Catching The Coral Cold

For many ocean dwellers marine diseases are on the increase, as climate change and direct human influence put animals under increasing stress. Many marine pathogens are found everywhere in the ocean, but don't infect their target until their host's immune system is compromised – something that is caused by changing environmental conditions.

With the current shift to both a warmer climate and warmer oceans, heat stress is has the potential to weaken the immune system of many marine animals. The rise in water temperatures also has the potential to propel pathogens to new areas, where they can infect populations that previously haven't been exposed to the disease.

The affect of heat stress on corals is well known: when these animals are exposed to temperatures just a few degrees above their optimum they bleach, i.e. they expel their symbiotic algae and die. If a coral colony is under more moderate heat stress its immune system can weaken, making it more vulnerable to infection. The pathogens that take this opportunity are known, aptly, as opportunists – and unlike other pathogens, opportunists can survive outside their host, biding their time until their host's immune system is down.

Prolonged exposure to high water temperatures in 1998 caused the most widely spread and severe bleaching event ever recorded and the large loss of coral may not have been due to bleaching alone. Why? Because under heat stress, pathogens take the opportunity to break through the host's defences, meaning some coral colonies may have been weakened by the heat and died of infection.

It's not only climate that can affect coral diseases, but also inputs to the marine environment. In the mid-90s purple spots plagued sea fans in the Caribbean and Florida Keys. These lesions in the coral tissue were caused by a fungal infection called aspergillosis. A terrestrial fungus had washed into the Caribbean basin, bringing new diseases into contact with the reef. This caused the mass mortality of many sea fans, leaving only those resistant to the disease behind. The sea fan population in the Florida Keys has since recovered due to these disease-resistant sea fans repopulating the reef.

Coral communities can also be affected when other animals are suffering from an outbreak of disease too. When 95% of the population of Diadema, the Caribbean urchin, died, many Caribbean reefs shifted from being coral dominated to being overgrown with algae. This is because urchins are important herbivores in reef ecosystems and their grazing prevents algae from taking over the reef.

Other diseases can also cause large ecological changes, such as White Band and White Pox Disease. These diseases affect some reef building corals and not others. When an outbreak of White Pox and While Band Disesase occurred in the Caribbean, the coral community changed from one dominated by Acropora, which is susceptible to these diseases, to one dominated by Agaricia, which is unaffected. The change in the reef community persists today.

We know little about how habitat degradation and pollution due to human activity affects many marine animal disease epidemics (a.k.a. epizootics) because coastal waters are affected by so many different pollutants and few studies have been done on habitat degradation. Corals are important in improving our understanding of the causes of marine disease because they are stationary, so can help pinpoint origins of diesease. We also have the benefit of being able to peer into the geological record to find out if large epizootics have occurred in the past. Over several thousand years, there were no shifts from Acropora to Agaricia in Belize, nor was there evidence of other species shifts due to disease suggesting that that the recent outbreaks are due to present environmental conditions.

With pathogens so prevalent in the marine environment and climate changing at an unprecedented pace, it's important for us to keep watch on coral reefs, work out what else is behind the outbreaks and prevent putting ecosystems under further stress.

Corals aren't the only creatures that can ‘catch a cold' – can you think of any other marine animals – or plants – that have been affected by disease outbreaks? How might the way diseases are transferred between fish compare to the way they are transferred between marine mammals?

References

Burge, C. A., Kim, C. J., Lyles, J. M., & Harvell, C. D. (2013) Special Issue Oceans and Humans Health: The Ecology of Marine Opportunists Microbial ecology 65 869-879.

Harvell, C. D., Kim, K., Burkholder, J. M., Colwell, R. R., Epstein, P. R., Grimes, D. J., Hoffmann, E. E., Lipp, E. K., Osterhouse, A. D. M. E., Overstreet, R. M., Porter, J. W., Smith, G. W. and Vasta, G. R. (1999) Emerging marine diseases-climate links and anthropogenic factors Science 285 1505-1510.

Ward, J. R., & Lafferty, K. D. (2004) The elusive baseline of marine disease: are diseases in ocean ecosystems increasing? PLoS Biology 2

Update: I didn't use this paper here, but there's a neat study on tracking coral disease using GPS in PLOS ONE – definitely worth checking out!

Images

1) Sea fan aspergillosis. The purple spots are characteristic of the disease and are caused by Aspergillus sydowii. Credit: Sierra S.

2) Bleached (foreground) and healthy (background) Acropora in the Great Barrier Reef. Credit: Wikimedia Commons user Acropora.

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