Variations in Total Species Richness and the Unevenness of Species Abundance Distribution between Two Distant Conus Communities (Neogastropoda): A Case Study in Mannar Gulf (India)

The genus Conus forms a conspicuous and rather homogeneous group within marine Gastropods. This makes it all the more interesting to focus on the sub-communities formed by Conus species and to analyze the potential specificities in the internal organization of species in these communities, in particular species richness, species abundance distribution and the effect of geographical distance between communities on differences in their respective species composition. Accordingly, two Conus communities along the coast in Mannar Gulf (India), separated by 80 km, are considered. Reliable analysis requires, first, to treat exhaustive data from complete samplings or, else – as here – to implement an appropriate extrapolation procedure to complete numerically the partial samplings. After numerical completion, substantial differences were highlighted between the two communities, not only in terms of true (total) species richness but, even more, as regards the profile and the average unevenness of the distributions of species abundance. Also, significant dissimilarity in species composition was found between the two communities, that may be tentatively attributed to either deterministic distance decay in similarity of species composition or, alternatively, to the persistence in the stochastic process of species recruitment from the regional stock of Conus planktonic larvae. This preliminary study yet requests to be complemented by other similar case studies, before drawing any safer interpretative conclusions.

Early growth, mortality, and partial migration of striped bass (Morone saxatilis) larvae and juveniles in the St. Lawrence estuary, Canada

After being extirpated from the St. Lawrence Estuary (SLE), striped bass (Morone saxatilis) were reintroduced in 2002, and by 2008 they were naturally reproducing. The increase of this reintroduced population prompted the need to document its nursery habitats utilization, which contribute to species recruitment. We analysed mortality-dispersion rates and the growth of larvae and juveniles in relation to their occurrence and migration patterns. From June to September 2014, we collected striped bass distributed in four estuarine habitats: the upstream freshwater section (UP), the oligohaline (O-ETM) and the mesohaline (M-ETM) estuarine turbidity maximum zone, and the downstream polyhaline section (DOWN). Based on otolith microstructure and microchemistry, 305 back-calculated growth and 36 migration trajectories have been reconstructed. The UP and the O-ETM provided optimal conditions wherein which larvae and juveniles exhibited fastest growth. In the SLE, we emphasized the co-existence of a freshwater resident contingent and two migrant contingents to the M-ETM and DOWN. We propose that migrants adopt an adaptive migration behaviour to avoid suboptimal conditions and strong intraspecific competition from resident in the upstream habitats. The potential advantage of a downstream migration later in the season might be an adaptative strategy to promote their survival during the early life stages.