The freshwater crab genus ''Seychellum'' is endemic to the granitic Seychelles, and a further 26 species of crabs and five species of hermit crabs live on the islands. The Aldabra giant tortoise now populates many of the islands of Seychelles; the Aldabra population is the largest remaining. These unique reptiles can be found even in captive herds. The granitic islands of Seychelles may support distinct species of Seychelles giant tortoises; the status of the different populations is currently unclear. Seychelles hosts some of the largest seabird colonies in the world, notably on the outer islands of Aldabra and Cosmoledo. In granitic Seychelles the largest colonies are on Aride Island including the world's largest numbers of two species. Sooty terns also breed on the islands. Other birds include Cattle egrets (''Bubulcus ibis'') and Fairy terns (''Gygis alba''). More than 1,000 species of fish have been recorded.