Friday, July 26, 2013

Epialtid Spider Crabs (Phylum Arthropoda: Family Epialtidae) of Singapore

Epialtid spider crabs (phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, 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 (families Majidae and Inachidae) 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. They feed mainly on marine algae or other plant materials.

Like other true crabs, epialtid spider 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.

Epialtid spider 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.

I have only personally seen one species of epialtid spider crab in the wild in Singapore.

One-horned Spider Crab (Menaethius monoceros)
The One-horned Spider Crab (Menaethius monoceros) is usually found living among seaweeds. Its cryptic coloration and the algae and other debris attached to its body often allow them to camouflage well with the surrounding. It has a triangular carapace, with three teeth on each side margin. The chelipeds of the males are longer than any of its legs, and the beak-like structure at the front is long and slender. The carapace gets to about 1cm wide.

One-horned Spider Crab (Menaethius monoceros)
Sometimes, it may be overgrown with algae, and it is hard to confirm the identify in the field.

Spider Crab
I have also seen smaller spider crabs, some of them with very long extensions protruding from the head. I am not sure if they are of the same species though.

  • 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.
  • 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
  • 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.
  • 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

No comments: