ECOLOGICAL AND PHYLOGENETIC CHARACTERIZATION OF TWO MACROBRACHIUM SPECIES IN TWO SELECTED RIVERS IN WESTAFRICA

Abstract

Aspects of ecology, biology, morphology and phylogenetic characterization of the two most popular Macrobrachium species endemic to West Africa were studied in two selected rivers (Ogun-Osun River, Nigeria and Volta River, Ghana) to update available scientific information needed for stock management and aquaculture production of these prawns. Observations revealed that M. macrobrachion and M. vollenhovenii were the two species of prawns foraging around the studied section of Ogun-Osun River Basin Channel, Nigeria while M. vollenhovenii was the only prawn population foraging around the designated upstream and midstream sections of the Lower Volta River Basin Channel, Ghana. Also, mixed population of Atya gaboniensis and M. vollenvovenii were prawn species observed around the downstream section of this river. Observations on river physico-chemical properties revealed mild seasonal fluctuations for the two rivers. However, there were no significant differences (p<0.05) in parameters values between seasons for each of these rivers while values ranged within recommended thresholds for surface water and aquaculture. River sediments at both study locations vary between sandy loam and sandy clay loam and exhibited weakly acidic pH.  Average water depth inhabited by these prawns connotes that they are sub-littoral in nature. Seasonality was observed to influence relative abundance of these two species as M. vollenhovenii was observed to exhibit higher relative abundance (53.38%) during dry seasons than rainy seasons while M. macrobrachion was higher in relative abundance (57.18%) during rainy seasons. Morphological indices employed in identification of the two prawns revealed variations in the number and arrangement of spines on the dorsal and ventral parts of rostrum and showed differences in anatomical traits on second pereiopods, telson and uropods as major morphological separations. Subsequently, the only distinguishing morphological character observed between specimens of M. vollenhovenii collected in Nigeria and Ghana was the length of rostrum as correlation analysis revealed negative correlation co-efficient (r) for rostral length comparism of the two species. Higher fishing pressure was recorded for Macrobrachium species around the study location in Nigeria as compared to Ghana as maximum total length recorded for M. macrobrachion and M. vollenhovenii in Nigeria ranged between 110mm and 120mm while M. vollenhovenii in Ghana had maximum total length of 155mm. Sex ratios analysis of these prawns revealed that seasonality does not influence sex ratio and that variable sex ratios were observed at different sampling periods. Seasonality was observed to influence condition factor (k) of the three prawn populations but had more effect on M. macrobrachion. Fecundity was observed to be significantly influenced by seasons, body size and age of prawns. Phylogenetic observations based on the S1358 RAPD primer investigation revealed that morpho-types of M. vollenhovenii collected from Nigeria and Ghana are similar. Furthermore, morpho-types of M. vollenhovenii and M. macrobrachion co-existing on Ogun-Osun River Basin, Channel, Nigeria, were observed to be morphological and phylogenetic different. In conclusion, the results of this study revealed great possibilities and potentials for easy domestication of these two prawn species.