Friday, July 26, 2013

Box Crabs (Phylum Arthropoda: Family Calappidae) of Singapore

Box crabs (phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, suborder Pleocyemata, infraorder Brachyura, superfamily Calappoidea, family Calappidae) got their common name from their box-like appearances (with some imagination). They can be recognised by the somewhat rounded or oval carapace, the relatively narrow gap between the eye, and the laterally compressed chelipeds (which are usually held close to their mouthparts).

Box crabs (superfamily Calappoidea, family Calappidae)
Box crabs have asymmetrical chelipeds. The right pincer has a curved tooth near the base of the upper finger, which the crab will stick into the opening of a snail, and use it much like a can-opening to cut a channel around the opening. The left pincer, which is more slender with forcep-like fingers, will then be used to extract the exposed snail from its shell.

Like other true crabs, box crabs have a broad carapace, and a very short and flattened abdomen which is usually folded underneath the body. They also have five pairs of "legs" (including the clawed arms), and hence they are placed in the order Decapoda ("deca" means "ten", while "poda" means "feet"). The gills are leaf-like - a distinctive characteristic of decapods from the suborder Pleocyemata. And as with other crustaceans from the class Malacostraca, their body comprises three main parts - a head with five segments, a thorax with eight segments, and an abdomen with six segments. The head is fused to the thorax, forming a cephalothorax. They have a tough exoskeleton strengthened with calcium carbonate, and the carapace covers the gills but not the abdomen.

Spectacled Box Crab (Calappa philargius)
Most box crabs are good burrowers, and are found on sandy to muddy substrates. The bigger ones are often collected for food by the locals but seldom sold in market due to the limited numbers caught.

They reproduce sexually, and have separate sexes. The females can usually be distinguished from the males by having a broader abdomen. This is an adaptation to allow them to carry the eggs under the abdomen until they hatch.

Here are some of the box crabs that can be seen Singapore.

Spectacled Box Crab (Calappa philargius)
The Spectacled Box Crab (Calappa philargius) is usually found in sandy to slightly muddy substrates. It can be recognised by the two dark spots on each chelipad, and the dark patch around each eye (which gives it its common name).

Spectacled Box Crab (Calappa philargius)
It is usually of a light orangy-brown colour, and the back margin of the carapace is lined with spines. Dark markings can be seen on the inner sides of the chelipeds. The Spectacled Box Crab can grow to about 12cm broad (carapace only). It is often collected for food, but is rarely sold in the market.

Reef Box Crab (Calappa hepatica)
The Reef Box Crab (Calappa hepatica) is sometimes seen on sandy and shelly substrates, often in or near reefs or among seagrass. It can be distinguished from the previous species by the numerous small bumps on its carapace and chelipeds. This crab grows to about 8cm wide (carapace only).

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