Friday, July 26, 2013

True Crabs (Phylum Arthropoda: Infraorder Brachyura) of Singapore

True crabs (phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, suborder Pleocyemata, infraorder Brachyura) are a group of crustaceans that most people in the region are familiar with, since many species are widely consumed in the region.

Crabs (Infraorder Brachyura)
They are of the order Decapoda, and like other members of this order, they have five pairs of "legs", including the clawed arms ("deca" means "ten", while "poda" means "feet"). They also have leaf-like gills, 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 with the thorax to form a cephalothorax. They have a tough exoskeleton strengthened with calcium carbonate, and the carapace covers the gills but not the abdomen.

They can be distinguished from other similar-looking decapods by having a broad carapace, a very short and flattened abdomen which is usually folded underneath the body, a pair of clawed arms, and four pairs of walking legs. Sometimes, one or more pairs of legs may be modified for swimming or other purposes. Their antennae are relatively short, and their eyes are on stalks.

They have separate sexes, and reproduce sexually. 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 the abdomen until they hatch.

Here are some of the families of marine crabs that I have photographed in local waters. I will elaborate on the freshwater crabs in a separate page.

A) Elbow Crabs (Family Parthenopidae)

Elbow Crabs (Family Parthenopidae)
Elbow crabs (superfamily Parthenopoidea, family Parthenopidae) typically have long to very long chelipeds (clawed arms) that they usually cannot hide completely beneath the carapace. These long chelipeds gave them their common name, as the joint in the middle resembles an elbow. Despite the chelipeds being so long, the fingers (or pincers) at the tip are usually unable to reach the back of the carapace. Many species have fine hair on the exoskeleton, which traps sediment and help the animal to camouflage. The surface of the exoskeleton is often uneven with ridges, bumps and spines. Studies suggest that they have a omnivorous diet, and possibly hunt for small invertebrates by ambush using their long chelipeds, with aid from their excellent camouflage.
B) Box Crabs (Family Calappidae)

Box Crabs (Family Calappidae)
Box crabs (superfamily Calappoidea, family Calappidae) got their common name from their box-like appearances (with some imagination). They can be recognised by the somewhat rounded or oval carapace, the relatively narrow gap between the eye, and the laterally compressed chelipeds (which are usually held close to their mouthparts). The chelipeds are asymmetrical. The right pincer has a curved tooth near the base of the upper finger, which the crab will stick it into the opening of a snail, and use it much like a can-opening to cut a channel through the shell. The crab will then use the left pincer, which is more slender and has forcep-like fingers, to extract the exposed snail from its shell. Most species are good burrowers, and are found on sandy substrates.
C) Moon Crabs (Family Matutidae)

Moon crabs (superfamily Calappoidea, family Matutidae) are mostly nocturnal, and got their common name from their typically pale round carapace. A long spine is usually present on the left and right sides of the carapace, and all their legs are distinctly flattened laterally into paddle-like structures. They are good swimmers and burrowers, and are usually found on sandy substrates. Most moon crabs feed on small invertebrates or scavenges.
D) Swimming Crabs (Family Portunidae)

Swimming crabs (superfamily Portunoidea, family Portunidae) are so-named as the last pair of legs are paddle-like, making them good swimmers. The carapaces of most species are somewhat hexagonal, and the margin of the front half is often lined with sharp teeth. The various species live in a variety of habitats, from soft substrates like mudflats and sandflats to harder bottoms like coral reefs and rubble. The diet varies between the species as well, ranging from slow-moving prey such as snails and annelids to fast-moving ones such as fish and shrimps.
E) Rubble Crabs (Family Xanthidae)

Rubble crabs (superfamily Xanthoidea, family Xanthidae) are mostly found living among coral rubble, in coral reefs, and rocky shores. They come in a variety of shapes, and the carapace can be hexagonal to rounded. Some species may have numerous sharp spines, while others may have blunt lobes or are unarmed. The fingers of the pincers of some species may be spoon-tipped to aid with the scraping of algae from rocks. For most species, the frontal margin between the eyes usually have a notch in the middle. Many species feed on algae, and as a result a number of them are known to be poisonous, due to the toxin in the algae they feed on. Some of the most poisonous crabs in the region are of this family.
F) Stone Crabs (Family Menippidae)

