Red mangrove (Rhizophora mangle) roots serve as additional valuable habitat for the eastern oyster (Crassostrea virginica) in a subtropical estuary

Abstract

Occasionally, two foundation species co-exist within seascapes, where one is nested within, or near, the other. However, we often lack an understanding of interactions between co-existing foundation species and how such interactions may affect their demographic rates. Oysters and mangroves are foundation species that often co-occur in subtropical estuaries, with oysters assembled into reefs found near mangrove-rimmed shorelines or oysters growing attached to mangrove prop roots. We evaluated demographics and potential ecological mechanisms affecting eastern oysters (Crassostrea virginica) living on intertidal reefs compared with those growing on red mangrove roots (Rhizophora mangle) using a series of field surveys and by measuring evaporation rates as proxy for desiccation. We found that total oyster density on mangrove roots was over two times higher per unit sample area compared to intertidal reefs. This difference occurred despite slightly higher oyster settlement rates to reefs than mangrove roots, suggesting higher post-settlement mortality in reefs compared to roots. Higher oyster mortality on reefs was consistent with our observations of more predators, higher percentage of gaping oysters, and greater abiotic stress on oysters on reefs versus mangrove root habitats. The comparative demography of oysters on reefs and mangrove roots provides insight into mechanisms by which mangrove roots represent valuable habitats for eastern oysters. Moreover, these findings suggest that future oyster restoration efforts may benefit by considering restoring oyster reefs in close proximity to red mangroves given their additional source of potential oyster settlers.