Assessing the diversity and species composition in macroinvertebrate assemblages thriving in shallow water macroalgal habitats: Structural complexity is not always better.

The assemblages of motile and sessile macroinvertebrates [mainly arthropods (peracarids), mollusks and annelids] were studied in three shallow macroalgal habitats from the NW Mediterranean Sea. Habitats were respectively dominated by three macroalgae: Ericaria mediterranea, a canopy-forming fucoid with repeatedly ramified cylindrical branches and spine-like projections; Cystoseira compressa, a canopy-forming fucoid with occasionally ramified compressed branches; and Ellisolandia elongata, a turf-forming, repeatedly branched, geniculate calcified alga. A total of 175,800 macroinvertebrate specimens, belonging to 228 taxa were collected. Macroinvertebrate assemblages in E. mediterranea habitat showed the highest abundances and number of taxa (104,412 specimens and 184 taxa), followed by those from C. compressa (49,128 specimens and 157 taxa) and E. elongata habitats (22,260 specimens and 144 taxa). Annelids were the most diverse group (43% of taxa) and amphipods the most abundant (78% of individuals). Macroinvertebrate abundances, alpha-diversity and species composition significantly differed among habitats and/or sites, while the number of taxa did not show significant differences neither among sites nor among habitats. Despite the simpler morphology of E. elongata respect to fucoid species, E. elongata habitat registered the lowest macrofauna abundance values but the highest alpha-diversity. Amphipods were the most abundant group in fucoid algae habitats (82-85%), while bivalves dominated in E. elongata one. Annelid polychaetes were the third more abundant group. Macroinvertebrate assemblages thriving in E. elongata habitat were always different to those found in fucoid algae habitats. Amphipods Protohyale schmidtii, Jassa herdmani, Jassa morinoi and Stenothoe gallensis were the most abundant species in fucoid algae habitats, while several species of Mytilidae, together with Caprella liparotensis, S. gallensis and nematodes dominated in E. elongata turfs. Differences observed are explained according to differences in algal morphological complexity and natural variability.