Le peuplement de Cyprinodontes du bassin de l'Ivindo, Gabon
Authors: Brosset André 
Published: November 2022

The structure and dynamics of the community of Cyprinodont fishes living in the forest creeks and pools of the Ivindo Basin (N.E. Gabon) have been studied for three consecutive years. Field observations have been supplemented by laboratory observations and experiments carried out in France for six years.

The abiotic and biotic parameters of the niche of the various species were studied. Samples were collected every month during three consecutive years in five different locations to ascertain the diet, the breeding cycle, and the demographic and social structures of the populations. The reproductive strategies, growth, longevity and social behavior were studied in laboratory conditions at Brunoy ; experiments were carried out to demonstrate how reproduction might be socially controlled in some species.

The main results of this study may be summarized as follows :

— Eight sympatric species of Cyprinodont fishes live in the forest streams of the Ivindo Basin ; their community remains fairly stable in composition over the years.

— The environmental conditions are very uniform in such forest creeks and pools. The black shallow waters are almost devoid of mineral and organic components. The water temperature remains stable (20-23° C) throughout the year. The water remains level, except for brief swellings. Primary production is almost nil.

— The only environmental requirement common to all sympatric Cyprinodont fishes is the presence of a thick foliage screen close to the water surface, keeping the stream on the shade. Six of the species are usually found in still waters, and one in running waters, the last one can be found in both.

— All the Cyprinodont species, except those belonging to genus Diapteron, feed almost exclusively on small invertebrates falling from the overhanging foliage, mostly ants and spiders. No less than 49 species of ants have been identified in their stomach contents. This diet remains the same for all species, irrespective of the seasons and locations. Diapteron species have a different diet ; they feed on aquatic organisms only, which they hunt at dusk, whereas other sympatric Cyprinodonts are diurnal feeders.

— The spatial distribution of the community members is not apparently dependent on definite preferences of the various participating species, but is influenced by certain characteristics of the habitat. Social groupings of small-sized adults belonging to all the species of the community, all mixed together, are found in very small creeks, less than 3 cm deep. Large-sized individuals of several species usually occur together in the mainstream of the creek. Large monospecific social groups are restricted to pools.

— No regular, seasonal changes in community structure were noticeable. However, sudden swellings due to tropical storms, as well as changes in stream course produced by trampling by elephants, can completely change the community structure at a given locality within a very short time.

— The social groupings of the sympatric Cyprinodonts are extremely variable, ranging from monogamous pairs to unstructured promiscuous groupings as well as harems, multimale groups and unisexual groups. The social structure appears to be influenced more by local environmental conditions than by innate specific mechanisms.

— Whereas Aphyosemion and Epiplatys are somewhat seasonal in their reproduction, Diapteron breeds throughout the year. Egg size, as well as the rhythm of spawning, vary from species to species. An optional and long pre-and post-incubatory diapause can occur in the egg of Raddaella batesi. However, the length of incubation (17 days) and the average number of eggs produced per adult female per year (200 to 300) remain similar for all species.

— Sexual maturity is reached in most species at the age of 120 days and the maximum lifespan is 3 to 5 years. R. batesi matures earlier, when 60 days old, and becomes senescent when 16 to 18 months old. In all populations the sexually reproducing individuals are more numerous than the young ones. The adult sex-ratio is usually balanced, except for two harem-forming Aphyosemion species, among which females are more numerous than males. — The Ivindo Basin Cyprinodont community is entirely dependent upon the rain forest ecosystem, and can be considered as one of its component parts. When the mature rain-forest is felled, the Cyprinodont community immediately disappears. Some of the ecological and behavioural characteristics of the terrestrial rain-forest vertebrates are also apparent in Cyprinodont fishes as illustrated by the high species diversity, low reproduction rate, prolonged lifespan and diverse social habits.

Obviously, the principle of competitive exclusion does not operate in the Ivindo Basin Cyprinodont community. The situation described here closely approximates the «interspecific lottery for living space » condition found by Sale (1978) in coral reef fishes. In our case, the «random factors » are represented by such «catastrophic » events as sudden and violent swellings of the streams or disturbance of the habitat by forest elephants. As a consequence of such random habitat changes, community structure can change drastically in the same habitat at different times, and vary in similar habitats at a given period. Whether such a situation can be related or not to the high phenotypic variability of these fishes and to the polymorphism of some of the species still remains an open question.