32. The Fauna of the African Lakes: a Study in Comparative Limnology with special reference to Tanganyika

Abstract

The special interest attaching to this comparative study of African lakes is due to the remarkable nature of the fauna of Tanganyika. That lake was discovered in 1858 by Burton and Speke, the latter bringing back with him shells considered to have a distinctly marine appearance. Subsequent collections emphasised this point, and interest was further increased by the discovery of a medusa by Böhm. A scientific expedition to investigate the fauna was despatched in 1895 in charge of J. E. S. Moore. The rich and unusual nature of the fauna then collected led him to formulate the hypothesis that Tanganyika represents an old Jurassic sea. In order to test the validity of this hypothesis, a second expedition, on which Moore was accompanied by Fergusson, left England in 1899. The result was held by Moore to justify the theory, and he embodied his conclusions in a work entitled “The Tanganyika Problem,” published in 1903. As the aquatic flora had not been taken into account, a third expedition was despatched to rectify that omission and make a further collection of animals. This left in 1904 in charge of the writer, returning in 1905. More recently still, in 1912-13, the Belgian expedition of Louis Stappers visited the lake and obtained additional information.The scope of this paper includes, besides Tanganyika, the five adjacent lakes of most interest, viz.:–Victoria Nyanza, Nyasa, Albert Nyanza, Edward Nyanza, and Kivu. All these, with the exception of Lake Victoria, occupy portions of the Great Rift Valley, which has probably been formed by trough-faulting on a stupendous scale. They lie in long narrow depressions bounded by escarpments rising to a height of two or three thousand feet above the level of the water. Nyasa and Tanganyika are very deep, the former reaching to over 780 metres and the latter to no less than 1435 metres. Victoria Nyanza has the largest area, but occupies only a shallow basin bounded by low hills.In all the lakes, but especially the largest, conditions are almost oceanic. Climatic differences are negligible, but water temperatures are uniformly high, showing an average of about 26o C. Analyses of the water have been made in very few instances. The water of Tanganyika, while fresh, is unusually rich in salts of magnesium, and that of Kivu contains excessive quantities of the latter as well as sodium salts. It is likely that the salinity of Tanganyika was greater formerly, and may still be subject to variation. Evidence exists of a considerable rise and fall in the level of the lake, yet it seems probable that rainfall and evaporation are very nearly balanced. There is reason to believe that Tanganyika had no outlet until it received an additional water supply from the Kivu basin, which was cut off from the Nile, and added to the drainage area of the big lake by the formation of a volcanic dam in recent geological times. Tanganyika would thus have been completely isolated and its waters more saline until an outflow was established. The present effluent appears to have been formed as an affluent, its bed being finally captured by a tributary of the Congo. A periodic rise and fall in the lake level may be caused by a temporary damming of the bed of the effluent, indirectly due to irregularities in the rainfall.As regards fauna, it is probable that at the present time all the six lakes have received fairly equal investigation. Only strictly aquatic animals are considered in this paper, but forms obtained from the neighbourhood of a lake are included in the totals. Tanganyika exhibits by far the most remarkable features, containing some 402 species of which 293 (nearly 73 per cent.) are endemic. Nyasa has a total second in point of size, but this has been artificially swollen by extra-lacustrine records which are wanting for other lakes. It has only 24 per cent, of endemic forms, while Victoria Nyanza with a smaller total has a larger percentage of endemics, namely 38 per cent. The three smaller lakes show a great reduction both in number of types and number of endemics. Tanganyika is further distinguished in that 57 out of 168 genera are peculiar to its waters, whereas Nyasa can only muster 6 endemic genera and the other lakes fewer still.The fishes of Tanganyika are of outstanding interest, comprising 146 species, of which 121 are endemic. The most notable feature is the number and high degree of specialisation of the Cichlidse, which with 27 genera (21 endemic) and 89 species (84 endemic) is the richest Cichlid fauna in the world. A species of incrusting gymnolsematous Polyzoon occurs, such forms being mostly marine. There is a large molluscan fauna, and of the Gasteropods more than two-thirds exhibit a marine-like appearance. These are known as the thalassoid or halolimnic group and are without exception endemic. There are no thalassoid Lamellibranchs. ‘Twelve species of prawns are known’, typically fresh-water in character, but specialised and all peculiar to the lake. There is an endemic genus of crabs, with 3 species. The Eucopepoda, Branchinra, and Ostracoda are well represented, each showing a large proportion of endemic species. The Cladocera are conspicuously absent from the lake and the Rotifera are relatively few in number. This may be related to the salinity of the water. The medusa originally described from Tanganyika has now been discovered in Victoria Myanaa and the Niger. There are four endemic species of fresh-water sponges. Only 5 