CHA Phy Io ge o gr üphic Pqtterns in Populations of Cichlid Fishes from Rocky Habitats in Lake Tangznnyika

The cichlid species flocks of the great East African lakes represent the most diverse assemblages of freshn'ater fishes in the world. Lake Tanganyika is by far the oldest of the three major East African rift lakes with an estimated age of about 9 to 12 million years (Cohen et al., 1993). Its geological history is relatively well known (reviewed in Tiercelin and Mondeguer, \997). The lake is formed of three basins which have been fused to one large lake about 5 to 6 million years ago. Seismic data show that about 200,000 (Tiercelin and Mondeguer, 7991) to 75,000 (Scholz and Rosendahl, 7988; C. A. Scholz personal communication) years ago the level of Lake Tanganyika dropped 600 m below its present levei, possibly even splitting the lake into three sublakes for several tens of thousands of years. This vicariant event must have had severe effects on several habitats and their fish populations. After this period the lake levei rose again with additional minor fluctuations in the more recent history. At present times the lake has reached its largest extension due to addition of a large tributary river, the Fluztzi in the northern edge of the lake. It was formed about 10,000 years ago by the formation of the Virunga volcano chain in Rwanda, blocking the formal connection of this area with the Nile system. The influx of the Ruzizi River also ended a long period of isolation from the Zaire River system and caused an overflow of Lake Tanganyika via the Lukuga into the Lualaba, the upper reaches of the Zaire River. Although the cichlid flock of Lake Victoria is considered to be monophyletic (Meyer et al., 7990), tine Malawi and Tanganyika cichlid flocks are probably of polyphyletic origin (Greenwood, 1987; Nishida, 1991; Sturmbauer and Meyer, 1993; Moran et a1.,7994; Sturmbauer et a1.,7994; Kocher et ttl., 1993,1995). Lake Malawi harbours a small sub-flock of five endemic species of Tilapiine cichlids (Eccles and Trewavas,7989; Axelrod, 1993), in addition to its subflock of "haplochromines" which is considered monophyletic (Moran et ttl., 7994). The Lake Tanganyika cichlid flock is composed of several lineages, assigned to 12 tribes (Poll,