Geographical setting: The Malagarasi basin is situated in western Tanzania and extends across some 9.2 million hectares, encompassing five substantial rivers and extensive riparian wetlands. It is the largest drainage system into Lake Tanganyika, a recognized center of world biodiversity. Due to the exceptional size and importance of the ecosystem, the core of the basin has been designated a site of international significance under the Ramsar Convention on Wetlands (http://www.ramsar.org). This Ramsar location is second only to the Okavango in geographic scale.
The Malagarasi drainage encompasses a wide array of biotopes including fast flowing streams, slow rivers, open lakes, ponds choked with macrophytes and vast papyrus swamps. During the dry season wetlands cover some 450,000 hectares, but the annual rains expand the flooded area significantly. Before the opening of the East African Rift during the Miocene, the lower part of the Malagarasi was the headwater of the Congo River. This is effectively still true, although regional tectonics have modified the geography of area substantially since that time.
Now Lake Tanganyika, the largest of the East African rift lakes, acts as a holding basin for Malagarasi waters before they are discharged into the lake’s outflow, the Lukuga River. These waters then move through the Congo, and finally reach the Atlantic (Beadle 1974). The upper reaches of the Malagarasi have a remarkable geological history. The upper northern regions of the basin were formerly within the Nile drainage and flowed northward toward the modern Lake Victoria drainage until tectonic uplift reversed the direction of flow. It has been suggested, based on similarities between the fish species in the two regions, that this reversal brought the Nilotic fauna in contact with that of the Congo (de Vos et al. 2001).
Past research in the region: There is a paucity of information on the region in the scientific literature. For instance, a Web of Science search using the term ‘Malagarasi’ returns no recent published papers, (whereas ‘Okavango’ returns 212 and Everglade* 1,118!). Though numerous birds, mammals and large reptiles are recorded from the area, little is known of the freshwater fauna. The few studies of the region that have focused on aquatic organisms have concentrated on fish, although even this comparatively well-studied group yields unexpected surprises. For example, a recent study reported the remarkable finding of an endemic species flock of fluvial ‘goby’ cichlid fishes, an endemic clariid, and at least three undescribed other catfish species (de Vos & Seegars 1998, De Vos et al. 2001). A recent publication (Nkotagu & Ndaro, 2005) addresses limnological aspects of the large shallow lakes, Nyamagoma & Sagara, but not the larger river area.
Biodiversity and Biogeography: There are a number of controversies regarding the evolutionary origins of the region’s fish and mollusks that we are now in a position to address with our recent collections from the Malagarasi. For example, Potadomoides pelseneeri, a taxon last collected alive over 50 years ago (West & Michel, 2000; Brown, 1994) has been suggested as the key to the origins of the spectacular gastropod radiation found with Lake Tanganyika. We now have fresh specimens and molecular data to address this evolutionary puzzle. Similarly, our collection of specimens with several new species under description from this region may help to provide insight into the origins of Lake Tanganyika’s cichlid fish species flocks, and also possibly provide a link with the haplochromine radiation in Lake Victoria.