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seagrass lagoon

Flower crabs Portunus pelagicus Family Portunidae
Waiting to mate... The blue male crab (on top) is holding on to the female below

Hidden flowers: Flower crabs are common in the seagrass lagoon, but they are not easy to spot. If trapped in a pool at low tide, they bury themselves under sand or mud. Some have a layer of seaweed and other debris stuck to their bodies. Some even have barnacles growing on their shells to complete their disguise. So you need a keen eye to spot them! Besides the larger adults, there are also lots of smaller juvenile flower crabs hiding among the seagrass and seaweeds.

The difference between boys and girls: Male flower crabs have long pincers, twice to three times longer than their bodies are wide. Males are also more decorative, with blue markings and attractive patterns on their bodies. Females are better camouflaged in dull green and brown. The male's abdomen is more narrow while the female's is broader as she uses the abdomen to carry her eggs.

Flower power: These crabs hunt fish and other swimming animals at high tide, snagging them with their long pincers which are armed with sharp teeth. Flower crabs are excellent swimmers. Their last pair of legs are paddle-shaped and rotate like propellers. Flower crabs are fully marine crabs and cannot live long out of water.

Moult ‘n’ mate: Like many other crabs, Flower crabs can only mate just after the female moults. Often, a male is seen protectively holding on to a female. He does this because she is either just about to moult; or he has just mated with her and wants to make sure no other male gets to her before her shell hardens. The male transfers his sperm into a special receptacle in the female. When the female is ready to spawn, she will use his sperm to fertilise her eggs. The fertilised eggs are attached as a big mass (called a sponge) to her abdomen where she cares for them until they hatch. They hatch into free-swimming larvae that drift with the plankton, changing into yet another form before settling down and developing into miniature flower crabs.

Human uses: These crabs are edible and a favourite dish for many Singaporeans. They are caught with nets or baited traps. In the US, a related member of the Family Portunidae, the Blue crab (Callinectes sapidus) is important commercially.

Caught in the act!
A rare photo of
flower crabs mating!

Hidden in sand

Camouflaged
with seaweed

quick facts
Body to about 20cm wide, common in the seagrass lagoon

Classification:
OrderDecapoda
Class Malacostraca
Subphylum Crustacea
Phylum Arthropoda

See also ...
Why are there so many 'dead' crabs on Chek Jawa? About moulting
About swimming crabs in general

Links
Blue-Crab Archives by Steve Zinski: a fabulous site packed with info and photos on a relative Portunid Callinectes sapidus found in the US. Details on moulting, reproduction and development with photos of all stages of growth.
Portunus pelagicus on the International Commission for the Scientific Exploration of the Mediterranean Sea website: rather technical fact sheet with photo. This crab is considered an introduced species in the Mediterranean.

Other references

Barnes, Robert D. & Ruppert, Edward E., 1996. Invertebrate Zoology. Harcourt College Publishers. 6th Edition. pp. 1056, G-1-16, I-1-30.
Tan, Leo W. H. & Ng, Peter K. L., 1988. A Guide to Seashore Life. The Singapore Science Centre, Singapore. 160 pp. online version
Lim, S., P. Ng, L. Tan, & W. Y. Chin, 1994. Rhythm of the Sea: The Life and Times of Labrador Beach. Division of Biology, School of Science, Nanyang Technological University & Department of Zoology, the National University of Singapore. 160 pp.
Ng, Peter K. L. & N. Sivasothi, 1999. A Guide to the Mangroves of Singapore II (Animal Diversity). Singapore Science Centre. 168 pp. online version
Chuang, S. H., 1961. On Malayan Shores. Muwu Shosa, Singapore.225 pp., plates 1-112.
Davey, Keith, 1998. A Photographic Guide to Seashore Life of Australia. New Holland, Australia.144 pp.

a companion website to the chek jawa guidebook website©ria tan 2003 www.wildsingapore.com

a companion website to the chek jawa guidebook
website©ria tan 2003 www.wildsingapore.com