The impact of environment on ontogenesis variations in Ruggieria and Chrysocythere (Ostracoda)

The ontogenetic development of Ruggieria lekkii, Ruggieria triangulata, Ruggieria martinsoni and Chrysocythere ornata (Ostracoda) was investigated on two different continental shelves (Congo and Senegal). For one species, the ontogenetic trajectories obtained through length and height of valves differed considerably in distinct areas. In the first case, trajectories showing well-separated development stages and slight intraspecific variability in length and height were seen. In the second, trajectories showing valves which are difficult to relate to one stage or another and a high intraspecific variability were noted. In the latter, the uncertainties in the determination of the development stages were resolved by using a statistical method. Furthermore, this allowed for the comparison of the variations in ontogenesis (augmentation or reduction in the development), in relation to abiotic (detrital supply and upwelling processes) and biotic ecological factors (faunal density and interspecific competition). Comparisons were also made with ornamental variations observed on Chrysocythere ornata, which are directly linked with bioprecipitation (environmental) conditions. This comparison highlights the role played by abiotic and trophic factors on the developmental variations of ostracods in some continental shelves areas. This work shows that ontogenetic variations depend either on biotic or abiotic factors. They can act in different ways on distinct species and one can be predominant in a particular biota. Once the architectural characters have appeared, their variations no more depend on ontogenetic development. The study of ontogenesis reveals the adaptation capacities of one species in an environment with variable conditions.