Hidden Fish Assemblages in Mediterranean Posidonia oceanica Meadows Are Less Diverse and Abundant than in the Cryptic Spaces of Neighboring Habitats

The present research provides the first quantitative comparison of the hidden fish assemblages in Posidonia meadows and neighboring non-Posidonia habitats. The data and samples were collected at sixty sampling points at three locations on the south side of Brač Island in the eastern Adriatic Sea from October 2023 to June 2025. The gradient of a significant increase in fish abundance and average fish species richness in cuboids and the increase in the frequency of occurrence of fish species were observed from habitats inside Posidonia meadows, over the Posidonia meadow edge, to the habitats outside Posidonia meadows. The primary influence on the abundance was the rarity of species from the family Gobiidae within Posidonia habitats. The markedly different species composition between the Posidonia and non-Posidonia habitats was driven by the high species richness of the family Labridae in the Posidonia habitat compared to the high species richness of the family Gobiidae in the non-Posidonia habitats. The Posidonia meadow edge showed overlap with the two other habitat types, sharing a number of species. The sampling protocol developed in this study is suitable for the quantitative assessment of fishes inhabiting hidden Posidonia microhabitats and provides a methodological basis for future research. The current knowledge of fish in Mediterranean Posidonia meadows, as well as the conservation consequences of still limited knowledge, are discussed.