Johngarthia lagostoma is a species of terrestrial crab that lives on Ascension Island and three other islands in the South Atlantic. It grows to a carapace width of 110 mm (4.3 in) on Ascension Island, where it is the largest native land animal. It exists in two distinct colour morphs, one yellow and one purple, with few intermediates. The yellow morph dominates on Ascension Island, while the purple morph is more frequent on Atol das Rocas. The species differs from other Johngarthia species by the form of the third maxilliped.
Johngarthia lagostoma lives in burrows among vegetation, at altitudes of up to 400 m (1,300 ft), emerging at night to feed on plant matter and occasionally on animals. From January to March there is an annual migration to the sea to release the planktonic larvae. The species was first described (as Gecarcinus lagostoma) by Henri Milne-Edwards in 1837 from material sent to him by the naturalists Jean René Constant Quoy and Joseph Paul Gaimard, collected by the French ship Astrolabe.
Johngarthia lagostoma is found on Ascension Island, Ilha Trindade, Fernando de Noronha and Atol das Rocas. On Ascension Island, J. lagostoma is restricted outside the breeding season to the slopes of Green Mountain, where there is sufficient moisture and vegetation, the rest of the island being too arid for the crab to survive. All the land above 400 metres (1,300 ft) is suitable habitat for the crabs, as is much of the land above 200 m (660 ft). They are occasionally found at lower altitudes, including the well-watered gardens of Georgetown, and the sooty tern breeding colony in the south-west of the island (known as the Wideawake Fairs). On Ilha Trindade, J. lagostoma is abundant wherever plants grow, included the highest parts of the island.
The geographic distribution of J. lagostoma across a small number of islands in the southern Atlantic Ocean is very unusual, and difficult to explain by planktonic dispersal. Its nearest relatives are the other species of Johngarthia, two of which (J. malpilensis and J. planata) inhabit islands in the Pacific Ocean off Central America, and one (J. weileri) is found on islands in the Gulf of Guinea. Some authors have suggested the existence of former islands, now submerged, which could have acted as "stepping stones" for the colonisation of Ascension Island; the isolation of Ascension Island from any other land mass makes transportation of either larvae or adults difficult.
Mature specimens of J. lagostoma are typically 70–110 millimetres (2.8–4.3 in) wide across the carapace on Ascension Island; individuals from Atol das Rocas are somewhat smaller. In the family Gecarcinidae, species are normally separated by the form of the first pleopod (gonopod), which is used by males during mating, but there is no difference in the gonopod between J. lagostoma and J. planata. Instead, J. lagostoma differs from other species in the genus by the form of the third maxilliped; it has a fissure which is a narrow slit, but which gapes open in other species. The third maxilliped is also larger, covering the epistome and the antennules in J. lagostoma but not in other species.