Friday, July 26, 2013

True Spider Crabs (Phylum Arthropoda: Family Majidae) of Singapore

True spider crabs (phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, superfamily Majoidea, family Majidae) can be distinguished from other spider crabs (families Inachidae and Epialtidae) by having eyes with nearly complete or complete orbits (i.e. eyeholes). A pair of sharp spines can usually be found at the front near the eye, and sometimes the spines may bear smaller spinelets.

Philyra Spider Crab (Micippa philyra)
The carapace is usually triangular or pear-shaped, often with spines by the sides. The entire exoskeleton is usually covered in hooked hairs which trap sediment and other sessile organisms such as ascidians and sponges, allowing it to blend into the surrounding.

Unlike most other crabs, true spider crabs can move forward-backwards instead of just sideways. They are usually scavengers, though some are known to actively hunt small invertebrates as well. Some of the smaller species can be mistaken for sponge crabs (family Dromiidae). Spider crabs, however, attached the sessile organisms to the hooked hairs on their backs, unlike the sponge crabs which carry them using their rear legs.

Like other crabs, true spider 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.

True spider 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.

Here are the true spiders crabs that I have photographed in Singapore.

Philyra Spider Crab (Micippa philyra)
The Philyra Spider Crab (Micippa philyra) is named after a Greek mythological character, the ocean nymph, Philyra. This little spider crab is usually not more than 3cm wide (carapace only), and is often hard to spot due to the ascidians and sponges that it has attached to its body, allowing it to blend nicely into the surrounding.

Philyra Spider Crab (Micippa philyra)
When the crab is turned over, the hook-like legs are revealed. This crab is usually found on coral rubble flats or in the coral reef.

Sea Toad (Schizophrys aspera)
The Sea Toad (Schizophrys aspera) is another master of camouflage that is often overgrown with ascidians. The carapace is somewhat pear-shaped, and the margins are lined with shape spines. It has a pair of horn-like structures extending from the front of the carapace. Two species of Schizophrys are recorded from Singapore, and they can be distinguished by looking at the pair of horns. The above species can be recognised by having only one spinelet branching out near the middle of the horn, while the other species, Schizophrys dama, has horns with two spinelets. The maximum carapace width for both species is about 6cm, and they are usually found on rocky shores, coral rubble flats and coral reefs. These spider crabs are sometimes collected for consumption by locals.

The above photo features an unidentified Schizophrys species. Note that the size of the chelipeds varies even within members of the same species.

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