Stone crabs (superfamily Eriphioidea, family Menippidae) are commonly found in rocky areas or areas with coral rubble. Their carapace is usually somewhat hexagonal to ovate, and the margin near the front is usually marked with crested teeth (i.e. the top surface of the teeth is not flat, but ridge-like, somewhat like the corner of a roof). They typically have large and powerful asymmetrical chelipeds, and a large molar-like tooth can be found near the base of the top finger, allowing them to crush and break shells of molluscan prey.
G) Forcep Crabs (Family Oziidae)

Forceps crabs (superfamily Eriphioidea, family Oziidae) typically have asymmetrical chelipeds, with the larger one having a gently curved tooth at the base of the top finger, and the smaller cheliped with slender fingers, much like a pair of forceps. Like the box crabs, the curved tooth is for cutting into the opening of snails, while the forcep-like pincers extract the flesh from the shell. Some species may develop a molariform tooth as the crab matures, which allows the crab to crush snail shells as well. The carapace may be hexagonal or ovate, the front half of the margin is lined with numerous teeth. Forceps crabs are usually found hiding among rocks or roots.
H) Red-eyed Crabs (Family Eriphiidae)

Red-eyed crabs (superfamily Eriphioidea, family Eriphiidae) typically have red eyes, though this is not a useful distinguishing feature since may other crabs share this characteristic. They have hexagonal to ovate carapace, and the chelipeds are usually large and powerful. Both carapace and chelipeds are usually covered in fine, spiky bumps, and the front margin of the carapace is usually marked with lobes or spines. They typically have large and powerful asymmetrical chelipeds, and a large molar-like tooth can be found near the base of the top finger, allowing them to crush and break shells of molluscan prey. Studies suggest that some species include algae as part of their diet as well.
I) Hairy Crabs (Family Pilumnidae)

Hairy crabs (superfamily Pilumnoidea, family Pilumnidae), despite the common name, do not always spot a hairy appearance. Most of the commonly seen species in Singapore have hairy or granulated exoskeletons though. The hair helps to break the shape of the animal and trap sediment, and hence allows it to camouflage with the surrounding. Some species can be confused with those from the previous family, but they can be distinguished by the lack of a large tooth at the base of the upper finger of the pincer. The carapace is usually hexagonal or somewhat ovate. Some hairy crabs are found to be poisonous, possibly due to their diet of marine algae and sessile cnidarians (such as zoanthids) which may harbour toxins.
J) Acropora Crabs (Family Tetraliidae)

Acropora crabs (superfamily Trapezioidea, family Tetraliidae) are often found living in Acropora corals (Acropora spp.). They were previously placed in the family Trapeziidae, but were recently placed in their own family. Acropora Crabs typically have a smooth trapezoidal to somewhat ovate carapace, usually with the back end narrower than the front end. One cheliped is conspicuously bigger than the other. The front margin of the carapace is straight but serrated (those of trapeziid crabs have four lobes or spines). They can be most readily distinguished from trapeziid crabs by having six movable abdominal segments instead of the third to fifth segments being fused. In addition, there is usually an obvious band of a different colour from the rest of the body running across the face of the crab. These crabs feed on the mucus produced by the corals, and in return defend the host with their powerful claws from potential coral predators. I have only seen one species in Singapore so far - the Zorro Crab (Tetralia nigrolineata) which got its common name from the black band on its head.

K) Trapeziid Crabs (Family Trapeziidae)

Trapeziid crabs (superfamily Trapezioidea, family Trapeziidae) are known to live several genera of hard corals. The chelipeds are usually equal or somewhat equal in sizes. They can be distinguished from the previous group by having the third to fifth segments of the abdomen being fused. The carapace may be trapezoidal, hexagonal, octogonal or somewhat ovate, with the surface being smooth, polished, or finely granular, while the front margin is marked with four lobes or broad teeth. The outer surfaces of the pincers are usually smooth. The species seen in Singapore are mostly found on the hard coral, Pocillopora damicornis. Apologies for the poor picture feature above, as I have yet to get good photos of this group. It features a Red Coral Crab (Trapezia cymodoce).

