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Phylum Arthropoda > Subphylum Crustacea > Class Malacostraca > Order Decapoda > Brachyurans
Swimming crabs
Family Portunidae
updated Dec 2019
if you learn only 3 things about them ...
Unlike other crabs, they swim very well!
They move quickly to hunt fast-moving prey like fish.
They can give a nasty pinch. Leave them alone!

Where seen? Swimming crabs are commonly seen on all our shores. They particularly active at night, but are often also out and about during the day. Besides the large adults, small juvenile swimming crabs are also hidden among the seagrass and seaweed, and other nooks and crannies. These active crabs come in all kinds of colours. Some can react fiercely by waving spiny pincers, and may even give you a nip. So don't touch these crabs.

Features: Body width 5-15cm. Swimming crabs are among the few crabs that are swift and agile swimmers. They usually swim sideways, but can also swim backwards and sometimes forwards. They swim with their last pair of legs which are paddle-shaped. These rotate like propeller blades when they swim. However, these crabs are essentially bottom-dwellers and don't swim about all the time. They often hide among the vegetation and slip under rocks and into other narrow crevices.

Swimming crabs have a streamlined shape for racing through the water. They have long pincers armed with sharp spines to snag fish and other fast moving prey. Often, one pincer is slightly larger than the other.

Here's more on how to tell apart the swimming crabs commonly encountered on our shores.

Fierce Crabs: If disturbed, swimming crabs often fearlessly wave their pincers menacingly. This is not an idle threat. If you come too close, this crab might just give a good nip that draws blood!

Mating swimming crabs
Pulau Sekudu, May 04

Brown crab mating with a red one.
Both are Red swimming crabs.
Terumbu Pempang Tengah, May 11

Flower crab (top) next to
its moulted shell (bottom)
Sentosa, Jul 04
What do they eat? They eat fish, worms, other crustaceans (including other crabs), clams and snails. They may also nibble on seaweed.

Swimming Babies: Like other crabs, swimming crabs often mate when the female is moulting.

Monstrous crabs: In Greek Mythology, Scylla is a female sea monster, who lives along a narrow channel, opposite the dwelling place of Charybdis, another female sea monster, sometimes a whirlpool. Thus arose the idiom: "between Scylla and Charybdis" meaning a precarious position where avoiding of one danger exposes oneself to another danger.

Human uses: Swimming crabs are edible and enjoyed by people everywhere. In our part of the world, from Asia to Australia., the Flower crab (Portunus pelagicus) is one of the commonly eaten members of this family.

Threat posture.
Pulau Semakau, Nov 09

Eating a heart urchin.
Terumbu Pempang Tengah, May 11

Eating a jellyfish
Pulau Semakau, Dec 04

Some Swimming crabs on Singapore shores
more on how to tell apart the swimming crabs on Singapore shores

Crucifix swimming crab

Family Portunidae recorded for Singapore
from Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore
in red are those listed among the threatened animals of Singapore from Davison, G.W. H. and P. K. L. Ng and Ho Hua Chew, 2008. The Singapore Red Data Book: Threatened plants and animals of Singapore.

+from The Biodiversity of Singapore, Lee Kong Chian Natural History Museum.
*from Ng, Peter K. L. & N. Sivasothi, 1999. A Guide to the Mangroves of Singapore II (Animal Diversity)
**from WORMS

  Family Portunidae
  Charybdis affinis
Charybdis anisodon
Charybdis annulata
(Banded-leg swimming crab)
Charybdis anisodon
(Orange-clawed swimming crab)
Charybdis brevispinosa
Charybdis callianassa
**Charybdis feriata=Charybdis feriatus
(Crucifix swimming crab)
Charybdis granulata
Charybdis hellerii
(Purple-legged swimming crab)
Charybdis natator
(Ridged swimming crab)
Charybdis orientalis
Charybdis ornatus
Charybdis truncata=**Charybdis (Goniohellenus) truncata
Charybdis variegata

+Lupocycloporus gracilimanus

Lupocyclus inaequalis
Lucocyclus rotundatus
(NE: Presumed Nationally Extinct)

Podophthalmus vigil
(Sentinel swimming crab)

Portunus brocki=**Portunus (Xiphonectes) brockii
+Portunus (Portunus) convexus
Portunus gladiator=**Portunus (Monomia) gladiator
Portunus gracilimanus=**Portunus (Lupocycloporus) gracilimanus
Portunus hastatoides=**Portunus (Xiphonectes) hastatoides
+Portunus (Xiphonectes)
cf. hastatoides
Portunus innominatus=**Portunus (Lupocycloporus) innominatus
Portunus pelagicus (Flower crab) including tiny ones
Portunus rubromarginatus=**Portunus (Monomia) rubromarginatus
Portunus sanguinolentus
(Blood-spotted swimming crab)
Portunus tenuipes=**Portunus (Xiphonectes) tenuipes
Portunus tweediei=**Portunus (Xiphonectes) tweediei

Scylla sp. (mud crabs)
*Scylla olivacea
(Orange mud crab)
*Scylla paramamosain
(Green mud crab)
*Scylla tranquebarica (Purple mud crab)

Thalamita sp. (eyes-wide-apart swimming crabs)
Thalamita admete
+Thalamita chaptalii
Thalamita crenata
(Powder blue-clawed swimming crab)
Thalamita danae
(Blue swimming crab)
cf. pelsarti
Thalamita prymna
(Blue-spined swimming crab)
Thalamita sima
Thalamita spinimana
(Red swimming crab)
Thalamita stimpsoni=**Thalamita danae
Thalamita sp. (Mottled swimming crab)

Links References
  • Hua Kai & Adib Adris. 30 October 2020. Sentinel swimming crab (Podophthalmus vigil) at Seringat-Kias. Singapore Biodiversity Records 2020: 175. The National University of Singapore.
  • 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.
  • Wee, D. P. C. & Ng, P. K. L., Swimming crabs of the genera Charybdis De Haan, 1833, and Thalamita Latreille, 1829 (Crustacea: Decapoda: Brachyura: Portunidae) from Peninsular Malaysia and Singapore. Pp. 1-128. Raffles Bulleting of Zoology, Supplement Series No. 1 (31 December 1995)
  • 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.
  • Davison, G.W. H. and P. K. L. Ng and Ho Hua Chew, 2008. The Singapore Red Data Book: Threatened plants and animals of Singapore. Nature Society (Singapore). 285 pp.
  • Wee Y.C. and Peter K. L. Ng. 1994. A First Look at Biodiversity in Singapore. National Council on the Environment. 163pp.
  • 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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