Cichlid diversity, speciation and systematics: examples for the Great African Lakes

The cichlid faunas of the large East African lakes pro vide many fascina ting research tapies. They are unique because of the large number of species involved and the ir exceptional degree of endemicity. In addition, certain taxa exhibit a substantial degree of intra~lacustrine endemism. These features al one make the Great African Lakes the largest centers of biodiversity in the vertebrate world. The numbers of cichlid species in these lakes are considered from different angles. A review is given of the data available on the tempo of their speciation, and sorne of the biological implications of its explosive character are discussed. The confusion in the definition of many genera is illustrated and the current methodology of phylogenetic research briefly commented upon. Theresults of the systematic research within the SADC/GEFLake Malawi/NyasaBiodiversity Conservation Project are discussed. It is argued that systematic research on the East African lake cichlids is entering an era of lesser chaos but increasing complexity. INTRODUCTION The main value of the cichlids of the Great African Lakes is their economie importance as a readily accessible source of prote in for the ri parian people. In addition, these fishes are important to the specialized aquarium trade as one of the more ex ci ting fish groups to be kept and bred by many hobbyists ali over the world. The economie importance of the cichlids by no way diminishes their enormous scientific value as ever-stimulating research subjects for systematists, evolutionary biologists, ecologists, ethologists, geneticists, morphologists, fisheries biologists, etc. While many scientific studies are of direct relevance to the economie importance of these fishes and may contribute to a better fisheries management, others are more focussed on purely scientific aspects oftheir fascina ting biology. JouRNAL OF AQUARICULTURE AND AQUATIC SciENCES ClcHLm REsEARcu: STATE OF THE ART VOLUME IX Page 150 Grea ter awareness of the scientific and economical value of these fishes has led to the establishment of varioüs recent research projects such as the three GEF (Global Environmental Facility) projects on the larger lakes (Victoria, Tanganyika, Malawi/Nyasa). Though based on sorne common grounds and concerns, these three projects each have different emphases and time schedules. Of these three, the largest emphasis on biodiversity research was placed on the SADC/GEF Lake Malawi/Nyasa Biodiversity Conservation Project, the first core program of which ended in July 1999. As senior systematist of this project, I often had to deal with questions concerning the numbers of (described and undescribed) fish species in the lakes and the speed of the ir speciation. Also the poor knowledge of current cichlid systematics was repeatedly questioned, often with a certain amount of scepticism. It is indeed hard to exp lain to non-taxonomists how difficult it is to estimate the number of species present in the lake and to produce simple descriptions and classifications. It is equally difficult to explain that the ,(/ ,,/ ,.( .! /~ ,y y large, present day taxonomie confusion in the se fishes is not necessaril y the result of po or quali ty research in the past (a truism easily forgotten). Many problems are simply due to the inherent difficulty of the research subject or, in simple words, to the fact that these fishes look so similar. It is from a certain feeling of frustration when trying to answer these FAQ's (Frequently Asked Questions) as weil as possible that this manuscript originated. A unique vertebrate fauna Systematics is essen ti ail y the study of the diversity of life, in this particular case; of the East African cichlids. Hence, it is a key discipline in understanding the unique features of these fishes. A first observation is that the speciose nature of cichlids in the Great African Lakes is unmatched in the world of vertebrates (table 1). This is a direct consequence ofthe explosive speciation within these lakes, the results of which complete! y overshadow the classic textbook example of t~e Galapagos finches. The underlying evolutionary processes that have resulted in this multitude of species are subject matter for many specialized publications and will not be discussed here. A second feature of this unique fauna is that the majority of the species are endemie to a lake, i.e. they iive only in one lake system and nowhere else (see below). The two main features (species richness and endem1sm) àlone make the Great African Lakes the largest centres ofbiodiversity in the vertebrate world. The African cichlids: the view of the accountant Cichlids are reported to be the third im;gest f~mily of fishes (marine, brackish and freshwater) in the world (Nelson,J994). However, the figure given byNelson is without any doubt an underestimate . Based on current estimates (Kullander, 1998;Snoeks, in press), the total number of cichlids might weil be around 2,400 species, which would make it the large_st family of fishes, and even of vertebrates. Although many species occur in South and Middle America [estimated at 550 (Kullander, 199&)1, itis in the East African Lakes that cichlids have speciated in greatest profusion. In fact, the cichlids of these lakes constitute more than 10 % of the extant freshwater fishes of the world and each of the three larger lakes (Victoria, Tanganyika, Malawi/Nyasa) harbours more fish species than any other lake in the world. Taken in an African context, the described African cichlids constitute about a third of the total number of i ts fresh water fish fau na ( fig.l ). However, given the large numbers of undescribed species, more so in cichlids than in other families, this ratio will increase considerably when a more detailed inventory of the lake cichlids becomes available. In aU large East African Lakes, except Albert and Turkana, the majority of fishes belong to the family Cichlidae (fig. 2). Several hypotheses have been forwarded to explain the aberrantly low numbers in Lakes Albert and Turkana, but probably the intrinsic morphological features (Bauplan) of the ancestors were not the major determining feature in this process (Snoeks, in press). The relative importance of other intrinsic versus environmental factors (physical, chemical and biological) is still to be determined (Fryer, 1965;Fryerandlles, 1972;Greenwood, 1994; Snoeks, in press). While the highest numbers of cichlids are found in the three larger lakes, the situation is somewhat different for the non-cichlids (fig. 3). In view oftheir large size, the three largest lakes do not have a disproportionately large non-cichlid component (fig. 3a). However, while the smaller lakes (Turkana, Albert, Edward, and Kivu) harbour mo~ethan athird of the noncichlid fauna, they only comprise a smail fraction (6 %) of the cichlid fauna present in the East African Lakes (fig. 3b ). The aberrant position ofLakes Turkana and Albert also becomes apparent in a species 1 area plot (fig. 4 ). While the data for the non-cichlids are situated rela. tively close to the fitted linear flinction, the correlation is less in the cichlids, which are iimch more wi~ely scattered around the fitted line. Removing the data of Lakes Turkana and Albert resulted in a much narrower dispersal ofthe cichlid numbers aromid the regression line. Further exploration of these data through speeies/vailable volume plots slightly changed the . relative position of severallakes, but the aberrant position of Lakes Albert and Turkana remained (re-