Conspicuous endolithic algal associations in a mesophotic reef-building coral

Understanding how corals and their symbionts specialize across depth gradients allows us to understand biodiversity in shallow and mesophotic coral ecosystems. Here we determined the prevalence of endolithic algal in Agaricia undata (17–83 m) and examined community changes within (shallow, upper and lower zones) and among sites (oceanic vs. continental siliciclastic influence). We observed exposed filaments of endolithic algae in some colonies, which in some cases surfaced the coral as tubular pipelines bridging A. undata costae. We also found multiple cryptic species within the monophyletic group of Ostreobium -like algae (12 rbcL types). Rarely explored as symbionts, Ostreobium in A. undata highlights its potential role in facilitating a broader depth range.