Taxinomie et bio-écologie des crustacés décapodes dans le bassin versant du Nyong (zone forestière Sud Cameroun

Abstract

Decapod crustaceans are of high economic, ecological and health importance. These organisms are endangered by the degradation of the environment cause by various pollutions and the overexploitation of this resource. This biological group thus sees its diversity change and decrease over the years although it is poorly or little known in Cameroon. Few data is available on the diversity these organisms, systematics and even biology. Bases on the above reasons, it was essential to study the biodiversity of these organisms in the Nyong drainage basin in the forest zone of Cameroon. The purpose of this work is to assess the structural composition of decapod population in some rivers of the Nyong drainage basin, to determine their seasonal and longitudinal dynamics in this drainage basin, to evaluate the growth and reproduction of decapods and to assess the environmental conditions that influence the spatio-temporal distribution of decapods the drainage basin. The study was carried out from January 2017 to February 2018 in the Nyong and eleven of its tributaries. Sampling took place on a seasonal basis in 11 rivers, i.e. 3 sampling stations in each of tributaries and 5 stations along river Nyong making a total of 38 stations. The characterization of the habitat and the physicochemical analyzes were carried out following standard methods. The multihabitat approach was used for the specimens sampling using a hand net of 30 cm square and 400 μm mesh opening and the organisms were enumerated and identified under a binocular magnifying glass with the help of appropriate taxonomic and identification keys. Physicochemical analyzes reveal that the waters of the Nyong drainage basin are slightly basic to acidic (6.06 <pH <7.79 UC), moderately oxygenated (50% <O2 <70% dissolved oxygen), poorly mineralized ( 16.01 mg / l < electrical conductivity (<155.20 mg / l) and have low loads of organic matter (0.22 mg / l <oxidability <1.53 mg / L). this drainage basin is not very anthroponized and shows an good to very good ecological quality except for the high values of color and electrical conductivity at the Kongola river ( Kon 3 station) in the Mbalmayo area. A total of 4877 organisms were collected belonging to two Sub-Orders, Caridea and Brachyura, or 37 morphotypes of decapod were identified. Out of these, 30 species were Caridea shrimp of the genus Caridina (C. nilotica, C. africanus, Caridina sp.1 and Caridina sp.2), the genus Macrobrachium (M. latimanus, M. rude, M. niloticus, M. dux, M. macrobrachion, M. equidens, M. vollenhovenii, M. lepidactylus, M. idella, M. lanceifrons, M. rosenbergehii, Macrobrachium spp. (Comprising thirteen morphotypes), the genus Palaemon (Paleamon sp.) and the genus Automate (Automate sp.). 7 species of brachyuran crabs were identified among which 6 of the species belong to the genus Sudanonautes (S. africanus, S. faradjensis, S. aubryi, S. granulatus, S. chavanesii and S. floweri) and a species of the genus Potamonautes (Potamonautes sp.). The species C. africana was the most dominant (67.27%) in the entire drainage basin during the study period. The dominance of this species Caridina africana was observed in most streams except in streams of the littoral zone where dominance was reversed by Caridina sp. 2, Paleamon sp., M. idella in the Mibangue, Ongue and Nyong rivers respectively. In terms of diversity, the Nyong drainage basin is not very diversified with a very high abundance of Caridina africana (2.05bit / ind, P = 0.40). The diversity indices (H ’and P) increase from upstream to downstream of the Nyong drainage basin. Thus, the coastal zone is more diversified with a clear even distribution of species. Macrobrachium sp.8, S. faradjensis and S. aubryi species have isometric growth while M. equidens, and S. floweri have positive allometric growth on the other hand, S. africanus, M. vollenhovenii, Macrobrachium sp.3 and Macrobrachium sp. 4 have negative growth in the Nyong drainage basin. The M. equidens is the most prolific species (FT = 1643 eggs / g) compared to M. dux, Macrobrachium sp.3 and Macrobrachium sp.12 and its breeding period extends from mid raining season into mid dry season. M. dux and Macrobrachium sp.3 breed twice a year during the long dry season and the short dry season. The low fertility mean rates in M. dux, Macrobrachium sp.3 and Macrobrachium sp.12 suggest a case of early fertility in the juvenile state. Statistical analyzes (Canonical Correspondence Analysis and Spearman correlation tests) showed that physicochemical parameters (temperature, pH, alkalinity, dissolved oxygen, suspended matter, turbidity, nitrate, ammoniacal nitrogen), the nature of the substrate, the depth of the water and the type of microhabitats are the main environmental factors that influence the spatial distribution of the different decapod species in the Nyong drainage basin.