Ostreobium comprise endolithic algae commonly seen in conjunction with scleractinian corals. In the past, it was solely recognized as a coral skeleton bioeroder. Yet, their relationship with corals is critical because they give photosynthetic byproducts and help the coral when it loses its primary symbionts due to stress. The variety of these algae among coral species of the genus Porites in the tropical easter Pacific and western Atlantic was investigated here. We extracted Ostreobium samples from seven Porites species including two from the Tropical Easter Pacific-TEP ( P. panamensis, P. lobata) and five from the Caribbean (P . furcata, P. porites, P. colonensis, P. branneri, and P. astreoides). We also compared the new rbcL sequences from algae found within various coral species from other parts of the world. A biogeographic analysis and two methodologies, PTP (Poisson tree process) and GMYC (General Mixed Yule-Coalescent), were used to delineate the different species. The findings revealed a significant degree of genetic diversity within Ostreobium, with more than 15 groups of no more than three individuals and 40 individual lineages. Its origins date back to the Ordovician, 500 Ma, and it does not appear to preserve host-specificity. The sampled locations include a wide variety of Ostreobium still, biogeographically, varied patterns were confirmed, making it impossible to pinpoint the precise origins of most clades. The ancestry analyses revealed convergent events for the emergence of Ostreobium in a few genera of local corals, but the phenomenon also occurred in genera from far-off places.

Vastly understudied, mesophotic coral ecosystems lie below shallow reefs (> 30 m depth) and comprise ecologically distinct communities. Brooding reproductive modes appear to predominate among mesophotic-specialist species and may limit genetic connectivity among populations. Using reduced representation genomic sequencing, we assessed spatial population genetic structure (at 50 m depth) in an ecologically important mesophotic-specialist species, Agaricia grahamae among locations in the Southern Caribbean. We also tested for hybridisation with the closely related (but depth-generalist) species Agaricia lamarcki, within their sympatric depth zone (50 m). In contrast to our predictions, no spatial genetic structure was detected between the reefs of Curaçao and Bonaire (~ 40 km apart) within A. grahamae. However, cryptic taxa were discovered within both nominal species, with those in A. lamarcki (incompletely) partitioned by depth and those in A. grahamae occurring sympatrically (at the same depth). Hybrid analyses and demographic modelling identified contemporary and historical gene flow among cryptic taxa, both within and between A. grahamae and A. lamarcki. These results (1) indicate that spatial genetic connectivity in these ecologically important mesophotic species may be maintained over large geographic distances and (2) highlight that gene flow links taxa within this relativity diverse Caribbean genus.