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Phylum Arthropoda > Subphylum Crustacea > Class Malacostraca > Order Decapoda > Brachyurans > Family Portunidae
Blue-spined swimming crab
Thalamita prymna*
Family Portunidae
updated Dec 2019
Where seen? This colourful swimming crab with bright blue spines is commonly seen on many of our shores, especially those with coral rubble and reefs. It is particularly active at night.

Features: Body width 5-7cm. Body rectangular, body sides with 5 bright blue-black tipped spines, the fourth spine is much tinier than the others so it may appear to have only 4 spines. The eyes are wide apart. Between the eyes are 6 rounded equal-sized lobes. Last pair of legs are paddle-shaped. The pincers also have bright blue-black tipped spines. Body is not very hairy, usually smooth and shiny, colours usually yellowish, orange with brown blotches.

Thanks to comments by Ondrej Radosta on Marcus Ng's photo on flickr who shared that Thalamita pelsarti looks very similar; T. prymna has a smooth and glossy upper body, while T. pelsarti has a granular almost 'hairy' upper body.

Sentosa, Aug 04

6 rounded equal-sized lobes between the eyes.

Fourth spine tinier than other spines
so it may appear to have only four spines.

A mating pair.
Kusu Island, Jul 04

*Species are difficult to positively identify without close examination.
On this website, they are grouped by external features for convenience of display.

Blue-spined swimming crabs on Singapore shores
On wildsingapore flickr

Other sightings on Singapore shores

East Coast (PCN), May 21
Photo shared by Vincent Choo on facebook.

Terumbu Selegie, Jun 11
Photo shared by Jerome Pang on facebook.

St John's Island, Oct 20
Photo shared by Loh Kok Sheng on facebook.

Pulau Hantu, Jul 20
Photo shared by Dayna Cheah on facebook.

East Coast Park, Jun 15
Photo shared by Loh Kok Sheng on facebook.

East Coast Park, May 16
Photo shared by Dayna Cheah on facebook.

Terumbu Pempang Kecil, Dec 15
Photo shared by Jianlin Liu on facebook.

Pulau Salu, Apr 21
Photo shared by Jianlin Liu on facebook.

Grateful thanks to Crabhunter for identification of some of these crabs.

Links References
  • 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.
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