Sponge crabs (phylum Arthropoda, subphylum Crustacea, class Malacostraca, order Decapoda, superfamily Dromioidea, family Dromiidae) are so-named for their habit of carrying a piece of living sponge or colonial ascidian for camouflage purposes. They are mostly scavengers, and hence help to clear the sea floor of dead animals.
Some sponge crabs may be mistaken for spider crabs (family Majidae), 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.
Like other crabs, sponge 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.
Sponge 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 sponge crabs that I have photographed in Singapore.
The Cannonball Sponge Crab (Lauridromia indica) is so-named for its round body. It is sometimes called the Pink-fingered Sponge Crab for its pink fingers, but despite that, it is not the only sponge crab with pink fingers. It is, however, the only one recorded in local shallow waters so far that has pink fingers and can grow to about 8cm wide. There is a possibility that other similar species may occur in local waters though, but just not discovered yet. The above photo features one with a brown colonial ascidian.
Occasionally, some Cannonball Sponge Crabs without holding a sponge or ascidian can be seen. Its carapace is usually wider than long, and on each side of the carapace, there are six teeth of about the same size. The rostrum (beak-like structure between the eyes on the carapace) is made up of three pointed spines. Newly moulted specimens may have a pinkish body in addition to the pink fingers. This very common species is sometimes collected for consumption, but is generally not popular due to the poor quality of the meat.
The above is possibly the Edward's Sponge Crab (Dromidiopsis edwardsi), which also has a rostrum made up of three pointed spines but the pincers bear whitish fingers. Its carapace is approximately as long as wide, and the last pair of legs are relatively short. The above photo features one carrying a red colonial ascidian, and its carapace is about 1.5-2cm wide.
The Tuberculate Sponge Crab (Cryptodromia tuberculata), or Bumpy Sponge Crab as I call it, has numerous tubercles (i.e. pointed bumps) on its chelipeds. The carapace is wider than long, and the front margin between the eyes has three very blunt and inconspicuous teeth, looking more like a wavy structure. The centre teeth is usually more prominent that the two by the sides. Sometimes, the carapace may be marked with dark patches. This small sponge crab gets to about 1cm wide (carapace only). The above photo features one carrying a yellowish sponge.
The Hairy Sponge Crab (Lewindromia unidentata) is generally covered in fine hair. The carapace is approximately as long as wide, and the sides have no spines. The rostrum has three points, with the one in the middle being smaller, but usually this is hard to see in the field as the crab is covered in hair. The pincer fingers are strongly curved downwards, and whitish in colour. The maximum carapace width of this small crab is about 1cm.
References
Some sponge crabs may be mistaken for spider crabs (family Majidae), 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.
Like other crabs, sponge 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.
Sponge 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 sponge crabs that I have photographed in Singapore.
The Cannonball Sponge Crab (Lauridromia indica) is so-named for its round body. It is sometimes called the Pink-fingered Sponge Crab for its pink fingers, but despite that, it is not the only sponge crab with pink fingers. It is, however, the only one recorded in local shallow waters so far that has pink fingers and can grow to about 8cm wide. There is a possibility that other similar species may occur in local waters though, but just not discovered yet. The above photo features one with a brown colonial ascidian.
Occasionally, some Cannonball Sponge Crabs without holding a sponge or ascidian can be seen. Its carapace is usually wider than long, and on each side of the carapace, there are six teeth of about the same size. The rostrum (beak-like structure between the eyes on the carapace) is made up of three pointed spines. Newly moulted specimens may have a pinkish body in addition to the pink fingers. This very common species is sometimes collected for consumption, but is generally not popular due to the poor quality of the meat.
The above is possibly the Edward's Sponge Crab (Dromidiopsis edwardsi), which also has a rostrum made up of three pointed spines but the pincers bear whitish fingers. Its carapace is approximately as long as wide, and the last pair of legs are relatively short. The above photo features one carrying a red colonial ascidian, and its carapace is about 1.5-2cm wide.
The Tuberculate Sponge Crab (Cryptodromia tuberculata), or Bumpy Sponge Crab as I call it, has numerous tubercles (i.e. pointed bumps) on its chelipeds. The carapace is wider than long, and the front margin between the eyes has three very blunt and inconspicuous teeth, looking more like a wavy structure. The centre teeth is usually more prominent that the two by the sides. Sometimes, the carapace may be marked with dark patches. This small sponge crab gets to about 1cm wide (carapace only). The above photo features one carrying a yellowish sponge.
The Hairy Sponge Crab (Lewindromia unidentata) is generally covered in fine hair. The carapace is approximately as long as wide, and the sides have no spines. The rostrum has three points, with the one in the middle being smaller, but usually this is hard to see in the field as the crab is covered in hair. The pincer fingers are strongly curved downwards, and whitish in colour. The maximum carapace width of this small crab is about 1cm.
References
- 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.
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- Debelius, H. 1999. Crustacea - Guide of the world. IKAN, Frankfurt. 321pp.
- ETI BioInformatics. 2012. Marine species identification portal. Retrieved Jul 8, 2013, from http://species-identification.org.
- McLay, CL (2001) The Dromiidae of French Polynesia and a new collection of crabs (Crustacea, Decapoda, Brachyura) from the Marquesas Islands. Zoosystema 23(1): 77-100.
- McLay, C. L. & P. K. L. Ng. 2005. On a collection of Dromiidae and Dynomenidae from the Philippines, with description of a new species of Hirsutodynomene McLay, 1999 (Crustacea: Decapoda: Brachyura). Zootaxa. 1029, 1–30.
- National Science Council of Taiwan. 2004. BiotaTaiwanica. Retrieved Jul 10, 2013, http://biota.taibif.org.tw
- 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.
- Tan, L. W. H. & P. K. L. Ng. 1988. A Guide to Seashore Life. Singapore Science Centre. Singapore. 160 pp.
- World Register of Marine Species. 2012. Retrieved Jun 10, 2013, from http://www.marinespecies.org.
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