Benthic scavenger assemblages along a depth gradient: megafauna-macrofauna divergence

Benthic scavengers mediate rapid carrion removal and nutrient regeneration in soft-bottom shelf ecosystems. Despite their importance, depth-related patterns, distribution and ecology remain under-characterized. This study assessed community assemblages along a soft-bottom depth gradient by deploying baited traps targeting megafauna and macrofauna at 30, 50, and 70 m depth in the Coast of Viana do Castelo (NW Iberian Peninsula). Megafaunal assemblages varied significantly with depth, and the response influenced the pattern observed: abundance-based structure separated 50 m from 30 to 70 m, whereas biomass-based structure separated 30 m from 50 to 70 m. Depth effects were driven by the dominance and abundance of the Nassariid gastropod Tritia reticulata at mid-depths and by the biomass of the swimming crab Polybius henslowii at shallow sites, with Conger contributing substantially to deeper-water biomass. In contrast, macrofaunal assemblages did not differ significantly across depths and were overwhelmingly dominated by lysianassid amphipods, and cirolanid isopods. These results provide a depth-resolved baseline for soft-bottom scavenger biodiversity in North Portugal and underscore complementary insights from abundance versus biomass when interpreting faunal responses to environmental gradients, as the metrics can decouple along environmental gradients, and of using abundance for size-constrained macrofauna.