Studying the distribution and demography of marine species is challenging due to the open nature of the oceans. This latter was historically believed to facilitate extensive gene flow among populations when clear barriers to gene flow were not apparent. Gene flow between populations could be mediated directly by adult migration across large distances for reproduction purposes, feeding and habitat preference or via larval dispersal with the aid of ocean currents, in order to recruit to optimal areas for development and to increase survival. Hence, gene flow among localized populations across large geographic distances most likely results in weak or no genetic differentiation. This recognized model has changed in recent years, as limited gene flow has been demonstrated for many species even in the absence of physical barriers to genetic mixing.
To understand fish population dynamics and to manage marine resources sustainably, assessing the degree of population connectivity has become an important focus. It has been reported globally that many marine species are exposed to intensive fishing activities. One such species from the South West Indian Ocean (SWIO) region is the widespread fish species, Otolither ruber. Based on our knowledge, there are no genetic investigations conducted for this species to date. Hence, I studied the evolutionary and ecological processes influencing genetic diversity and the population genetic structure of O. ruber throughout the region based on mitochondrial and nuclear data analyses.
Before attempting large scale data generation, critical consideration should be given to marker selection. I was able to find the most suitable mitochondrial gene from three evaluated genes. The cytochrome b gene gave consistently good amplification and showed the most variation within and among O. ruber sampling locations. The advantageous cross-species amplification approach yielded seven microsatellites, previously developed in other sciaenid fishes, with relatively high levels of polymorphism in O. ruber.
Two distinct O. ruber populations in the north and the south of the SWIO region were identified based on statistical analyses of cytochrome b sequences and microsatellite genotypes. Low, unidirectional, female gene flow (based on mtDNA) and overall asymmetrical gene flow (based on nuclear DNA markers) were inferred from north to south between these evolutionary units. The isolation with migration model demonstrated a recent past population divergence and a low level of ongoing gene flow. The observed genetic differentiation in this shallow-water demersal sciaenid is mostly likely determined by the oceanography of the region, historical processes and life history traits, such as male-biased dispersal. The biology of the species in terms of reproductive and migratory behaviour should be further investigated to substantiate these findings.
The results obtained from this study contribute to improved biological knowledge of O. ruber and its genetic status in the SWIO region, and will be used for future studies undertaking a management assessment of O. ruber. The observed variation among these populations should be maintained by management interventions. The study will also add to comparative phylogeograpic studies of co-distributed species, particularly other sciaenids.