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Boxer crab

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Boxer crab
Lybia5.jpg
Scientific Classification
Species
  • Lybia caestifera
  • Lybia edmondsoni
  • Lybia plumosa
  • Lybia tessellata

Boxer crab is the common name for any crab belonging to the taxonomic genus Lybia, which are also known as the pom pom crab. They are perhaps best known for their behavior of using their pincers to hold sea anemones, which look like gloves or pom poms. The anemones' stinging tentacles will deter a predator. In turn, the anemones are provided with mobility and food.

Lybia are also capable of regenerating lost limbs through molting. A crab may gradually regrow a leg after molting several times. They are a very reclusive species, usually only emerging in dim lighting conditions to feed themselves. If threatened they may fake a "jab" at their enemy like a boxer. This feint might repel an attacker that has been stung before.

Contents

Anatomy

Boxer crabs are tiny crabs, many less than 1 in across the carapace. They have two antennae, five pairs of legs, and two chelipeds, or pincers, on the front two legs. A shell covers the crab's body that is made primarily out of chitin, a natural polymer material. Around the carapace of the crab, the chitin forms a thick, protective coat. Around the joints of the legs, however, a thinner, more yielding layer is used to allow mobility as well as protection. Since its shell prohibits growth, it must periodically molt when the shell becomes too small. Molting provides an advantage in that the crab may regrow a lost limb. After an encounter with a nosy fish or fellow crab, the crab may lose a leg. Once the crab has molted a few times, the limb will usually have regenerated, though it may need to molt multiple times before the limb is the same size again. [1]

The gender of the crab may be determined by the size of a tail-like projection, which in females is used to carry a large egg mass. While Lybia is a fairly small genus, the species are quite diverse in appearance. Certain species of Lybia, such as L. edmondsoni, and L. tesselata, have colorful mosaic-like patterns on the dorsal surface of its carapace. Others, such as L. caestifera, have spines covering its carapace and seem to blend in with the debris around it. The legs may seem almost translucent with L. caestifera's paler color.[2]

This crab displays a symbiosis with tiny sea anemones. The anemones, usually only about 5 mm in diameter, and about 8 mm in height are held in the crab's pincers. It is for this reason that it is called the pom pom crab - the anemones having the appearance of a set of pom poms. The crab may use the anemones stinging cells (cnidocytes) to defend itself from predators. There are at least three species of anemones used by these crabs, including Triactis producta and Bundeopsis species. Interestingly, both of these species of anemone are found free living and are not dependent upon the crab. When molting, the crab is vulnerable to attack, as it must drop the anemones. It wriggles out of its shell quickly, and picks up the anemones once more before going back into hiding until its new shell hardens. [3]

Reproduction

Lybia, like most crabs are oviparous. While the eggs are carried by the female, 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 3 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 she has 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 much as 250 larvae released in an hour. These larvae, called zoea in their beginning stage of development, are free-swimming and tend to travel in a cloud-like formation with the other zoea of the hatch. The only resemblance these early forms have to the adult are two large eyes, and small antennae. They are characterized by a large horn of the top of their head. [4] The early zoeal stages of the larvae usually feed on drifting phytoplankton and detritus. Stages of morphology within the larvae are advanced through molting, usually once every few days. Lybia larvae will the reach the final, or megalopa, stage at about 17 days old. Megalopae resemble deformed versions of the adult, featuring all limbs including small pincers. After a final molt, they will begin to settle. The young crab will hide and retreat into shelter at the bottom, waiting for its new shell to grow. [5]

Ecology

Map of the Indo Pacific range

Food Acquisition

Lybia, like many other Xanthid crabs, are scavengers and omnivores, usually feeding on detritus. Lybia's unique symbiosis with certain small anemones shows an interesting design. Firstly, the crab's pincers, normally used as weapons, are small enough they can only be used to grasp anemones. Secondly, the smaller variant of Triactis producta, lack zooxanthellae, a key feature in the design of free-living variants. Free living Triactis producta are much larger and possess small, tentacle-like projections, hanging from vesicles, lining the column of the anemone. These zooxanthellae, or pseudo-tentacles, carry out most of the photosynthesizing process in the right lighting conditions. It is rather odd then, to see the smaller variants held by the crab lacking such key features in nourishing itself. [6] 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 it's mouth parts, 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, however this means that the crab will not have to put down its anemones, were it to use its pincers to tear apart food. [7]

Habitat

Boxer crabs live in a wide range of coral reefs anywhere in the warm waters of the Indo-Pacific range. This range includes the waters near Mozambique, the Cook Islands, the Philippines, Indonesia, Samoa. They are also found around Hawaii, Japan, Thailand and other areas where the water temperature averages about 75 °F(24 °C) throughout the Pacific and Indian Ocean. The coral provides many rocky caves and crevices that the crab can hide in. This means that the crabs are rarely seen, emerging only in dim lighting conditions to feed, and only when the crab is certain that it is safe. In fact, some home aquarists keeping this animal have reported to have only seen the crab emerge a few times per year. [8]

Symbiosis by Design

Sybiosis is any long term interaction, or cooperation between organisms of different species. In nature there are many examples of symibotic realtionships, usually because each organism has a use and need for the other. They can supply basic demands, such as Lybia feeding its anemones as well as itself. In turn the anemone provides a defensive measure for the crab that would otherwise be easy prey to many reef dwellers, even other crabs. What makes this relationship fascinating, is how well the crab and anemone are adapted to each other, to the point of giving up some key design features on many other anemones an crabs. It has already been mentioned that the crab's claws, usually a measure of defense themselves, are very small and useless in defense. The anemone seems to lack pseudo-tentacles around its column, which free-living variants use for photosynthesizing. Considering the crab's tendency to avoid light, it would make no sense for the anemone, which would normally rely on light, to live in the dark with the crab. [9] A lack of these key features is evidence of design. Normally, the lack or loss of natural defensive measures like a crab's pincers, or a primary means of nourishing itself, would be actually put both creatures at a serious disadvantage. The fact that the anemones used by the crab are nearly half the size of the free-living variants, and lack structures in the way of the column that the crab grips, shows this creature was designed for a relationship with the crabs of Lybia. Likewise, the crabs abnormally small claws are the perfect size for the anemone. Interestingly, if an anemone is lost, the crab will split the anemone and it will regenerate into a clone, replacing the lost anemone.[10]

Gallery

References

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