Friday, July 26, 2013

Varunid Crabs (Phylum Arthropoda: Family Varunidae) of Singapore

Varunid crabs (phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, superfamily Grapsidoidea, family Varunidae) typically have smooth, squarish or trapezoidal carapaces with the front margin lacking any lobes or teeth. The abdomen of the males rarely covers the whole space between the last pair of legs, and all abdominal segments are movable. These features are hard to examine on the field, and hence it is probably easier to memorise the few commonly seen species instead.

Varunid Crabs (Family Varunidae)
Most varunid crabs live in the mangrove or mudflats, though some may also be found on drift woods and flotsam out in the sea. Some are known to be able to survive in both fresh and salt water. Their legs are broad and often lined with hair, allowing them to swim for short distances. Varunid crabs are omnivorous, and some have been observe to scavenge.

Like other true crabs, varunid 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.

Varunid 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 varunid crabs that I have photographed in Singapore.

Paddler Crab (Varuna litterata or Varuna yui)
The Paddler Crabs (Varuna litterata &Varuna yui) have a smooth, squarish carapace with round corners and the legs are flattened and somewhat paddle-like. The colour can be light brown to brownish grey, sometimes with darker patterns on the carapace. Two species are recorded from Singapore, Varuna litterata and Varuna yui. These two species are very similar, and can only be distinguished by differences in the reproductive parts of the males. Generally, Varuna litterata prefers areas facing oceanic waters, while Varuna yui prefers the continental shelf waters. Both species can be found far away from the sea in freshwater, in estuarine areas, or even out in the sea on drift wood and flotsam. They are sometimes collected for consumption by villagers, either pickled in vinegar and/or salt or deep fried.

Orange Signaller Crab (Metaplax elegans)
The Orange Signaller Crab (Metaplax elegans) can be recognised by their distinctive orange claws. The claws of males are much larger than those of the females, and they wave their huge claws to attract the attention of the latter. The carapace is somewhat trapezoidal, with the front end wider than the back end. The maximum carapace width of this species is about 2cm. They are often found in large groups on mudflats and in mangrove forests during low tide. Their diet comprises mostly of worms. The above photo features a male.

Orange Signaller Crab (Metaplax elegans)
The female Orange Signaller Crab has much smaller claws.

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