Role of fish herbivory in structuring the vertical distribution of canopy algae Cystoseira spp. in the Mediterranean Sea

Biological interactions can strongly influence the distribution of organisms. In the Mediterranean Sea, macroherbivore densities vary with depth in a highly predictable manner, but the direct effects of grazing on the vertical distribution of macrophytes remain largely unknown. Species of the genus Cystoseira provide a unique system in which to test the effects of herbivores on algal distribution in the Mediterranean Sea, since they form highly prominent forest-like assemblages along the entire sublittoral zone, with different species dominating the canopy at different depths, We measured the algal cover of Cystoseira spp. in habitats characterised by dominance of these algae at different depths, and quantified the density of herbivorous fish at each habitat. The shallow sublittoral (ca. 5 m) was the only habitat where abundant Cystoseira forests coexisted with high densities of herbivorous fish. Reciprocal in Situ feeding experiments showed that Cystoseira spp. from wave exposed shores (0 m) and deep habitats (40 m) are highly susceptible to being eaten when placed within reach of herbivorous fish and are, thus, restricted to spatial refuges with locally reduced herbivory. In contrast, Cystoseira spp. from shallow rocky bottoms (5 m) and wave sheltered shores (0.2 m) are resistant to consumers and are not affected by transplantation to areas of high fish densities. Our findings challenge the widely accepted view that herbivorous fish have only a modest effect on structuring algal assemblages in temperate waters and show that these consumers can have a particularly important effect on the vertical, distribution of algal assemblages in the Mediterranean Sea.