Secondary engineering of rhodolith beds by the sand tilefish Malacanthus plumieri generates distinctive habitats for benthic macroinvertebrates and fish

Whilst the importance of foundation species for biodiversity conservation has been largely acknowledged (e.g., coral and bivalve reefs, seagrass, kelp and rhodolith beds), the role of their associated species as secondary habitat engineers has just begun to be appreciated. Here, we evaluated whether secondary engineering of rhodolith beds by the sand tilefish Malacanthus plumieri influences the composition and β-diversity of benthic macroinvertebrates and reef fish assemblages. Our findings indicate that, by selecting, relocating, and rearranging rhodoliths into mounds, M. plumieri creates a distinctive habitat for macroinvertebrates and fishes. M. plumieri mounds increase fish abundance by 57% in rhodolith beds, with an 82% species turnover rate between mounds and non-mounded areas. In contrast, the macroinvertebrates in M. plumieri mounds are largely a subset of the species from non-mounded areas, with an 86% species nestedness rate. Despite decreasing the abundance of macroinvertebrates in the mounds by half, M. plumieri increases the heterogeneity and structural complexity of rhodolith beds, affecting the composition of associated fish assemblage at a larger spatial scale. Our results suggest that, by increasing the structural complexity of rhodolith beds and shaping their associated biodiversity, the abundance of M. plumieri mounds could be a useful proxy to define priority areas for conservation across the South Atlantic rhodolith beds, especially in the light of ongoing impacts related to offshore oil exploitation, overfishing and carbonate mining.