Étude bioécologique des dictyoptères semi-aquatiques dans le bassin versant de la Mefou (région du centre, Cameroun)

Abstract

Water is a habitat, a nutrient, a means of production, transport of goods and is the basis of life. Aquatic ecosystem is well structured milieu which matter and energy exchanges occur due to the ecological interactions that are established between living organisms and their environment. Furthermore, they harbour a fauna and flora that provide information on its quality and the various phenomena that occur there. Therefore, the community structure of benthic macro invertebrate communities is the most widely used biological group for assessing the overall health status of aquatic ecosystems. Nevertheless, this bio evaluation has many shortcomings in the Cameroonian context where several groups of living organisms even though present, are not included in the water quality assessment scale. This is the case of semiaquatic Dictyoptera which colonise several forest hydro systems, although poorly documented. In order to contribute to the elaboration of biological tools enabling the evaluation of the ecological health status of Cameroonians hydrosystems, a descriptive, ecological and biological study of the semi aquatic Dictyoptera of few forest watercourses in the watershed of Cameroon. Mefou was conducted. This study was carried out from January 2016 to January 2017. Hydro morphological, granulometric and physicochemical analysis were performed according to conventional methods. Sampling of benthic macroinvertebrates was done by the multihabitat approach using a square shaped flask with a mesh size of 400 μm mesh. The results of the hydro morphological and granulometric analysis present variations in the sedimentary composition and structure, indicating the heterogeneous distribution of the substrate. The physicochemical quality of the water reveals that turbidity, electrical conductivity, suspended solids, colour, dissolved oxygen, ammonia nitrogen, nitrate and nitrite ions, orthophosphate and pH are in the range of good quality waters. This observation is in agreement with the Organic Pollution Index (IPO), which indicates a level of pollution that varies from low to none in all sub-watersheds. A total of 143 samples were obtained for a total of 9647 individuals amongst which 2002 dictyopterans. The observation of the larvae enable us to show the presence of swimming bristles on the internal surface of the legs and two rows of respiratory stigmata at the base of each abdominal segment which is an indispensable element for aquatic life. The dissection of the sub-genital plaques of the males revealed the structural variations of the genitals. Three morphotypes have been described. The "black" morphotype is characterized by a black coloration, an asymmetrical sub genital plate bearing a single style, a large right phallomere with a short, blunt prepuce, a median scleritis and a spiral left phallomere. The morphotype "brown" has a maroon colour, a genital symmetrical plate with two short and thick styles, a large right phallomere, with an elongated and pointed foreskin, a medial filiform scleritis and a spiral left phallomere. In the morphotype "brown", the plate under the genital is asymmetrical and carries a style. Its right phallomere is small and carries a short foreskin, its median scleritis is slightly curved and its left phallomere spiral. Functionally, the dictyopterans and the other sympatric benthic macroinvertebrates such as Perlidae, Ephemeridae, Hydropsychidae and Ephemerellidae present similar affinities and prefere watercourse with high levels of dissolved oxygen and a substrate large in size. Physicochemical and biological analysis show that the watercourses prospected are of good ecological quality and harbour a diversified dictyopteran fauna whose harmonious cohabitation with the macroinvertebrates indicators of good water quality will enable the construction of biological models of being use in the evaluation of the state of health of forest watercourse in Cameroon.