of the crab, but around the joints of the legs, a thinner, more yielding layer allows for mobility as well as protection. Since its shell prohibits growth, the crab must periodically molt when the shell becomes too small. They are also capable of regenerating lost limbs through molting. A crab may gradually regrow a leg after molting several times.

Pincers

The boxer crab’s pincers are small and so well adapted to holding anemones that they are rendered nearly useless for defense, yet they are able to take advantage of the anemones for this purpose. The stinging anemones act as a deterrent to predators by giving an extra punch to the crab's defense tactics. If threatened, they may fake a "jab" at their enemy like a boxer. This feint might repel an attacker that has been stung before.

These specialized pincers can be beneficial and have proven useful, as there are a number of different Lybia species that carry anemones. There are disadvantages, however. One is that the pincers are small and aren't strong enough to tear up food. To overcome this, boxer crabs have adapted their second pair of legs for this purpose, tearing food into small pieces and moving it toward their mouth.

Life Near Anemones

Lybia, like many other xanthid crabs, are scavengers and omnivores, usually feeding on detritus. Their unique symbiosis with certain small anemones represents an interesting design. The crab's pincers are too small to act as weapons and are used to grasp anemones instead. Free-living anemones are much larger and possess small tentacle-like projections, hanging from vesicles, lining the column of the anemone. These pseudo-tentacles contain zooxanthellae that carry out the process of photosynthesis in the right lighting conditions and provide nutrients to the anemone.

It is rather odd then to see the smaller variants held by the crab lacking this key source of nutrients. Instead, the anemone's tentacles may be used like mops, gathering debris and food particles from the ground around the crab's hiding place. The crab pulls the particles from the anemone's tentacles with its mouthparts, and the anemones are guaranteed a consistent supply of food. Also, the crab may use its first pair of walking legs to move food toward its mouth. Using its legs is unusual, but this means the crab does not have to put down its anemones to tear apart food.

Breeding

The gender of the crab may be determined by the size of a tail-like projection, which in females is larger and used to carry a large egg mass. Lybia, like most crabs, are oviparous. While the female carries the eggs, they are fertilized and hatch completely externally. After fertilization, the female will carry the eggs on the ventral surface of her body, which will appear as a large red-orange clump. The eggs will remain like this for anywhere from 13 to 15 days. During this time, the female consumes significantly more. This can be dangerous, as such exposure makes a female pom pom crab vulnerable to larger predators.

Finally, about three days before the eggs hatch, the female will retreat back to the safety of her lair, emerging only to begin releasing the hatched larvae. When the female emerges, the bright red-orange color of the eggs will have faded. Waiting for dim lighting conditions, the female crab will ascend higher than ever before on the rock formations. Once she has reached a high peak, she will begin releasing the hatched larvae. The process of releasing the larvae can take several hours, with as many as 250 being released in an hour.

Keep an Eye Out for Boxer Crabs

That might sound like a lot, but these tiny creatures are really rare. Be sure to take a closer look if given the opportunity, as this tiny creature, waving around pom-pom-like anemones and trying to knock you out like Muhammad Ali, is something truly unique to behold.