Biodiversity associated with Mediterranean fucalean forests and mechanisms regulating it: an assessment of the current knowledge

Forests formed by fucalean brown algae of the Cystoseira s. l. complex (genera Cystoseira, Ericaria and Gongolaria) are a typical feature of shallow Mediterranean seascapes. These seaweeds play a key role in shaping biodiversity on shallow rocky bottoms; due to their highly branched morphology, they increase tridimensionality and structural complexity of benthic assemblages, providing a high diversity of habitats for many sessile and mobile organisms. Studies concerning the biodiversity of Cystoseira s. l. forests have largely focused on the associated macroalgal and invertebrate assemblages. Inventories presented in these studies usually consist of floristic and/or faunal lists based on traditional morphological identifications. The majority of them was carried out at local spatial scales, i. e. an individual site or a few sites in a geographic area; Ericaria crinita is the species for which the largest sampling effort and geographical coverage is currently available. To date, information concerning the microalgal assemblages associated with Cystoseira s. l. is completely lacking. Knowledge of the microbiomes associated with Cystoseira s. l. is also extremely limited, despite of the role plaid by this component on the well-being of the macroalgae. Several hundreds of macroalgal and invertebrate taxa have been reported in association with Cystoseira s. l. species (up to 597 taxa recorded for Ericaria corniculata and E. brachycarpa), with substantial differences among species. For individual species, significant variation in associated biodiversity has been detected at regional scale. Experimental studies demonstrated that loss of Cystoseira s. l. canopies involves major changes in the structure of benthic assemblages. Density of the canopy and size of Cystoseira s.l. patches directly affect the number of associated benthic taxa. Future investigations should increase the number of Cystoseira s. l. taxa sampled and extend the geographic coverage (especially to eastern Mediterranean regions). Molecular data such as eDNA sequences should be incorporated in future studies. Finally, the microbiomes associated to Cystoseira s. l. forests should be investigated with more detail.