Staghorn coral (Acropora cervicornis) is found throughout the Caribbean. It forms an important part of the Florida Keys Reef Tract, which is the third largest living coral reef in the world and the only one in North America.
Coral reefs are under threat from ocean warming and acidification, which can result in coral bleaching and increased susceptibility to disease threats. There are several strategies for the conservation of coral reefs including banking coral sperm to preserve valuable genetic material that one day might be re-introduced to the reef, bringing new genes to the population. SEZARC is working with the Coral Restoration Foundation in Key Largo, USA to collect and freeze sperm from genetically diverse staghorn coral genotypes as part of a national effort to preserve genetic material from corals from the Florida Reef Tract.
The coral spawn once a year, about three days after the full moon in August, releasing gamete bundles that can be collected. After bundle breakdown, the sperm are separated, concentrated and frozen over liquid nitrogen before storing in a liquid nitrogen dewar. Once frozen, they can be preserved for hundreds of years or until the genetic material is needed back into the coral population.
SEZARC is investigating the optimal time for coral sperm cryopreservation after spawning. In vitro fertilization studies revealed declines in sperm quality over the course of the night, once the bundles have released the sperm and the eggs, and recent transmission electron microscopy studies demonstrated sperm intracellular changes that start to occur soon after spawning. This suggests a narrow window exists for optimal fertilization, so sperm cryopreservation before the sperm become too aged is critical.
Transmission electron micrograph of a transverse section through a staghorn coral sperm.
L Penfold, J Wyffels, K O’Neill, A Moura. (2022). Reprod., Fertil. and Dev. 34(2) 234 – 324.
Another strategy for coral conservation is large-scale production of coral embryos for larval propagation to help repopulate reefs. Larval restoration may be conducted in nurseries or may include seeding mass-produced larvae directly onto the reef. Advantages of larval propagation include the opportunity to select specific genetic genotypes, effectively ensuring maximum genetic diversity that may enable increased rates of adaptation and resilience to climate change.
In spite of the fact that larval propagation is a widely used strategy, less than 50% of larvae settle to form coral polyps. Maximizing the efficiency of coral larvae propagation and settling rates would increase available numbers of coral for reseeding the reefs. Previous studies have revealed that embryo development, the precursor to larval development, is affected by several factors including sperm concentration. Sperm concentrations that are too high can result in lower embryo development rates, possibly due to polyspermy, so the incubation time allowed for fertilization of oocytes by sperm could be critical to control.
SEZARC received an Association of Zoos and Aquariums Conservation Grants Fund award to study the timing of staghorn coral fertilization following co-incubation of eggs and sperm. This work will provide information toward generating the best embryos for coral propagation and will develop baseline data to compare against potential future environmental impacts on coral gametes and fertilization. SEZARC is collaborating with the Coral Restoration Foundation, University of Florida, and Smithsonian’s Conservation Biology Institute on this important project.