Friday, July 26, 2013

Rafting Crabs (Phylum Arthropoda: Family Plagusiidae) of Singapore

Plagusiid crabs (phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, superfamily Grapsidoidea, family Plagusiidae) 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.

Like other true crabs, plagusiid 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.

Plagusiid 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.

They are easily confused with grapsid crabs (family Grapsidae), 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 on Singapore's shore, Plagusia, which can be recognised by the numerous tiny bumps covering the exoskeleton. Plagusia crabs are commonly called Rafting Crabs as they are often found living on flotsam and even pelagic marine animals.

Two Plagusia species are recorded from Singapore, Plagusia squamosa and Plagusia immaculata. These two species are very similar, and the only way to tell them apart is by looking at the hairs on their legs - Plagusia squamosa has obvious rows of short, stiff black hairs on its legs, while in Plagusia immaculata, the hairs are sparse, few, hardly organised into rows or absent. There are scientists who suggest that they could be the same species, and hence I will not attempt to identify the exact species of the crabs featured in my photos below.

Rafting Crab (Plagusia sp.)
The above Rafting Crab (Plagusia sp.) has a purplish coloration. The carapace is about 5cm wide.

Rafting Crab (Plagusia sp.)
Some of the Rafting Crabs seen have a brownish coloration instead.

  • 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.
  • Schubart, C. D. & P. K. L. Ng. 2000. On the identities of the rafting crabs Cancer depressus Fabricius, 1775, Cancer squamosus Herbst, 1790, Plagusia immaculata Lamarck, 1818, and Plagusia tuberculata Lamarck, 1818 (Crustacea: Decapoda: Brachyura: Plagusiidae). The Raffles Bulletin of Zoology 48(2): 327-336.
  • World Register of Marine Species. 2012. Retrieved Jun 10, 2013, from

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