All you Need is Love… and Gametes
(February 24th, 2016) A run of bad luck brought the elkhorn coral in the Caribbean Sea close to extinction. But scientists have come up with unusual ways to save the marine invertebrates.
The decay of the elkhorn coral, Acropora palmata, in the Caribbean Sea is material for a Greek tragedy. During the 1970s, an infection, known as white-band disease, struck the population and killed many of the important reef-builders. A. palmata probably would have recovered nicely because the long-spined sea urchin, Diadema antillarum, would have kept the competing macroalgae in check. Except that an unidentified pathogen wiped out 93% of the urchin population between 1983 and 1984. You may think that’s still okay because macroalgae are also controlled by herbivorous fish. Except that we overfished them. One could only hope that at least the weather would have mercy on poor A. palmata. Weeeell, a couple of hurricanes thought otherwise. The surviving corals had some good days after the storms, which ended when they faced predation by corallivorous snails and the infamous damselfish. The elkhorn coral was left with no option but to enter the IUCN critically endangered list of species in 2008. But the coral’s story could have a happy ending, after all.
As A. palmata plays an essential role in the Caribbean ecosystem and can’t be replaced by any other coral, marine biologists have been working on ways to beat the hurricanes, the human over-exploitation of the Caribbean Sea and, of course, the infamous damselfish.
“Coral gardening” is one such strategy. Small fragments from coral colonies are grown in nurseries and returned to the reef once they have reached the right size. This, however, is clonal expansion. Each new colony is genetically identical to the others. In evolutionary terms, that’s not a good strategy for surviving infections. Come the “right” pathogen and the entire population will be destroyed, since all individuals are equally vulnerable.
Valérie Chamberland et al. looked for a different strategy, a strategy that guarantees genetic diversity. Just like Mother Nature, they decided to go for meiosis. Instead of scrapping coral colonies, they collected the gametes that A. palmata releases synchronously three days after the full moon in August. Using carefully designed nets that surround the spawning colonies (see photo), eggs and sperm are caught, brought to the lab, and fertilised in vitro. Coral embryos developed into swimming larvae within days and eventually settled onto clay substrates designed for their transfer into the wild. In September 2015, the laboratory-bred corals released their own eggs and sperm along with the endogenous reef population. For the first time, Chamberland et al. successfully raised laboratory-bred A. palmata colonies to sexual maturity. It only took them four years and involved institutions from Curaçao, Germany, the Netherlands and the USA.
“We now need to take the next step forward to apply our findings on a larger scale in Curaçao and elsewhere in the Caribbean”, explains Dirk Petersen (from Rotterdam Zoo), co-author of the study and founder of SECORE (SExual COral REproduction), a non-profit organisation, dedicated to coral reef conservation.
Finally, there’s a good glimpse of hope for Acropora palmata! Maybe these scientists should consider working with pandas, too…
Photos: Paul A. Selvaggio