Friday, July 26, 2013

Sand Bubbler & Soldier Crabs (Phylum Arthropoda: Family Dotillidae) of Singapore

Dotillid crabs (phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, superfamily Ocypodoidea, Family Dotillidae) are typically small crabs with rounded carapaces.

Dotillid Crabs (Family Dotillidae)
Their eyestalks are elongate, giving them a good view of the surrounding, so that they can quickly burrow into the soft substrate to hide from predators. They are usually found on sandy or muddy upper shores, hiding in their burrows during high tide, and emerge only during low tide to feed. Dotillid crabs are typical deposit feeders, and feed on the detritus (i.e. tiny decaying organic matter) deposited on the substrate.

Sand Bubbler Crab (Scopimera intermedia)
They will put the sand into their mouthparts to extract the layer of detritus covering the sand grains, and then push the cleansed sand grains out of the mouthparts to be gathered into a sand ball.

Sand Bubbler Crab (Scopimera intermedia)
Eventually when the sand ball gets too big, the crab will toss the sand ball aside.

Sand balls made by Sand Bubbler Crab (Scopimera intermedia)
The numerous tiny sand balls found on sandy beaches are usually made by dotillid crabs. The bigger sand balls are the sand excavated by the crab while building its burrow, while the smaller ones are the discarded sand balls from its feeding activities.

Like other true crabs, dotillid 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.

Dotillid 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 dotillid crabs that I have photographed in Singapore.

Soldier Crabs (Dotilla myctiroides)
The Soldier Crabs (Dotilla myctiroides) are more easily recognised by their behaviour - they usually move around in large groups like a troop of soldiers during low tide. This species often inhibits muddier sand banks. They have a round body with relatively long legs and chelipeds. The carapace grows to a maximum width of about 1.5cm.

Wichman's Soldier Crab (Dotilla wichmani)
The Wichman's Soldier Crab (Dotilla wichmani) prefers sandier areas, and does not wander far from its burrow. It is smaller than the previous species, with a maximum carapace width of about 1cm. The two species are generally hard to tell apart in the field other than by observing their behaviour.

Sand Bubbler Crab (Scopimera intermedia)
The Sand Bubbler Crabs (Scopimera spp.) are very tiny, and the carapace seldom exceeds 1cm wide. Recent studies suggest that there is probably only one species in Singapore - Scopimera intermedia, as featured in the above photo. This species has long and slender fingers, unlike others which usually have an obvious tooth in on the inner edge of the upper finger.

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