Friday, July 26, 2013

Hairy Crabs (Phylum Arthropoda: Family Pilumnidae) of Singapore

Hairy crabs (phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, superfamily Pilumnoidea, family Pilumnidae), despite the common name, do not always spot a hairy appearance. Most of the commonly seen species in Singapore have hairy or granulated exoskeletons though.

Hairy Crabs (Family Pilumnidae)
The hair and granules help to break the shape of the animal and trap sediment, and hence allow it to camouflage with the surrounding. Some hairy crabs are found to be poisonous, possibly because they feed on marine algae and sessile cnidarians (such as zoanthids), some of which may contain toxins. Hairy crabs have been observed to actively hunt other small animals to feed on as well.

Like other true crabs, hairy 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, or chelipeds), 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.

Hairy crabs reproduce sexually, and have separate sexes. They mate face-to-face, usually with the male on top and the female below. 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 their abdomen until they hatch.

Some species can be confused with red-eyed crabs (family Eriphiidae), but they can be distinguished by the lack of a large tooth at the base of the upper finger of the pincer. The carapace is usually hexagonal or somewhat ovate. They are also sometimes confused with some of the hairier rubble crabs (family Xanthidae), but they usually lack the distinctive notch found on the frontal margin of the carapace of the latter.

Here are the hairy crabs that I have photographed in Singapore.

Common Hairy Crab (Pilumnus vespertilio)
The Common Hairy Crab (Pilumnus vespertilio) is very commonly seen in the intertidal areas of Singapore in a variety of habitats, ranging from seagrass meadows to coral rubble flats. The long hair covering its exoskeleton traps sediment, allowing it to blend nicely into the surrounding sand. Some people call it the teddy bear crab due to its hairy appearance. Its carapace gets to a maximum width of about 3cm.

Common Hairy Crab (Pilumnus vespertilio) eating cardinalfish
The Common Hairy Crab has been observed eating algae and small fishes. The above photo features one which had caught a small cardinalfish and broke the latter's body into two.

Short-haired Crab (Actumnus setifer)
The Short-haired Crab (Actumnus setifer) lives in burrows it has excavated in living or dead coral fragments. Like a hermit crab, it carries this mobile home from place to place. The crab is covered with short fine hairs (as the common name suggests) and numerous granules. It is reddish in colour, and grows to about 2cm wide.

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