Distribution and population dynamics of key ascidians in North Carolina harbors and marinas

Ascidians have successfully invaded marinas and harbors around the world. Despite broad knowledge of their global ranges, in some locations, including the state of North Carolina, U.S.A., little is known about the community composition and distribution of native and introduced ascidians. We conducted field surveys at 16 harbors and marinas along the coast of North Carolina (33-35 °N) and documented the diversity, distribution and relative abundance of all ascidian species. Ascidians were identified using morphological observations and barcode sequencing of the mitochondrial gene cytochrome c oxidase subunit I. Distribution patterns of native and introduced ascidians were analyzed using presence-absence and relative abundance matrices in relation to latitudinal position (South versus North) and geographic distance among harbors. Finally, we monitored the dynamics of a well-established ascidian community at Wilmington over 1.5 years using photo transects. For each ascidian species, we calculated percent cover and abundance and then related those values to temperature fluctuations using cross-correlation analyses. Overall, we found three introduced, two cryptogenic, and eight native ascidian species. Geographic location and distance between survey sites had no effect on ascidian community composition in terms of presence-absence of species. However, the relative number of individuals per species present at each harbor was significantly related to the distance between sites. The ascidian community at Wilmington (three native, one cryptogenic, and one introduced species) showed some seasonality, with abundance and/or percent cover significantly correlated with temperature values recorded during the same month or a few months beforehand.