Friday, July 26, 2013

Sentinel Crabs (Phylum Arthropoda: Family Macrophthalmidae) of Singapore

Sentinel crabs (phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, 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.

Like other true crabs, sentinel 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.

Sentinel 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 sentinel crabs that I have photographed in Singapore, but unfortunately, I have not been able to identify them to the species.

Sentinel Crab (Macrophthalmus sp.)
This fairly big Sentinel Crab (Macrophthalmus sp.) has a bluish grey carapace of about 5cm wide. There are several spines on its chelipeds. It was found on a sand bar next to a seagrass meadow.

Sentinel Crab (Macrophthalmus sp.)
This Sentinel Crab (Macrophthalmus sp.) has a pale brownish grey carapace which is about 3cm wide. It was found on coarse sand mixed with broken shells.

Sentinel Crab (Macrophthalmus sp.)
The above Sentinel Crab (Macrophthalmus sp.) may resemble the previous species, but the carapace (about 3cm wide) is not plain but covered in darker markings. It was found on coarse sand.

Sentinel Crab (Macrophthalmus sp.)
This small Sentinel Crab (Macrophthalmus sp.) has a greyish carapace (about 2cm wide) marked with black patterns.

Sentinel Crab (Macrophthalmus sp.)
The above Sentinel Crabs (Macrophthalmus sp.) were seen in a muddy seagrass meadow. The body is mostly orange, and the chelipeds are white. The carapace is about 3cm wide.

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

No comments: