Diversité, phylogénie et écologie des Monogènes parasites des poissons du genre Synodontis Cuvier, 1816 (Siluriformes - Mochokidae) dans quelques bassins hydrographiques du Cameroun et impact sur la santé des hôtes

Abstract

Human societies are facing with a great challenge: providing food and livelihoods for a population which will largely surpass nine billion by mid XXI century. FAO (2014) estimates that populations are increasingly consuming fish and are highly dependent on this sector. The low abundance and small size of fishes caught in capture fisheries show depletion of natural stocks due to overfishing. In this context, fish farming can ensure continuous growth in the supply of fish for human consumption, but is threatened by fish mortality partially due to parasitism. On the other hand, parasites can play an important role in ecosystem equilibrium. Monogenea appear to be among parasite groups capable of causing great losses in fish populations, but also good hosts markers due to their often narrow specificity. We undertook this research as part of the improvement of fisheries production dynamics of populations of the Monogenean gill parasites of Synodontis fishes, which are good candidates for domestication due to their wide distribution, great diversity and great economic importance. The present work was done in the watersheds of Dibamba River, Sanaga River, Nyong River and Boumba River. Five fish species belonging to genus Synodontis have been harvested. Fish were caught by gill nets or angling. They were then kept refrigerated or fixed in the field in a 10% formalin solution according to the study later targeted. Each fish was then measured, weighed and their sex determined. Gill arches were removed by dorsal and ventral sections, and then placed in a Petridish containing tap water. The parasites were dislodged from the gill filaments with the aid of a dissecting needle. Monogeneans were fixed between slides and cover slips into a drop of glycerin ammonium-picrate mixture for systematic studies, introduced in 95% alcohol for genetic studies or fixed between slides and cover slips in a drop of water for ecological studies. Parasites in 95% alcohol were amplified, then their DNA sequenced. The resulting DNA sequences were cleaned up and used to construct a consensual phylogenetic tree. The systematic studies led to the description of ten new species of Monogenes belonging to genus Synodontella as well as the redescription of Synodontella melanoptera. The Principal Component Analysis made from the sclerotized parts of these species shows that parasites are different mainly by the length of the ventral and dorsal hooks. The other ten parasite species found only on the gill filaments of one host specieare oioxenous, while Synodontella melanoptera parasitizing the gill filaments of three different host species of the same genus is mesostenoxenous. Synodontella, Protoancylodiscoides and Schilbetrema appear as belonging to a monophyletic group. Horseshoes-shaped makes it possible to separate Synodontella species from Cameroon in two different subgroups. This difference can be explained by a long isolation period of the hosts, living in two different basins, followed by the divergence of the parasite populations (vicariant speciation). During the study of the population dynamic of monogeneans gills parasite of Synodontis rebeli from the downstream of Sanaga River, 257 fish specimens were exanimated and we found 3683 monogeneans belonging to three species ie: Synodontella melanoptera, Synodontella sanagaensis and Synodontella extensiocirrus. All the three species presented an aggregate distribution. Their mean intensities vary from low to very low. Synodontella melanoptera and Synodontella sanagaensis are dominant while Synodontella extensiocirrus is rare, satellite or dominant, depending on the host capture period. The mean intensities of the monogeneans gill parasite of S. rebeli increase during the rainy season and with the size of the hosts. Parasitism by Synodontella melanoptera, Synodontella sanagaensis and Synodontella extensiocirrus does depend neither on the sex nor on the host gill side, but increases with specie richness. In the natural environment, Monogeneans gill parasites of S. rebeli do not seem to cause damage to their hosts, but these parasites can greatly reduce the weight of their hosts in fish farming.