The Impacts of Boring Sponges on Oyster Health across Multiple Sites and Tidal Heights

Abstract

Clionaid boring sponges (Cliona sp.) may affect individual oyster growth and condition while destroying oyster habitats, which can impact restoration efforts and aquaculture for eastern oysters (Crassostrea virginica). Since both oysters and boring sponges are affected by similar abiotic variables (i.e., salinity, tidal exposure), this study explored the potential trade-offs between these abiotic variables and sponge presence on growth, condition index, and survival of oysters using a fully-factorial in situ experiment across 3 sites in coastal GA, USA. Across all sites and tide heights, oysters with sponge grew 20% slower and had 10% lower condition index than oysters without sponge. Savannah State, the site with the lowest salinity (mean ± SD = 24.5 ± 2.1, range 20–28), exhibited growth rates that were 1.4–1.7 times faster than the other two sites, with even sponge-infested oysters exhibiting higher growth rates than uninfested oysters at the other two sites. In addition, growth rates of sponge-infested oysters were reduced by 26% relative to uninfested individuals in the low intertidal tier, but only by 12% in the upper intertidal zone, suggesting the negative impact also varies with tidal location. Therefore, oysters exposed to air for prolonged periods and placed in areas with polyhaline salinity exhibit fast growth and high relative condition index, although salinity may be confounded by other variables (food availability, fouling organisms). This study highlights the importance of considering site and tidal location for oyster restoration and culture.