groups of animals contain no endemic types.Tanganyika is one of the most remarkable lakes in the world, the only Cases comparable being the Caspian Sea and Lake Baikal. Recent figures are difficult to ascertain, hut while Baikal may even surpass Tanganyika in the number of unique animal forms, it appears that the Caspian is less striking.There is reason to believe that the relations between marine and fresh-water orgiuisms are intimate and due to a community of descent. The barriers which prevent a change of medium are not insurmountable. Organisms originated in the ocean, and have attained their distribution in fresh water in various ways. Moore regarded many of the Tanganyika types as relicts from a former ocean.The aquitic plants of Tanganyika are of less interest than the animals. The higher plants show no peculiarities, but the Algá differ from those of the remaining lakes. A number of species are endemic and others are usually marine or brackish in habit. The phytoplankton is rich in species, and more than 70 per cent. of the form do not occur in Nynsa or Victoria.Recent discoveries do not favour Moore's hypothesis of a marine Jurassic origin for Tanganyika. Neither his comparison of shells from the lake with marine fossil shells, nor his views on the primitive nature of the halolimnic Gasteropods, have been accepted by leading experts. No members of his halolimnic group, save the Polyzoon and the medusa, can be regarded as peculiarly marine. The Polyzoon is allied to a species Still living in Indian seas and the medusa is Known from other parts of Africa. The endemic animal types are held to be specialised rather than primitive in nature. Geological evidence is not more favourable. The extensive beds of sandstone and conglomerate which occur in the lake regions were probably formed under fresh-water and terrestrial conditions. They are considered by some to be of Triassic age, but may belong to a much earlier period, i. e. Devonian. Thus there is no support for the view that the ocean at one time extended over the Congo basin. Further, there is much to show that the trough in which Tangnnyika lies was not formed until Middle Tertiary times.A comparison has been made between thalassoid shells from Tanganyika and a fresh-water Cretaceous geniis on the one hand and fresh-water Pliocene shells on the other. Since the thalassoid shells have been held to resemble types from such different sources, they offer little indication as to the origin of the lake fauna. The quasi-oceanic conditions in Tangmyika may have produced an effect on the organisms it contains. Germain asserts that Gasteropods of marine aspect occur in other regions besides Tanganyika, and are derived from the fresh-water types of a former vast lake basin. On the present lakes becoming isolated, the conditions in Tanganyika produced a more striking series of such forms than elsewhere. This view is not regarded as accept- able. It has been suggested that the salinity of the Tanganyika water has produced marine-like forms, but further evidence is needed. The view that Titnganyika owes its remarkable organisms to a prolonged period of isolation is regarded as the most likely suggestion. It does not run counter to geological conceptions.This theory does not account for the medusa. Boulenger suggests that it may have survived from an Eocene sea in Northern Africa,. Gravier considers, on the contrary, that it may be a recent migrant from the ocean.The faunas of the remaining lakes are of less interest. Victoria Nyanza is next in importance to Tanganyika. The most conspicuous group is the fishes, with a large proportion of endemic species. The Mollusca also are noteworthy. The fauna of Nyasa is similar in character, but with fewer peculiar types. There are, however, 5 endemic genern of fishes. The smaller lakes contain representatives of fewer animal groups as well as fewer species. Kivu is the extreme case, with only ?3 species, of which 4 tire endemic. The poverty of its fauna may be associated with its exceptional salinity. Albert Nyanza. displays the smallest proportion of endemic types, viz. 13 per cent. It is suggested that periods of isolation would account for the peculiarities of Victoria Nyanza and Nyaaa.Certain animal types are unexpectedly absent from the African lakes. Such are the fresh-water Crustacea Asellus and Gammarrus and the fish parasite Echinorhynchus. No explanation of this is forthcoming.The number of specific forms in these lakes appears to be proportional to the size. This would seem to hold good also for the number of genera and families and even for the number of groups represented. it is possible that this principle is of general application. It is thought to be analogous to the phenomenon exhibited by the flora of oceanic islands.The affinity between African and Indian fresh-water types is recognisable in several instances among the inhabitants of the lakes. This affinity is explained by the former existence of a. continent which embraced them countries in the Carboniferous and subsequent periods.Little is known of the deeper regions of any of the lakes. It remains to be discovered whether associations of abyssal animals exist in them. Neither the vertical distribution of plankton forms, nor their seasonal variations have yet been studied. Further information is needed on the salts dissolved in the water, the depth and nature of the lake basins, water temperatures, etc. It is nevertheless possible from established facts to form a true conception of the nature of the lakes and their organisms.