Articles

Crabs on Islands

Seychelles has an incredible 32 species of land, shore, brackish water and freshwater crabs. These do not include all the marine crabs that inhabit the sea. We were interested in knowing which species of crabs inhabit the dry or terrestrial part of different islands and in what kind of numbers because crabs can be a good indicators of environmental changes.
 

Little red hermit crab


Purple hermit crab


Red hermit crab


Round-eyed ghost crab


Strawberry hermit crab


White pink ghost crab


Yellow nipper
All photos by Sarah Brook


People are familiar with the ghost crabs that dig their burrows along the shoreline, and scurry around on the hunt for food, when they think they’re not being watched.  Providing hours of fun for children who think they can outrun these incredibly fast little creatures, the crabs are more than just a source of entertainment in Seychelles.

A recent survey by Nature Seychelles of what are considered to be true land crabs has revealed that different islands of Seychelles have different numbers of these crabs. The coconut or robber crab once found on some of the surveyed islands no longer occurs there. 

Ghost crabs and hermit crabs are the most easily recognisable and are an important part of small island ecosystems.  The Seychelles doesn’t have any native omnivorous mammals on land and with the demise of the wild herds of giant tortoises, their role on small islands is filled to a large extent by land crabs and lizards.  The crabs will feed on anything from fallen fruit and plant material, to dead birds.  They provide an important service to the islands by keeping them clean.

The survey has identified 3 species of ghost crabs inhabiting the beaches of Seychelles, one of which digs its burrows greater than 200m inland.  Three species of hermit crabs have also been found, hiding under vegetation and in rocky crevices by day and becoming active in the evening in the vegetation surrounding the beach.  A further two species of ‘true’ land crabs also occur on some of the islands and emerge from their hiding places at night.

Surveys of these land crabs on  6 granite and coral islands have shown that the number of crabs differs greatly between islands and habitat types.  Islands like Cousin and Cousine with a plateau, native vegetation and no alien predators have very high numbers of ghost and hermit crabs.  On islands that have been drastically altered by humans, or have suffered from invasive species like rats and crazy ants, there are much fewer crabs. Hermit crabs in particular are found in very small numbers. 

One of the most important things people can do to increase the number of hermit crabs on small islands, is to stop collecting shells on the beach.  Without lots of available shells of different sizes, there will be very few hermit crabs. A study in the Maldives found that hermit crab populations have declined because of lack of shells and the crabs were forced to use all sorts of receptacles like bottle tops as shells!

Despite their charisma and obvious presence on beaches and oceanic islands throughout the tropics, not enough is known about these intriguing crustaceans.  Nature Seychelles hope to continue further work into the importance of land crabs on oceanic islands, as this survey has highlighted they are clearly strongly affected by human impacts on the environment
 

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Seychelles Nature, Green HealthClimate Change, Biodiversity Conservation & Sustainability Organisation

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