Friday, July 26, 2013

Inachid Spider Crabs (Phylum Arthropoda: Family Inachidae) of Singapore

Inachid spider crabs (phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, superfamily Majoidea, family Inachidae) often have velcro-like hooked hairs on their exoskeleton, which helps to hold on to sponges, asidians and small bits of debris for camouflage purposes. The hooked hair is present in other spider crabs (superfamily Majoidea) though.

Inachid spider crabs can be distinguished from other spider crabs (families Majidae and Epialtidae) by their visibly long eyestalks and the lack of obvious orbits (i.e. eyeholes). The carapace is triangular or teardrop-shaped, with a short rostrum that may come with a single spine. Studies suggest that they either scavenge or feed on small invertebrates. Unlike most other crabs, these crabs can move forward-backwards instead of just sideways.

Like other true crabs, inachid 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.

Inachid 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.

I have only personally seen one species of inachid spider crab in the wild in Singapore.

Decorator Crab (Camposcia retusa)
The Decorator Crab (Camposcia retusa) is so-named for their habit of attaching various items on their body, ranging from debris such as shell fragments and loose rocks, to sessile organisms, such as sponges, ascidians and algae. It is sometimes also called Velcro Crab for the hook-like hairs on its exoskeleton, which helps to hold on to the items attached to it. The photo above features one which was covered mostly with ascidians and sponges. This species can grow to about 5cm wide (carapace only).

Decorator Crab (Camposcia retusa)
The Decorator Crab above covered itself with some red coraline algae in addition to some asidians.

Decorator Crab (Camposcia retusa)
When sessile organisms are scarce, the Decorator Crab may attach shell fragments and small stones to itself as well.

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