Coral Reef Diversity - Sometimes Less Is More
August 29, 2012

Inflexibility Of Coral Host Leads To Higher Resistance To Environmental Stresses

Brett Smith for - Your Universe Online

A general rule of Darwinian evolution is that diversity provides for a more robust population that is capable of withstanding a higher degree of stress than a more homogenous population.

However, researchers at the University of Hawaii at Manoa (UHM) have found that less diverse coral populations lead to reefs that are less sensitive to environmental disturbances, according to their report in Proceedings of the Royal Society B.

"This is exactly the opposite of what we expected," said lead author Hollie Putnam, a PhD candidate at UHM. "Our findings suggest more (diversity) is not always better.”

Coral reefs are the product of a symbiotic relationship between an animal host, called a polyp, and single celled algae (Symbiodinium) that live within the corals' tissues. This mutually beneficial relationship provides protection and a stable environment for the algae while the single-celled organism produces energy through photosynthesis. This energy is then used by the polyp to construct and reinforce the coral´s exoskeleton. Studies have shown that coral receive up to 90 percent of their energy from their algae, or endosymbionts.

"The relationship of coral species to their algal symbionts is fundamental to their biology," said David Garrison, program director in the National Science Foundation (NSF)'s Division of Ocean Sciences, which funded the research. "This study gives us a new understanding of how corals are likely to respond to the stresses of environmental change."

Corals can host different types of endosymbionts, but according to the latest study, this ability to host a variety of single-celled organisms does not transfer to a more robust reef population. To reach this conclusion, Putnam and other scientists from UHM obtained tissue samples from 34 species of coral in Moorea, French Polynesia.

The team was then able to survey the different types of Symbiodinium living within each coral based on a genetic analysis. This revealed that some coral species host a single Symbiodinium type and that others host many types that vary among individuals within that coral species.

They were also able to determine which coral species are more resilient to environmental changes and how that relates to the diversity of endosymbionts it can host.

"The corals we sampled spanned a range of environmental sensitivities from resistant to susceptible, and we were able to link, for the first time, patterns in environmental performance of corals to the number and variety of symbionts they host," reported Putnam.

The study results show that corals hosting diverse Symbiodinium populations tend to be more environmentally sensitive. Conversely, environmentally resilient corals typically hosted one or few specific types of Symbiodinium.

The research team said that future studies will attempt to identify the mechanism that transfers the differences in success between corals that are flexible and inflexible in their Symbiodinium associations. They also plan to widen the scope of their studies to understand this relationship on a much larger scale.

The health of Hawaiian coral reefs is an important topic that has many experts making plans to save and develop the fragile aquatic ecosystem. These plans include harvesting sperm and eggs from the animals for a possible future use.