The Importance of Crabs!!!

Denis Island at approximately 14,000 years of age is a youthful island when compared to its truly ancient granite neighbours in the central Seychelles archipelago. This “youth” coupled with its flat terrain means the island’s ecosystem is also relatively simple. Denis does not support the abundant species of vertebrate that an equivalent area of continental landmass would. Denis, for example has no mammals or snakes, the ecological niches of scavengers and detrivores being filled largely by the island’s lizard and crab populations.

Crabs therefore play a key role in the island’s ecosystem. Denis Island plays host to at least 7 species of land crab the Horn-eyed ghost crab (Ocypode ceratophthalmus), the Purple ghost crab (O. cordimana), the Pink ghost crab (O. ryderi), the Yellow nipper (Grassipes crinipes) and 3 species of hermit crab the Tawny hermit crab (Coenobita rugosus), Red hermit crab (C. perlatus) and the Purple hermit crab (C. brevimanus).

It had been anticipated that following the rat eradication on Denis in 2002 that the hermit crab populations in particular would expand rapidly but this has not been the case. In order to get a better overview of the current status of hermit crab populations and understand their occurrence and habitat preferences GIF is undertaking a study on the island.

Two volunteers, Ms Tori Yates and Ms Vicki Wheeler, have been on Denis since the end of May and are undertaking this work on behalf of GIF in addition to monitoring and gathering data on the Sooty Tern Project (see blog entry dated Wednesday July 2nd 2008). Vicki and Tori have been utilising quadrat surveys to monitor the occurrence of hermit crabs in the different vegetation types to be found along the coast of the island; and “capture-recapture” techniques to derive an understanding of the crabs’ population densities.

The findings of this study will help us understand the population dynamics and habitat preferences of the 3 species. This will be very important in better guiding the vegetation management regimes planned under the islands Environmental Management Plan 2008 – 2012, ensuring that hermit crab needs are incorporated into management measures.