L) Sesarmid Crabs (Family Sesarmidae)

Sesarmid crabs (superfamily Grapsidoidea, family Sesarmidae) typically have a squarish carapace that is rough and covered with granules. They are often good climbers, with the tips of their legs pointed and hook-like, allowing them to easily climb up trees or mud mounds. 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. The eyes are wide apart at the two front corners of the carapace. Also, the male's abdomen rarely cover the space between the last pair of legs. They are mostly 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.
M) Grapsid Crabs (Family Grapsidae)

Grapsid crabs (superfamily Grapsidoidea, family Grapsidae) have squarish, rectangular, trapezoidal or circular carapaces. Unlike the previous family, the short hair on the face of grapsid crabs do not form network-like patterns. In addition, the abdomen of the males generally fill up the whole space between the last pair of legs. They can be found in mangroves or rocky shores, feeding on both plant materials and also small shore animals. The sharply pointed tips of their legs allow them to climb up trees and over rocks effectively. Grapsid crabs can be confused with members of the next family, the plagusiids. They can be distinguished 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.
N) Plagusiid Crabs (Family Plagusiidae)

Plagusiid crabs (superfamily Grapsidoidea, family Plagusiidae) are easily confused with species of the previous family, except that the third to sixth segments of the abdomen are fused and immovable, while the third to fifth abdominal segments of grapsid crabs are freely movable for most species. In addition, the mouthparts are different - the gap between the third pair of feeding appendages (or maxillipeds) is not distinctly rhomboidal, unlike those of the grapsids. These differences can be hard to examine in the field, but fortunately, this family is only represented by one genus in Singapore, Plagusia, which can be easily recognised by the numerous tiny bumps covering the exoskeleton. They are usually found on rocky shores, and the relatively long legs allow them to move from rock to rock quickly. They feed mostly on seaweed, and occasionally small animals. They are commonly called Rafting Crabs as they are often found living on flotsam and even pelagic marine animals.
O) Varunid Crabs (Family Varunidae)

Varunid crabs (superfamily Grapsidoidea, family Varunidae) typically have smooth, squarish 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. Most species 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.
P) Sentinel Crabs (Family Macrophthalmidae)

Sentinel crabs (superfamily Ocypodoidea, family Macrophthalmidae) can usually be recognised by their extremely long and thin eyestalks. The carapace is usually quadrilateral, broader than long, and lined with teeth by the sides. The chelipeds are somewhat equal in size. They are usually found on soft substrates, such as mud and sand. The long eyestalks allow them to see all around (even behind), so that they can quickly burrow into the soft substrate to hide from predator. They feed on tiny decaying organic materials (or detritus) and small invertebrates. These crabs often build burrows with a distinctive opening - either rectangular or ovate in shape.
Q) Ocypodid Crabs (Family Ocypodidae)

Ocypodid crabs (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 are usually unequal-sized, especially in the males of some species, whereby one cheliped 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.
R) Silt Crabs (Family Camptandriidae)

Silt crabs (superfamily Ocypodoidea, family Camptandriidae) typically have rounded to trapezoidal carapace and long eye stalks. The margins of the carapace are seldom broken by teeth or spines. The chelipeds are either equal or somewhat equal in shape and sizes, though the ones in males tend to be more swollen. Their first and last pair of walking legs tend to be shorter than the second and third pair. They are usually found on the mud, feeding on tiny decaying organic matter on the surface. I have only photographed one species in Singapore, the Red Silt Crab (Paracleistostoma depressum). This small crab with a maximum carapace width of about 1.5cm has bright red claws. The trapezoidal carapace has a pale outline when viewed from the top.

S) Dotillid Crabs (Family Dotillidae)

Dotillid crabs (superfamily Ocypodoidea, Family Dotillidae) are typically small crabs with rounded carapaces. The eyestalks are elongate, giving them a good view of the surrounding. They include the Sand Bubbler and Soldier Crabs that 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, meaning they feed on the detritus deposited on the substrate. They will put the sand into their mouthparts to extract the layer of detritus covering the sand grains, then push the cleansed sand grains out of the mouthparts where they will gather into a sand ball, and toss the sand ball aside when it gets too big. The numerous tiny sand balls found on sandy beaches are usually made by them.
T) Porter Crabs (Family Dorippidae)

