Fig 1.
Three coral sampling sites are pictured to demonstrate the variability of reef environments across the island. On the western side, Butler Bay (photo by Corina Marks) has high visibility with a moderate reef slope. Cane Bay (photo by Kristina Black) is most notably defined by a “wall,” or a sudden cavernous drop-off adjacent to the north shore. Deep End (photo by Daisy Flores) is shallow and turbid. Some sites receive direct anthropogenic impacts from the capital Christiansted, which is highly developed, and the town of Frederiksted, where cruise ships bring tourism. The island also maintains two marine protected areas: Buck Island National Monument and East End Marine Park. Coastlines and district boundaries are plotted with GADM mapping data (https://gadm.org/index.html).
Fig 2.
Environmental clustering of 12 sites into “ecoregions”.
(a) Clustering of 16 environmental variables, after reducing multicollinearity. (b) Clustering of sampling sites based on those 15 environmental variables. Ecoregion is indicated by color and assigned to sites that cluster by environment. (c) Sampling sites and their ecoregion assignment on the map of St. Croix, USVI. Coastlines and district boundaries are plotted with GADM mapping data (https://gadm.org/index.html).
Table 1.
Sample size and genomic sites retained for six coral species.
Fig 3.
Identifying cryptic genetic lineages in six coral species in St. Croix.
(a) A. agaricites is composed of two distinct genetic lineages as shown in the hierarchical clustering tree (left) and PCoA (right) of genetic dissimilarities. (b) M. cavernosa contains six lineages, as confirmed by clustering with a larger sample set from Florida and the Gulf of Mexico. (c) O. faveolata contains two lineages, (d) P. astreoides contains two lineages, (e) P. strigosa contains four lineages, and (f) S. siderea contains three lineages.
Fig 4.
Spatial distribution of cryptic genetic lineages in six coral species from St. Croix.
Each pie chart represents the occurrence of distinct genetic lineages at each sampling site, and the size legend in the bottom right of each panel indicates the number of samples in each pie. Different lineages are indicated by color and correspond to points in PCoAs from Fig 2. Coastlines and district boundaries are plotted with GADM mapping data (https://gadm.org/index.html).
Fig 5.
Environmental drivers of cryptic coral communities.
(a) Estimated importance (cross-validation R2) of depth and four ecoregions to coral community composition across the seascape. (b) Top five out of 40 environmental predictors, based on cross-validation R2 in a gradient forest model. (c-d) Cumulative importance distributions show change in community composition along the range of the two most important variables. Tick marks on the x-axis indicate the environmental values at each site. (c) Coral communities demonstrate steep turnover around 5 meters deep, and (d) communities show graduate turnover across a gradient of yearly pH range. (d) Map of depths around St. Croix, using bathymetry data from GEBCO 2023 Grid (doi:10.5285/f98b053b-0cbc-6c23-e053-6c86abc0af7b). (e) Map of the maximum temperature (averaged from 2010–2022) and derived from interpolation of in situ monitoring data from the Virgin Islands DPNR. Coastlines and district boundaries are plotted with GADM mapping data (https://gadm.org/index.html).