Shell-shape and morphometric variability in Mytilus galloprovincialis from micro-tidal environments: responses to different hydrodynamic drivers

Tidal conditions differently influence inter-tidal organisms in terms of generalphysiological and metabolic responses. In this study we investigated the mor-phological response in shells of Mytilus galloprovincialis native to differentmicro-tidal coastal environments in the Northern Adriatic Sea. Our purposewas to highlight the ecophenotypic variability across tidal levels and to eluci-date how tidal currents and waves produced by anthropogenic activities mayplay a part in modulating shell morphology. Three sampling sites were selected:an open-sea area 15 km off-shore and two sites within the lagoon of Venice,the first near one of its three inlets, and the other one in the proximity of Ven-ice city centre. At each sampling site, organisms were seasonally collected atdifferent depths within their vertical zonation, either in the inter-tidal zone –i.e. at both the highest and lowest tide zonation limits, and subtidally. Themussel shells were analysed by investigation of their morphometric relation-ships (height/length and width/length ratios) and by elliptic Fourier analysis ofthe shell contours. Shell thickness and condition index were also evaluated fora better comprehension of energy allocation/partitioning. Estimates based onlong-term measurements, visual observation, wind statistics and wave growthlaws allowed an evaluation of the forces acting on shells. At the open-sea site,the observed phenotypic variability of both shell shape and thickness wasclearly related to the tidal vertical zonation. At the two lagoon sites, the cur-rents generated by tidal flow through the inlet and the waves caused by the fre-quent passage of boats influenced both shell shape and thickness. A trade-offbetween protection and growth was apparent along the tide gradient, asemphasized by the differences in the partitioning and allocation of energybetween shell and flesh production.