Porter crabs (superfamily Dorippoidea, family Dorippidae) are often not easy to spot on the seashore, as they usually camouflage themselves by carrying a leaf, broken shell or other small items on their back. Sometimes, they can also be seen swimming upside-down near the water surface, with the item they are carrying facing downwards, appearing just like a floating debris when viewed from below! Without the item they are carrying, they can be recognised by their last two pairs of legs, which are oriented upwards for carrying things instead of downwards for walking. The carapace is usually trapezoidal, with the front margin narrower than the rear margin, and longer than broad. They are mostly scavengers.
U) Epialtid Spider Crabs (Family Epialtidae)

Epialtid spider crabs (superfamily Majoidea, family Epialtidae) typically have a triangular, longer than broad, flattish carapace, and the entire exoskeleton is mostly covered in hooked hairs (which helps to cling on to encrusting organisms or collect sediment for camouflage). The rostrum (nose-like tip at the front) is relatively huge and beak-like, and may be simple or two-spined. Epialtid spider crabs can usually be distinguished from other spider crabs (superfamily Majoidea) found on our shores by their very shot eyestalks and the lack of eyeholes (or orbits). However, this is not a very useful feature in the field as they are usually overgrown with algae or other encrusting organisms. The epialtid spider crabs that occur in Singapore waters are usually very small (not more than one or two cm long), and hence, the small size, relatively huge rostrum and elongate triangular shape are often more useful. These crabs feed mainly on marine algae or other plant materials.
V) Inachid Spider Crabs (Family Inachidae)

Inachid spider crabs (superfamily Majoidea, family Inachidae) often have velcro-like hooked hairs on their exoskeleton, which helps to hold on to sponges, asidians and small bits of debris for camouflage purposes. The hooked hair is present in other spider crabs (superfamily Majoidea) though. They can be distinguished from other spider crabs by their visibly long eyestalks and the lack of obvious orbits (i.e. eyeholes). The carapace is triangular or teardrop-shaped, with a short rostrum that may come with a single spine. Studies suggest that they either scavenge or feed on small invertebrates. Unlike most other crabs, inachid spider crabs can move forward-backwards instead of just sideways.
W) True Spider Crabs (Family Majidae)

True spider crabs (superfamily Majoidea, family Majidae) can be distinguished from other spider crabs by having eyes with nearly complete or complete orbits. A pair of sharp spines can usually be found at the front near the eye, and sometimes the spines may bear smaller spinelets. The carapace is usually triangular or pear-shaped, often with spines by the sides. The entire exoskeleton is usually covered in hooked hairs which trap sediment and other sessile organisms such as ascidians and sponges, allowing it to blend into the surrounding. Unlike most other crabs, true spider crabs can move forward-backwards instead of just sideways. True spider crabs are usually scavengers, though some are known to actively hunt small invertebrates as well. Some of the smaller species can be mistaken for sponge crabs (see below). Spider crabs, however, attached the sessile organisms to the hooked hairs on their backs, unlike the sponge crabs which carry them using their rear legs.
X) Sponge Crabs (Family Dromiidae)

Sponge crabs (superfamily Dromioidea, family Dromiidae) are so-named for their habit of carrying a piece of living sponge or colonial ascidian for camouflage purposes. Some sponge crabs may be mistaken for spider crabs, but their method of carrying the ascidians and sponges are different. Sponge crabs carry the ascidians or sponges with their last two pairs of legs. These legs are inserted obliquely on the circular to hexagonal carapace, directed upwards. Hence, sponge crabs only use two pair of legs for walking. Spider crabs, on the other hand, stick the ascidians or sponges to the hooked hairs on their exoskeletons, and use all four pairs of legs for walking. They are mostly scavengers.
Y) Pebble Crabs (Family Leucosiidae)

Pebble crabs (superfamily Leucosioidea, family Leucosiidae) usually have circular, oval or pentagonal carapaces. Many species have a convex carapace, and appear much like round pebbles, and hence the common name. The eyes and antennae are very tiny and hardly noticeable, and the chelipeds are symmetrical. Most pebble crabs are adapted for burrowing, and are found in soft substrates like sand or mud. Many species feed on tiny invertebrates by probing the top layer of the sediment and capturing the ones disturbed. So far, I have not been able to identify the pebble crabs that I have seen yet. Apart from the one above, I have also seen another one with a more reddish brown tint.

