biodiversity; coralligenous reef; infralittoral; macroalgae; Mediterranean Sea; spatial variability.

Coralligenous reefs are calcareous structures edified mostly by coralline algae that characterize the circalittoral zone of the Mediterranean Sea. However, in some cases coralline algae can constitute peculiar infralittoral biogenic reefs which have been studied less than the circalittoral ones. This study aims to contribute to the knowledge of infralittoral coralligenous reefs by describing their macroalgal assemblages on a large rocky platform off the Tuscany coasts, northwestern Mediterranean Sea. To this end, a multifactorial sampling design was used to describe the structure of the assemblages and to evaluate the variability of the system at multiple spatial scales. A total of 71 macroalgal taxa were found on the coralline algae. Macroalgal assemblages were well structured, with high biodiversity values. The dominant taxa included both photophilous and sciaphilous species, guaranteeing peculiar characteristics in these assemblages, above all if compared with the typical infralittoral and circalittoral macroalgal communities of the same geographic area. The assemblages showed greater variability at a small and intermediate spatial scale than at a large scale. Although infralittoral coralligenous outcrops constitute a peculiar system, they are still poorly understood and should not only be the object of specific studies but also be included in monitoring programmes.