An ecological and systematic survey of fishes in the rapids of the lower Zaire or Congo River

_____________ ----------------------------------------------239 Acknowledgments ----------------------------------240 Introduction _______________________________________________ 240 Limnology ---------------------------------------------------------242 Collecting Methods and Localities __________________ 244 Tabulation of species ---------------------------------------249 Systematics -------------------------------------------------------249 Campylomormyrus _______________ 255 M ormyrus ____ --------------------------------_______________ 268 Alestes __________________ _ ________________ 270 Bryconaethiops -------------------------------------------271 Labeo ---------------------------------------------------_______ 27 4 Bagrus _________________________ 275 Chrysichthys ______ 276 N otoglanidium -------------------------------------------27 6 Gymnallabes _______________ _____ ____________________ 277 Atopochilus ______________________ __________________________ 278 Chiloglanis -------------------------------------------282 Lamprologus _______ ________________ _______________ 283 N anochromis ---------------------------------------284 Steatocranus _____________________ 292 T eleogramma ---------------------------------------300 M astacembelus ________________________ _____________________ 302 Reproduction --------------------------------------308 Adaptations ________________ __________________________ 309 Discussion ____ --------------------------------314 Literature Cited _____________________________________________ 315 ABSTRACT. Rapids habitats at nine localities along the mainstream of the Zaire River between Kinshasa and Matadi were fished extensively with rotenone during low water, yielding over 7000 specimens belonging to 129 species. A total of 17 families are represented, the most speciose being Mormyridae ( 19 species), Cyprinidae ( 19), Cichlidae ( 17), and Mochokidae ( 16). Limnological and ecological observations are reported for Rapids habitats at nine localities along the mainstream of the Zaire River between Kinshasa and Matadi were fished extensively with rotenone during low water, yielding over 7000 specimens belonging to 129 species. A total of 17 families are represented, the most speciose being Mormyridae ( 19 species), Cyprinidae ( 19), Cichlidae ( 17), and Mochokidae ( 16). Limnological and ecological observations are reported for 1 Museum of Comparative Zoology, Cambridge, Mass. 02138. 2 Laboratory of Limnology, University of Wisconsin, Madison, Wise. 53706. the rapids habitats, and the adaptations and mode of reproduction of the fishes discussed. Nineteen new species are described from the Lower Zaire rapids, belonging to the genera Mormyrus, Alestes, Labeo, Bagrus, Chrysichthys, Notoglanidium, Gymnallabes, Chiloglanis, Lamprologus, Nanochromis, Steatocranus, Teleogramma, and Mastacembelus, most of them with obvious modifications for life in the rapids. Caecomastacembelus is placed in the synonymy of Mastacembelus, and morphologically intermediate hybrids reported between blind, depigmented Mastacembelus brichardi and normally eyed, darkly pigmented M astacembelus brachyrhinus. The genera Campylomormyrus (elephant-snouted mormyrids), Steatocranus and Teleogramma are fully revised. Of 16 nominal species of Campylomormyrus, only three are regarded as biologically valid. In addition to one new Steatocranus from the mainstream of the Lower Zaire River, two new species of this rheophilic cichlid genus are described from elsewhere in the Zaire basin, making a total of 21 new fishes described in this paper. Leptotilapia is placed in the synonymy of Steatocranus, which now comprises eight species. Bryconaethiops yseuxi is shown to be a valid species, and data on the number of mandibular tooth families are used to distinguish the closely related Atopochilus guentheri and A. royauxi. Two ecological categories of highly specialized rapids fishes are recognized: strongly rheophilic or current-loving, comprising 58 of the species in our survey, and hyporheic or intrusive, comprising eight species which evidently avoid light as well as strong current by delving deeply into the interstices offered by jumbled piles of rocks, and perhaps, in some cases, burrowing into mud or loose rubble. The most consistent evolutionary modification of the rapids fishes is reduction (but not complete loss) of the eyes. Of 66 highly specialized rapids fishes, 26 are microphthalmic (with small or minute eyes superficial in position), five cryptophthalmic (eyes reduced in size and partially or completely covered over by skin and Bull. Mus. Comp. Zool., 147(6): 239-317, February, 1976 239 240 Bulletin Museum of Comparative Zoology, Vol. 147, No. 6 other tissues), and only one is anophthalmic (totally without eyes) . The most startling discovery of the survey is a cryptophthalmic cichlid of the genus Lamprologus. This is the first reported blind species of percoid fishes. Some relationships between the rapids fishes and the ichthyofauna of the Zaire basin and of Africa as a whole are pointed out in the discussion. The opportunities for speciation and adaptation provided by rapids habitats have contributed significantly to the diversity of the highly endemic ichthyofauna of the Zaire basin, a process much less noticeable elsewhere in Africa, where highly specialized rapids fishes are relatively few. A few rheophilic genera, namely Campylomormyrus, Atopochilus, and Steatocranus, presumably evolved in the Zaire basin, subsequently spreading to one or more river systems in the adjacent Lower Guinean and Nilo-Sudanic ichthyofaunal provinces. The great majority of species of rapids fishes in the Zaire basin are endemic, many of them apparently restricted to the 300-km stretch of the Lower Zaire River between Kinshasa and Matadi. Most of the rapids genera, however, are widely (if discontinuously) distributed in the Zaire basin, wherever suitable habitat occurs. Perhaps the most striking aspect of the taxonomic composition of the Zairean rapids fishes is the preponderance of Cichlidae, which cor.tributed more than a third of the endemic rapids species and also more than a third of the total number of individual specimens taken during our rapids survey. Such predominance of Cichlidae is a noteworthy contrast to the situation in rich riverine faunas everywhere else in Africa, including low gradient rivers in the Zaire basin, where cichlid species, endemic or not, are always few in number and seldom contribute substantially to the numbers of individuals present. Evolution in rocky rapids habitats may well have contributed to preadapt Zairean cichlids, including Lamprologus, to explosive lacustrine evolution along the rocky shores of Lake Tanganyika.