Monday 27 August 2012

The Christmas Island Blue Crab recognized as a distinct species.

Christmas Island in the eastern Indian Ocean is noted for its distinctive Crab fauna, most notably the abundant Red Land Crab, Gecarcoidea natalis. Another distinctive form on the island is the Blue Crab, which has generally been regarded as a colour variation on the widespread Discoplax hirtipes, which is found from the Andaman and Nicobar Islands in the west to Hawaii in the east. This lives in burrows beside freshwater pools and streams. 

In a paper published in The Raffles Bulletin of Zoology on 29 February 2012, Peter Ng of the Tropical Marine Science Institute and Raffles Museum of Biodiversity Research at the Department of Biological Sciences at the National University of Singapore, and Peter Davie of the Queensland Museum, formally describe the Christmas Island Blue Crab as a separate species, as a result of a genetic study of the Crabs. 

The Christmas Island Land Crab. Ng & Davie (2012).

The new species is named as Discoplax celeste, celeste implying sky, or heavens, in reference to the sky-blue colour of the Crabs. It is essentially similar to Discoplax hirtipes in form and behavior, and develops its distinctive colouration only as an adult.

Colouration during the growth of Discoplax celeste. (A) juvenile (7.9 × 7.1 mm); (B) female (15.9 × 14.1 mm); (C) female (22.3 × 19.0 mm); (D) female (27.5 × 23.5 mm); (E) female (39.0 × 34.0 mm); (F) male (44.4 × 38.7 mm); (G) female (42.4 × 35.8 mm); (H) female (47.1 × 39.2 mm). Ng & Davie (2012).

Individuals with the purplish-brown with orange claws colouration of Discoplax hirtipes from Sumatra and the Nicobar Islands are also found on Christmas Island. Genetic sampling of these individuals suggested that they were closely related to the Crabs of Sumatra, not greatly surprising as Sumatra is the closest landmass to Christmas Island. Ng & Davie refer to these Crabs as Discoplax aff. hirtipes, on the basis that they suspect that these are likely to be a separate species from the Pacific Crabs.

Discoplax celeste (top) and Discoplax aff. hirtipes (bottom) from Waterfall Bay on Christmas Island. Both about 80 mm in diameter. Ng & Davie (2012).

The recognition of the Blue Crabs as a separate species is important, as these Crabs are increasingly at risk on Christmas Island, due to habitat destruction and invasive species. Recent fieldwork on the island has found very few juvenile specimens, which suggests the Crabs could be in serious difficulties. Recognition of the Blue Crabs as a separate species should (hopefully) ensure better protection for them.

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