The land crab Johngarthia lagostoma is Ascension Island’s largest native land animal. This species only lives on four small, South Atlantic islands (including the islands of Trindade, Fernando da Noronha and Atol das Rocas off Brazil), of which Ascension is by far the largest. Despite being true crabs and thus originating from the ocean, land crabs are adapted to live in very dry climates and considerable distances from the sea. However, although they are adapted to the dry conditions found at Ascension Island, for much of the year the crabs are confined to the mountainous interior of the island where higher rainfall supports more vegetation and there is more moisture in the air.
Part of the adaptation to living on dry land is land crab behaviour, residing in burrows and often emerging only at night when the temperature falls. However, they will also take advantage of the moisture and comparative coolness following Ascension’s irregular rainfall and may be seen out in the open after spells of rain. Omnivorous, Ascension land crabs will eat almost anything from vegetation, to carrion, but their primary diet is fruit (such as guava and tungi), leaves and flowers.
The varying colours of land crabs frequently leads to confusion, with colours ranging from yellow-orange through to a deep purple, though all land crabs on Ascension are of the same species. Males may be distinguished from females by their considerably larger claws, this dimorphism possibly being a result of the larger claws being used for courting displays or even used by males physically competing over mates.
Despite adaptations for living on land, the land crabs of Ascension are still required to return to the ocean in order to breed. Between February and April, at certain phases of the moon, many thousands of crabs descend from the mountain and other inland areas to the shoreline to spawn. This journey may be over distances of several kilometres and take several days, with crabs usually moving at night and resting during the heat of the day. The main coastal area for this activity on Ascension is in the vicinity of North East Bay where, on peak spawning nights, a true natural spectacle can be witnessed. Hundreds of female land crabs, each carrying as many as 100,000 eggs they have incubated on the underside of their body for around two weeks, enter the surf and release their eggs. These eggs rapidly hatch into tiny larvae, being swept away by waves and currents to develop in the surface waters of the ocean.
Around 2-3 weeks after the young land crabs have been released, those fortunate enough to have remained in waters close to the island (the majority having been consumed by predators or swept offshore) they start to return to shore. Tiny juvenile land crabs, known as ‘megalops’, crawl out on to the shore and begin their terrestrial life, evidence showing that they may live for several decades. Such numbers of megalops may come ashore that in 1963 several houses in Georgetown were reportedly overrun by juvenile crabs.
Despite resilience and adaptations to living on land, Ascension Island land crabs populations are still under threat. Numbers are still only a fraction of that present when the island was settled in 1815. During the development of farming on the island crabs would often consume crops and so persecution by humans began. Bounties (1s 6d per 100) were offered for caught and killed land crabs, with a recorded 335,535 crabs killed from 1879 and 1887 alone. Even by 1887 a considerable decline in land crab abundance had been noted by the island commandant.
Today, land crabs on Ascension Island are protected by law, any harvesting, injury or intentional killing being entirely illegal. However, a number of crabs are still unintentionally killed each year, particularly around breeding periods, as crabs cross roads and are accidentally hit by vehicles. The Ascension Island Government Conservation Department continues to raise awareness and monitor populations so that these marvels of nature and adaptation remain part of Ascension Island, as they have for thousands of years.
Text by Dr Andrew Richardson.
Photography: Conservation Department, AIG
Printer: Cartor Security Printing
Process: Stochastic lithography
Stamp Size: 28 x 42mm
Sheet Format: 10
Perforation: 13 ½ x 13 ¼ per 2cms
Release Date: 24 January, 2018
Production Co-ordination: Creative Direction (Worldwide) Ltd