Exploring the acclimation to depth of <i>Posidonia oceanica</i> comparing the morphological variation to the histo-anatomical characteristics of the leaves

<i>Posidonia oceanica</i> is a marine plant that plays an important ecological role in marine ecosystems. Understanding the eco-physiology of this plant is fundamental for planning conservation and restoration actions covering this species and its habitat. The aim of this study was to explore the acclimation of <i>P. oceanica</i> to depth by examining the leaf morphological variation in a depth gradient. We measured leaf length, width and thickness, as well as the distance of fifth and sixth ribs from the margin and the central rib at different depths, comparing leaves morphology and histo-anatomy. Leaf length was the only morphological character clearly related to depth, whereas the overall morphological variation seems to be due to a change in the histo-anatomical characters. Depth-related characters showed a different variation pattern from that of not depth-related ones. Our results do not fit the model used to explain the seagrass responses to light variation; most morphological leaf traits are not directly linked to the acclimation of this species to depth. Finally, we hypothesise that the variation pattern of not depth-dependent characters is an expression of <i>P. oceanica</i> meadow genetic structure along the depth gradient.