Meiofauna is an important, yet often overlooked, component of biodiversity in the ecosystem formed by Posidonia oceanica

The ecosystem formed by the marine flowering plant Posidonia oceanica is a biodiversity reservoir and provides many ecosystem services in coastal Mediterranean regions. Marine meiofauna is also a major component of that biodiversity, and its study can be useful in addressing both theoretical and applied questions in ecology, evolution, and conservation. We review the meiofaunal diversity in the meadow ecosystem of P. oceanica by combining a literature review and a case study. First, we gathered records of 672 species from 71 published studies, as well as unpublished sources, highlighting 4 species exclusive to this ecosystem. Eighteen of those studies quantified the spatial and temporal changes in species composition, highlighting habitat-specific assemblages that fluctuate following the annual changes experienced by these meadows. Hydrodynamics, habitat complexity, and food availability, all three inherently linked to the seagrass phenology, are recognized in the literature as the main factors shaping the complex distribution patterns of meiofauna in the meadows. These drivers have been identified mainly in studies of Copepoda and Nematoda, and their effect may depend ultimately on species-specific preferences. Second, we tested the generality of these observations using marine mites as a model group, showing that similar ecological preferences might be found in other less abundant meiofaunal groups. Overall, our study highlights the high diversity of meiofauna in meadows of P. oceanica compared with algae and sessile macrofauna associated with this seagrass and shows the complexity of the interactions and habitat use by meiofauna associated with the seagrass.