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Thalamita crenata (Family Portunidae)

Notched Thalamita



A widely distributed species in Australia and elsewhere in the Indo-West Pacific Oceans, from Madagascar to Japan and Hawaii. Present on Cocos (Keeling) Is, Lord Howe Is, and from central New South Wales to Queensland, Northern Territory and north-western Western Australia. (WA, Cocos-Keeling Is., NT, Lord Howe Is., NSW, QLD)


The oval-shaped Notched Thalamita is highly aggressive, and the most common crab on the tropical reef flat. It grows to about 100 mm across the carapace, which is wide at the front, and tapers rapidly to the rear. It bears about five sharp spines along the sides - the distinct notches between the spines give rise to the species name crenata, meaning notched. The flattened hind legs are paddle-like and highly evolved for swimming, a characteristic of this family. The large, spined claws are strong, and are usually held high, ready for self-defence against all comers. They can inflict a nasty nip if you try to catch one. The colour of the carapace is quite variable, but is usually greenish with brown mottling, although green, blue and black are seen commonly. The tips of the legs and chelae fingers are brown, while the chelae are blue to white.

Ecology/Way of Life:

This littoral species lives under stones, boulders or mussel clumps, and is present also mud flats and mangroves, occasionally amongst coral. These crabs hunt for a range of prey, such as molluscs and other crabs within the mangroves and adjacent areas on incoming and outgoing tides. The Notched Thalamita occurs intertidally and subtidally, often on coral-rubble beaches and on soft or hard mud shores. After moulting and the shell of the female is still soft, the male mates with her, to produce as many as eight million eggs. These are carried on the limb-like appendages between her thorax and her folded back abdomen. After they hatch the small planktonic larvae go through several moults as they grow, before settling down as first stage crabs.

Preferred Image

Interaction with Humans/Threats:

This is a commonly occurring species and does not appear to be under threat through human activity.

Other Comments:

This species was described by E. Ruppell in 1830 from a crab collected in the southern Red Sea; the genus name is from the Greek: thalame = a den, hole, or lurking place and -ita = small, probably referring to the use by this species of small crevices in the reef at low tide; the species name is from the Latin: crenata = notched, referring to the distinctive notches on the edge of the carapace.

Further Reading:

Davey, K. (1998) A Photographic Guide to Seashore Life of Australia. p.49, New Holland Press, Sydney.

Jones, D. & Morgan, G. (1994). A Field Guide to Crustaceans of Australian waters. p.157, Reed.


Text, map & photograph by Keith Davey.

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Images and Multi-media:  
Attached Image  
image/jpeg 88664 bytes Notched Thalamita
Distribution Map  
image/gif 1033 bytes Distribution of Notched Thalamita

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