Z) Masked Crabs (Family Corystidae)

Masked crabs (superfamily Corystoidea, family Corystidae) can usually be recognised by their long and hairy antennae, which can be as long as or longer than their body. The carapace is ovate or somewhat circular, usually longer than broad. Most species are good burrowers, and are found on soft substrates such as sand or mud. They are called masked crabs due to the shape of their carapaces and the patterns on them, which somewhat resemble human faces. They usually feed on small invertebrates in the sediment. So far I have only seen one species in Singapore, and it was tentatively identified as the Masked Burrowing Crab (Gomeza bicornis).

  • Burnie, D. 2001. Animal. London: Dorling Kindersley. 624 pp.
  • Carpenter, K. E. & V. H. Niem (eds), 1998-2001. FAO species identification guide for fishery purposes. The living marine resources of the Western Central Pacific. Volumes 1 to 6. FAO, Rome. pp. 1-4218.
  • Castro, P., P. K L. Ng & S. T. Ahyong. 2004. Phylogeny and systematics of the Trapeziidae Miers, 1886 (Crustacea: Brachyura), with the description of a new family. Zootaxa 643:1–70.
  • De Grave, S., N. D. Pentcheff , S. T. Ahyong, T.-Y. Chan, K. A. Crandall, P. C. Dworschak, D. L. Felder, R. M. Feldmann, C. H. J. M. Fransen, L. Y. D. Goulding, R. Lemaitre, M. E. Y. Low, J. W. Martin, P. K. L. Ng, C. E. Schweitzer, S. H. Tan, D. Tshudy & R. Wetzer. 2009. A classification of living and fossil genera of decapod crustaceans. The Raffles Bulletin of Zoology, supplement 21, pp. 1-109.
  • Debelius, H. 1999. Crustacea - Guide of the world. IKAN, Frankfurt. 321pp.
  • ETI BioInformatics. 2012. Marine species identification portal. Retrieved Jul 8, 2013, from
  • Mendoza, J. C. E. & P. K. L. Ng, 2007. Macrophthalmus (Euplax) H. Milne Edwards, 1852, a valid subgenus of ocypodoid crab (Decapoda: Brachyura: Macrophthalmidae), with description of a new species from the Philippines. Journal of Crustacean Biology, 27(4): 670–680.
  • Ng, P. K. L., L. K. Wang & K. K. P. Lim (eds.). 2008. Private lives: An exposé of Singapore’s mangroves. Raffles Museum of Biodiversity Research, Department of Biological Sciences, National University of Singapore. 240 pp.
  • Ng, P. K. L. & N. Sivasothi (eds.), 1999. A guide to the mangroves of Singapore II: animal diversity. Singapore Science Centre. 168p.
  • Ng, P. K. L., S. S. L. Lim, L. K. Wang & L. W. H. Tan. 2007. Private lives: An exposé of Singapore's shores. Raffles Museum of Biodiversity Research, Department of Biological Sciences, National University of Singapore. 212 pp.
  • Ng, P. K .L., R. T. Corlett & H.T.W. Tan (eds.). 2011. Singapore biodiversity: An encyclopedia of the natural environment and sustainable development. Singapore: Editions Didier Millet. 552 pp.
  • Štev cic , Z., 1983. Revision of the Calappidae. Memoirs of the Australian Museum, 18: 165–172.
  • Tan, L. W. H. & P. K. L. Ng. 1988. A Guide to Seashore Life. Singapore Science Centre. Singapore. 160 pp.
  • Tan, S. H. 2004. A Systematic Revision of the Parthenopidae(Crustacea: Decapoda: Brachyura). Unpublished PhD Thesis, National University of Singapore. 729 pp.
  • Vasconcelos M. A., T. C. Mendes, W. L. S. Fortes& R. C. Pereira. 2009. Feeding and decoration preferences of the Epialtidae Crab Acanthonix scutiforms. Brazilian Journal of Oceanography, 57: 137-143.
  • World Register of Marine Species. 2012. Retrieved Jun 10, 2013, from


Anonymous said...

I am not a marine biologist but I do have interest in learning more about crabs. I found your article easy to follow and very informative. As one who always find articles on the Tide Chaser interesting, this one is very good. Thank you.
Ekhlas Abdel Bary
ESC. Qatar University

Ron Yeo said...

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