The Oligochaeta of the Nile Basin Revisited

Abstract

A hypothesis on the origin and migration routes of the oligochaetes of the Nile Basin is presented. Five routes (but probably more than five waves) are recognized. Species found far from their origin (e.g. Gordiodrilus from the Upper Nile, found in Selima Oasis and Tripolitania) may represent an earlier wave than those that live closer to their source (e.g. Alma nilotica in the Delta and at Bahariya Oasis). The occurrence of egg cases of Alma nilotica in Bahariya Oasis suggests that once a surface water connection between the Nile and that Oasis existed. Such migrations occurred when rainfall was 500 mm y(-1) or higher. Introduced species (probably in the nineteenth century), such as Allolobophora caliginosa, Pheretima spp., Branchiura sowerbyi, are excluded from this discussion. Moreover, millenia of basin irrigation in Egypt and northern Sudan have caused a drastic impoverishment of the original fauna. The oligochaetes of the Northern Nile are therefore a mix of species from different sources. The fauna of the Upper Nile (the Sudd region) is Afrotropical, with the Ugandan, Ethiopian and Kenyan faunas distinct from each other. There is no relation with the fauna of Cyprus, the island nearest to the Nile Delta. There is also little relation between the Chad and Nile basins. The Nile fauna is distinct from that of adjacent West African countries (Central African Republic, Congo, DR Congo), and from that of Southern Africa. Threats to the oligochaetes of the Nile include land reclamation, desertification, urban sprawl, soil and water pollution. There is a need for exploration in west and south Sudan, Ethiopia, Chad and the Central African Republic, before their oligochaetes become obliterated by environmental degradation.