Reproduction and Population Structure in Temperate and Tropical Corals in Relation to Environmental Parameters

In the marine realm, two of the main stressors causing significant changes are ocean warming and acidification. Of particular concern are organisms reliant on the generation of calcium carbonate, such as corals. Reproduction and population structure are strongly related to environmental parameters and may be indicators for stability or decline of coral populations. Part of my research focused on the reproductive cycle of the Mediterranean non-zooxanthellate coral Caryophyllia inornata in relation to seasonal variations of seawater temperature and photoperiod. Moreover, an unusual embryogenesis was found in females, males and inactive individuals during the entire year, indicating a possible agamic origin of embryos. This was a pilot study for a broader investigation along a wide latitudinal gradient of temperature and solar radiation. Reproductive traits of this coral do not vary along the gradient as observed for another non-zooxanthellate species, Leptopsammia pruvoti, indicating that these species could be quite tolerant to environmental changes, probably due to the lack of symbiosis with the zooxanthellae. A study on the spermatogenesis of the zooxanthellate coral Balanophyllia europaea living along a natural pCO2 gradient showed no significant variations with low pH. We hypothesized that high CO2 levels probably enhance zooxanthellae photosynthesis, leading to an increase of the available energy for gonadal development. The ability to reproduce in particular conditions is related to population structure, providing information on coral responses to the environment. I investigated the population structure of the tropical coral Montastraea cavernosa along a depth gradient in Bermuda. Deeper populations were characterized by smaller but more numerous colonies compared to shallow ones, with no variation in the percent cover among depths. Thus, mesophotic populations of M. cavernosa in Bermuda seem quite stable, indicating that these reefs may serve as a source of propagules to maintain shallower reefs and help guide future management and conservation strategies.