Friday, July 26, 2013

Ocypodid Crabs (Phylum Arthropoda: Family Ocypodidae) of Singapore

Ocypodid crabs (phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, superfamily Ocypodoidea, family Ocypodidae) typically have long eyestalks, but are usually not as long or as narrow as the ones found in the previous related family. The carapace may be squarish, rectangular or trapezoidal, and many species have a lobe at the front between the bases of the eyestalks. The chelipeds (i.e. the clawed arms) are usually unequal-sized, especially in the males of some species, whereby one chelilped is much larger than the other.

Ocypodid crabs are usually found on sandy or muddy substrates, hiding in their burrows with trapped pockets of air when the tide is high, and emerge only during low tide to feed. Some species feed mainly on detritus, while other scavenge or actively hunt for prey.

The long eyestalks allow them to see all around (even behind), so that they can quickly burrow into the soft substrate to hide from predators. Their diet comprise mostly of tiny decaying organic materials (or detritus) or other small invertebrates. They often build burrows with a distinctive opening - either rectangular or ovate in shape.

Like other true crabs, ocypodid 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.

Ocypodid 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 ocypodid crabs that I have photographed in Singapore.

Horned Ghost Crab (Ocypode ceratophthalmus)
The Horned Ghost Crab (Ocypode ceratophthalmus) can be recognised by the long eyestalks which extend beyond the eye, appearing like a pair of pointed horns. While it scavenges when there are dead animals around, it is also a fierce predator, hunting other small animals like crabs and baby turtles. It is a very fast runner and often disappears in a flash when disturbed. The fast movements, in addition to its pale coloration and nocturnal habits, give this group of crabs the common name of ghost crabs.

Horned Ghost Crab (Ocypode ceratophthalmus)
The Horned Ghost Crab usually builds its burrow on the high shore, and moves around during low tide. It still needs water to breathe though, and has to go into the water regularly to wet its gills. The burrows are often Y-shaped with one main entrance, while the other end is unopened with about 1cm of sand on top. When disturbed, the crab will bash through the sand at the top of the unopened end and use it as an emergency exit. This species is sometimes collected for consumption.

Horned Ghost Crab (Ocypode ceratophthalmus)
The above is probably a juvenile Horned Ghost Crab, which has not develop the "horns" yet. The carapace has a mottled pattern, possible to allow it to camouflage better with the surrounding sand.

Forceps Fiddler Crab (Uca forcipata)
The fiddler crabs (Uca spp.) are a prominent group of ocypodid crabs commonly found on seashores. This group of crabs are characterised by the males having unequal chelipeds - one cheliped is so much larger than the other, to the extent that they appear as if they only have one huge cheliped. Only the small cheliped is used for feeding - the crab will pick up the soil and putting it into its mouthparts, feed on the layer of detritus coating the sand and mud, and then discard the inedible bits. The constant action of moving its small cheliped up and down with the big cheliped remaining relatively still resembles the actions of playing a fiddle, and hence they were given the common name of fiddler crabs. The huge claw is used mainly for waving to the females (i.e. for courtship purposes) and to ward off competing males.

Female Fiddler Crab (Uca sp.)
The female fiddler crab has two small equal-sized chelipeds. As a result, it can eat faster than the males. This is especially important as the female needs to perform the energy-consuming task of laying eggs.

Forceps Fiddler Crab (Uca forcipata)
The Forceps Fiddler Crab (Uca forcipata) can usually be distinguished from most other species by looking at its larger pincer - there is a gape in the middle between the upper and lower fingers, but the inner edges level out towards the tip, becoming somewhat parallel, giving the pincer a forceps-like appearance. The maximum carapce size of this species is about 3cm. Another species, Uca rosea, has similar pincers, but is slightly smaller (about 2cm wide) and usually have rose-coloured pincers, though it is usually best to check the reproductive parts to confirm the identity. The colour of the Forceps Fiddler Crab is rather variable, ranging from brownish to bluish, depending on its diet. The females of this species are known to build a chimney-like mud structure on top of its burrow, possibly to avoid detection of the burrow by potential predators. This species is usually found more inland towards the back mangrove, often along mud banks of tidal rivers.

Rose Fiddler Crab (Uca rosea)
The above is possibly a Rose Fiddler Crab (Uca rosea), though it is usually best to check the reproductive parts to distinguish it from the previous species. While in some specimens there may be a teeth around the middle of one or both of the upper and lower fingers, the inner edges level out towards the tip, giving it a forceps-like appearance. This species is usually reddish in colour, and the carapace gets to about 2cm wide. It is usually found towards the back mangrove, sometimes along mud banks of tidal rivers.

Porcelain Fiddler Crab (Uca annulipes)
The Porcelain Fiddler Crab (Uca annulipes) can be recognised by the irregular brown to black horizontal streaks on its carapace. In addition, the outer surface of the pincers is relatively smooth. The upper finger of the big cheliped is curved and hook-like towards the tip, while the lower finger looks truncated at the tip. This species is usually seen toward the seaward side of mangroves and on sandy mudflats. The maximum carapace width is about 2cm.

Orange Fiddler Crabs (Uca vocans)
The Orange Fiddler Crabs (Uca vocans) usually have orange to reddish chelipeds. The outer surface of the big cheliped is covered in small granules, and the fingers are compressed, appearing like blades. The colour of the carapace can be quite variable, ranging from dull colours such as grey and green to brighter tones like blue. Sometimes, it may have pales spots on the back of the carapace. This species is usually found on sandy mudflats, such as in sheltered bays or lagoons, or at the seaward side of mangroves. The maximum carapace width is about 2.5cm.

Tetragonal Fiddler Crab (Uca tetragonon)
The Tetragonal Fiddler Crab (Uca tetragonon) is possibly the most marine of all the local fiddler crabs. It is usually found on soft substrates in sheltered areas near coral reefs. This pretty crab has a bright blue carapace with black patches. The big cheliped is relatively smooth and and stout. The maximum carapace width is about 3.5cm.

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  • World Register of Marine Species. 2012. Retrieved Jun 10, 2013, from

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