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Phylum Arthropoda > Subphylum Crustacea > Class Malacostraca > Order Decapoda > Brachyurans > Family Portunidae
Flower crab
Portunus pelagicus
Family Portunidae
updated Dec 2019
if you learn only 3 things about them ...
They are edible and among our favourite seafood.
They can swim fast using their paddle-shaped last legs.
They do not survive long out of water.

Where seen? Flower crabs are among our favourite seafood! It is commonly seen on many of our shores, especially the Northern shores and those with seaweeds and seagrasses. But it is not easy to spot. If trapped in a pool at low tide, it buries itself in the sand or mud. Besides the larger adults, there are also lots of smaller juvenile flower crabs hiding among the seagrass and seaweeds.

Features: Body width 5-7cm to about 20cm. Body fan shaped, body sides with 9 white-tipped spines, increasing in size from the eye outward, the last spine a very large protruding horizontal spike. The eyes are not very far apart. Last pair of legs are paddle-shaped and rotate like boat propellers, so the crab swims well in all directions. It is a fully marine crab and cannot live long out of water. Pincers long narrow, armed with sharp spines.

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 forming a pointed triangular shape, while the female's is broader as she uses the abdomen to carry her eggs.

Moult and 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.

Changi, Jun 05

Flower crab (top) next to
its moulted shell (bottom)
Sentosa, Jul 04

Mating crabs, male on top.
Pulau Sekudu, Jun 05
Smaller juvenile flower crabs (1-2cm) may have a wide variety of colours and patterns including bold bars and blotches.

What does it eat? The flower crab is a predator. In other parts of the world, it has been reported to eat mostly slow moving, bottom-dwelling creatures such as snails and clams, and worms. It may also eat fish, shrimps and other crabs.

Flowering crabs: Some flower crabs may have a 'garden' of various living seaweed and barnacles growing on their bodies, pincers and legs. These crabs are usually those infected by a parasitic barnacle (Thompsonia sp.)

Role in the habitat: Flower crabs are predators. In turn, they are eaten by animals higher up in the food chain.

Human uses: Flower crabs are edible and a favourite dish for many Singaporeans. They are caught with nets or baited traps. In Western Australia, they are an important fishery resource with 740 tonnes, valued at around $2.2 million, harvested in 1997/98.

Those infected with parasitic barnacles
tend to be covered with seaweeds.
Pulau Sekudu, Apr 06

Buried in sand.
Tuas, Oct 10

Tiny ones are also seen. This one is
hardly bigger than a Button snail.
Changi, Apr 05

Flower crabs on Singapore shores
On wildsingapore flickr

Other sightings on Singapore shores

Pasir Ris-Loyang, Oct 20
Photo shared by Loh Kok Sheng on facebook.

Pulau Tekukor , May 10
Photo shared by Neo Mei Lin on her blog.

Terumbu Pempang Laut, Apr 11
Photo shared by Loh Kok Sheng on his blog.

Berlayar Creek, Oct 15
Photo shared by Loh Kok Sheng on facebook.

Terumbu Berkas, Jan 10

Pulau Senang, Aug 10
Photo shared by Loh Kok Sheng on his flickr.

Links References
  • Joelle C. Y. Lai, Peter K. L. Ng and Peter J. F. Davie. 31 Aug 2010. A revision of the Portunus pelagicus (Linnaeus, 1758) species complex (Crustacea: Brachyura: Portunidae), with the recognition of four species. The Raffles Bulletin of Zoology 2010 58 (2): 199-237
  • Ng, Peter K. L. and Daniele Guinot and Peter J. F. Davie, 2008. Systema Brachyurorum: Part 1. An annotated checklist of extant Brachyuran crabs of the world. The Raffles Bulletin of Zoology. Supplement No. 17, 31 Jan 2008. 286 pp.
  • Chou, L. M., 1998. A Guide to the Coral Reef Life of Singapore. Singapore Science Centre. 128 pages.
  • 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.
  • Jones Diana S. and Gary J. Morgan, 2002. A Field Guide to Crustaceans of Australian Waters. Reed New Holland. 224 pp.
  • Edward E. Ruppert, Richard S. Fox, Robert D. Barnes. 2004. Invertebrate Zoology Brooks/Cole of Thomson Learning Inc., 7th Edition. pp. 963.
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