Macrofaunal assemblages structure three decades after the first report of the invasive Crassostrea gigas reefs in a soft-intertidal of Argentina

The macrofaunal assemblages of a soft-bottom intertidal area invaded by the Pacific oyster Crassostrea gigas (Thunberg, 1793) were studied at the site where reef formation was first reported for the Atlantic coast of South America (Los Pocitos; 40° 25′ 59.5″ S, 62° 25’ 23.2” W). Seasonal random sampling (winter, spring and summer) was conducted on oyster reefs and the nearby bare sediment and cordgrass areas. The habitat showed different structures with a wide size range of available spaces for macrofaunal colonization. Fifty-three macrofaunal species were recorded. Thirty-five percent of them were native, 3% were cryptogenic, 5% were exotic, and 54% need further taxonomic determination. All of the exotic species were found in reefs; though the amphipod Monocorophium insidiosum (Crawford, 1937) also settled on soft and cordgrass sediments. Species assemblages changed across seasons. Diversity was higher in reefs than in cordgrass or bare sediments during the warm season. There were no differences in the proportion of trophic groups. Our results suggest that C. gigas restructures the macrofaunal species assemblages and that this effect is driven by seasonality. In Los Pocitos, oyster reefs currently show a primary role as a suitable habitat for many species, but the net impacts of the invasion of this ecosystem engineer, including the cascading effects on the food web or the facilitation of further invasions remain unclear. The mechanisms and net effects of this restructuring force need further attention.