Friday, July 26, 2013

Sesarmid Crabs (Phylum Arthropoda: Family Sesarmidae) of Singapore

Sesarmid crabs (phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, superfamily Grapsidoidea, family Sesarmidae) typically have a squarish carapace that is rough and covered with granules. They are often good climbers, with the tip of their legs pointed and hook-like, allowing them to easily climb up trees or mud mounds.

Sesarmid Crabs (Family Sesarmidae)
Their "face" - the area below the eyes by the two sides of the mouthparts - is densely covered with short, stiff hair (or setae) in a network-like (reticulated) pattern. This pattern distinguishes them from grapsid crabs (family Grapsidae), which have stiff hairs that do not form network-like patterns. The eyes of sesarmid crabs are wide apart, situated at the two front corners of the carapace. Also, the male's abdomen rarely covers the space between the last pair of legs. They are usually found in mangroves, and feed mostly on plant materials. The inner edges of the pincers are usually quite sharp, allowing them to cut up leaves.

Sesarmid crabs are often seen moving and foraging 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, sesarmid 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.

Sesarmid 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 sesarmid crabs that I have photographed in Singapore.

The Violet Tree-climbing Crab (Episesarma versicolor) is often seen up on the tree trunk and branches during high tide, probably to avoid predatory fishes. This species can be distinguished from other tree-climbing crabs by its violet palm and white fingers. It is usually found on the seaward part of mangrove forests. The maximum carapace width of this crab is about 5cm.

The various species of tree-climbing crabs are often also called vinegar crabs, as they are sometimes pickled in vinegar and dark soy sauce, and eaten with porridge by the Teochews. They mostly feed on leaves, but have been observed to scavenge sometimes. Their dull colours allow them to blend into the surrounding to avoid both aerial and terrestrial predators such as kingfishers, monitor lizards and snakes. They are sometimes considered pests in mangrove plantations, as they feed on the young leaves, shoots and seedlings.

The above is probably a Singapore Tree-climbing Crab (Episesarma singaporense), with its red claws and purple legs. There are apparently other species with red claws as well, and hence it can be hard to say for sure without closer examination of the animal physically. It is commonly found towards the landward side of mangrove forests, especially around mud lobster mounds, where it builds its burrow. The maximum carapace width of this crab is about 5cm.

The identity of the above Pink-fingered Tree-climbing Crab (Episesarma sp.) is in dispute recently, awaiting a revision. This species is more commonly found on the seaward side of mangrove forests, and is easily recognised by a whitish upper finger, and a pinkish red lower finger. The maximum carapace width of this crab is about 5cm.

The Mangrove Tree-dwelling Crab (Selatium brockii) usually hides in crevices and under loose bark in the day, and emerges at night to feed on algae growing on the tree. It also feeds on leaves and small invertebrates sometimes. This species can be recognised by its greenish to brownish carapace, often with dark bands on the carapace and legs. The eyes often have a greenish tint. The carapace gets to about 2.5cm wide.

The Javan Mangrove Crab (Nanosesarma batavicum) is a small crab with a rectangular carapace (maximum width about 1cm). It is often found on trees, and can be recognised by the series of small ridges lining the upperside of the top finger of its purplish pincers. It also has numerous strong teeth and spines on its legs, which help to "hook" onto the tree trunk, so that it can stay on the tree and move around effectively.

Iridescent Crab (Perisesarma indiarum)
The Iridescent Crab (Perisesarma indiarum) is sometimes seen in the mangrove, and can be recognised by its electric blue body, legs, face and mouthparts (the mustache like pattern below the face band). Research shows that Perisesarma crabs can see colours, and the colours on their faces are species specific. Scientists believe that these colours help the crabs to recognise rivals and potential mates. Perisesarma crabs largely feed on fallen leaves, and as they feed, small bits and pieces that are not consumed drop onto the ground, which are in turn eaten by other smaller herbivores. The carapace of this species can get to about 4cm wide.

Face-banded Crab (Perisesarma eumolpe)
The Face-banded Crab (Perisesarma eumolpe) can be distinguished from the previous species by having black mouthparts instead of the blue ones found on the latter. The carapace can grow to about 4cm wide.

Dussumier's Mangrove Crab (Perisesarma dussumieri)
The Dussumier's Mangrove Crab (Perisesarma dussumieri) is usually found in the back mangrove, especially in areas with freshwater influence. Due to this niche preference, it is generally uncommon in Singapore, since most of such habitats have been destroyed. Unlike the previous Perisesarma species, the band on its face is a dull cream colour. The tips of its legs are orange in colour, and there is a broad black band just above the orange tip. The carapace can grow to about 4cm wide.

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Andrea V. said...

Very interesting reportage!

Ron Yeo said...