Friday, July 26, 2013

Grapsid Crabs (Phylum Arthropoda: Family Grapsidae) of Singapore

Grapsid crabs (phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, superfamily Grapsidoidea, family Grapsidae) have squarish, rectangular, trapezoidal or circular carapaces. They can be found in mangroves or rocky shores, feeding mostly on algae and plant materials, though they sometimes also hunt small shore animals. The sharply pointed tips of their legs allow them to climb up trees and over rocks effectively.

Grapsid Crabs (Family Grapsidae)
Grapsid crabs generally prefer to move around and forage in atmospheric air instead of in water. In order to breathe on land, they have to keep their gills wet by holding water in their gill chamber, and recirculate the water by pumping it across the hairs on their face and back to the gill chamber. As the water goes across the hairs, gas exchanges take place and the water gets re-oxygenated.

Like other true crabs, grapsid 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.

Grapsid 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.

Grapsid crabs can be confused with plagusiids (family Plagusiidae) and sesarmid ( family Sesarmidae) crabs. They can be distinguished from plagusiids by their mouthparts - the gap between the third pair of feeding appendages (or maxillipeds) is not distinctly rhomboidal, unlike those of the plagusiids. In addition, the third to fifth abdominal segments of grapsid crabs are freely movable for most species, while those of the plagusiids are fused and immovable. These features are not easy to observe in the wild, but fortunately the plagusiid species in Singapore are generally covered in small bumps, but not the local grapsid species.

The sesarmid crabs can be distinguished from the grapsid crabs by the network-like patterns form by the hair on their faces. While the grapsid crabs have the short and stiff hairs on their faces as well, they do not form the network-like patterns.

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

Sally-light-foot (Grapsus albolineatus)
The Sally-light-foot (Grapsus albolineatus) is usually seen on rocky shores and sea walls. This very agile climber often forages just above the waves. Its long legs with pointed tips allow them to move quickly from rock to rock. This species can be recognised by its round carapace that is marked with pale stripes. The maximum carapace width is about 5cm. It is sometimes collected by hand or nets for consumption.

Purple Climber Crab (Metopograpsus sp.)
The Purple Climber Crabs (Metopograpsus spp.) are a group of purple or purplish crabs that are commonly found on rocks and manmade structures on the seashore. They come in different patterns and different shades of purple, but it is generally hard to identify the species just by looking at the colour and pattern. They feed mostly on the algae growing on the rocks, scraping them off with their pincers. Occasionally, they also feed on other small animals on the rocky shore, such as sea slaters. The below are some examples of variations of Purple Climber Crabs, which may or may not be of the same species.

Purple Climber Crab (Metopograpsus sp.)
The above Purple Climber Crab has a black carapace marked with whitish patches.

Purple Climber Crab (Metopograpsus sp.)
This Purple Climber Crab is of a lighter purple, and its carapace is marked with a maze-like pattern.

Purple Climber Crab (Metopograpsus sp.)
For the above Purple Climber Crab, only the claws are slightly purplish, and the back of the animals is marked with patchy patterns.

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