Testing the strength of the Coconut Crab.

Coconut Crabs, Birgus latro, are the largest terrestrial Crustacean species, and indeed largest terrestrial Arthropod of any description. It is found across much of the Indian and Pacific Oceans, living on most landmasses where Coconuts are found (though in some areas they have been exterminated to protect commercial Coconut crops). They are descended from Hermit Crabs, and younger Coconut Crabs still use borrowed Snail shells for protection, but the adult Crabs have developed tough calcareous exoskeletons. Like other Decapod Crustaceans, Coconut Crabs have large chelae (claws) which are their main method of food manipulation and defence. Studies of other Decapods have been shown to be capable of exerting a considerably greater pinching force with their claws than the bite of similarly sized Vertebrates, making the pinching force of the Coconut Crab potentially very formidable.

In a paper published in the journal PLoS One on 23 November 2016, Shin-ichiro Oka, Taketeru Tomita and Kei Miyamoto of the Okinawa Churashima Foundation, describe the results of an experiment designed to test the strength of the claws of the Coconut Crab.

Oka et al. collected 29 Coconut Crabs from Ocean Expo Park on Northern Okinawa Island, ranging in size from 16.2 mm in length and 33 g in weight to 64.5 mm in length and 2120 g in weight and measured their pinching force with a device used to measure crushing force when an animal bites down on a stainless steel stick-shaped sensor, and in addition took measurements of their claws.

Measurement of the pinching force and claw morphology in the Coconut Crabs. (a) The force was measured with the SK-MBF-01F device (SkyScience Co. Tokyo, Japan) and related sensors and (b) demonstration of the method by which pinching force was measured. (c) Claw measurements of the Coconut Crab used in this study. The placement of the sensor used for pressure measurement is highlighted in green. The measurements used for claw length (CL), claw height (CH), and claw width (CW) are also indicated. L1: in-lever length from the fulcrum to the apodemes insertion; LBAE: out-lever length from the fulcrum to the tubercle (the contact point with the device sensor). Oka et al. (2016).

Oka et al. found that the pinching force of the claws related directly to the size of the Crabs, with the smallest Crab exerting a pinch of 29.4 Newtons, and the largest 1765.2 Newtons. However even the largest Crabs used in the study were considerably smaller than the maximum size for the species, about 4 kg. Assuming that the strength of larger Crabs continues to scale up in the same way, Oka et al. calculate that they could exert a pinch-force of 3300 Newtons, greater than the pinch-force measured from any other Crustacean, and higher than the bite-force recorded from any terrestrial predator except Alligators.

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Two new species of Vampire Crabs from Central Java.

Sesarmid crabs of the genus Geosesarma are large, often colourful, fully terrestrial Crabs found in Southeast Asia and Indonesia. In recent years they have been traded in the international aquarium trade under the name ‘Vampire Crabs’ (which is fanciful, the Crabs do not drink blood but feed largely on small Insects etc.), with several species unknown to science appearing in the aquarium trade. The origin of these has been hard to track down, with dealers claiming they originate from Sulawesi, Java, Krakatau (highly unlikely since Krakatau is an active volcanic island with no fresh water supply, which has been completely resettled by wildlife from outside following the destruction of the island by a violent eruption in the late nineteenth century, and its subsequent re-appearance) or the Riau Islands.

In a paper published in the Raffles Bulletin of Zoology on 16 January 2015 Peter Ng of the Lee Kong Chian Natural History Museum at the National University of Singapore, Christoph Schubart of Zoology and Evolutionary Biology at Universität Regensburg and Christian Lukhaup of Bittenfeld in Germany describe two new species of Vampire Crabs from Central Java, both of which have previously been encountered in the aquarium trade.

The first new specie described is named Geosesarma dennerle, in honour of the German company Dennerle, who supported Christian Lukhaup’s studies in Java. The Crabs have squarish bodies, a little wider than long, with the largest male located having a carapace 14.4 by 14.0 mm and the largest female 13.5 by 11.8 mm. They have a very distinctive colouration, with the front half of the carapace and legs being violet or purple and the rear of the carapace being cream or yellowish white. The crabs were found living on the slopes of a small valley, between rocks and in dense vegetation. The species has also been found being traded in Germany under the names ‘Vampir Vampirkrabbe’ and ‘Blaue Vampirkrabbe’.

Specimens of Geosesarma dennerle from Cilacap on Java. Christian Lukhaup in Ng et al. (2015).

The second new species described is named Geosesarma hagen, in honour of the Rolf C. Hagen Group of Companies, who are a major pet supplies group in Germany, and who supported both Christian Lukhaup’s studies in Java and subsequent work on the Crabs by Christoph Schubart. These Crabs are also squarish and wider than they are long; the largest male found had a carapace 13.9 by 13.3 mm, while the largest female was 14.5 by 13.4 mm. This species also has very distinctive colouration, with the front of the carapace and walking legs being brown while the rear of the carapace and claws are bright orange or yellow. This species has been traded in Germany as ‘Rot Vampirkrabbe’. The Crabs were found living in a Banana and Rubber plantation on the side of a small hill, about 300 m from a road.

Specimen of Geosesarma hagen from Cilacap on Java. Christian Lukhaup in Ng et al. (2015).

Ng et al. also note that both species are apparently being heavily collected for the aquarium trade, and that local people report that they have become scarcer in recent years. This strongly implies that they are being collected in an unsustainable way, and that if this persists it will be necessary to develop a conservation management plan for these Crabs, probably involving a captive breeding program. They also note that the conservation status of land Crabs as a group has not yet been assessed under the guidelines of the International Union for the Conservation of Nature’s Red List of Threatened Species, and that this needs to be done in the near future, particularly if their popularity in the aquarium trade persists.

See also…

A new species of Icriocarcinid Crab from the End Cretaceous of California.

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A new species of Palaeocorystid Crab from the Late Cretaceous of Poitou-Charentes in southwest France.

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A new species of Frog Crab from the Early Cretaceous of Colombia.

Frog Crabs, Raninoidia, are well represented in the fossil record across much of the globe in the Late Cretaceous and Cenozoic, but poorly recorded from the Early Cretaceous, when the group is thought to have originated. For a long time the majority of early Frog Crabs known were from Eurasia, leading to...

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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.

See also A new species of Barnacle from Taiwan, Two new species of freshwater Isopod Crustaceans from Lake Pedder in Tasmania and The Snail that eats Crabs.

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