Previously Overlooked “Coral Ticks” Weaken Degraded Reefs

July 27, 2018

A previously overlooked predator— a thumbnail-sized snail—could be increasing the pressure on coral reefs already weakened by the effects of overfishing, rising ocean temperatures, pollution and other threats. The snail attacks a key coral species that may offer the last hope for bringing back degraded Pacific reefs.

The snail damages coral by sucking fluid from it like a tick, and may have been ignored because it camouflages itself on reefs and doesn’t move around to leave obvious signs of its attack. In experiments done directly on Fiji Island reefs, scientists quantified the impact of the snails, and found that snail attacks could reduce the growth of Porites cylindrica coral by as much as 43 percent in less than a month.

Scientists at the Georgia Institute of Technology conducted the research and reported it July 26 in the journal Ecological Applications. The research was supported by the National Institutes of Health, the National Science Foundation and the Teasley Endowment to Georgia Tech.

“Once the reefs are down and nearly out, these snails are piling on,” said Mark Hay, a Regents and Teasley professor in Georgia Tech’s School of Biological Sciences. “The Porites coral is kind of the last man standing, the last hope for some of these reefs coming back, and they are the ones these snails selectively prey on. As you get fewer and fewer corals, the snails focus on the fewer and fewer of these colonies that remain. This is part of the downward spiral of the reefs.”

In areas protected from fishing, Postdoctoral Fellow Cody Clements never found more than five of the creatures – whose scientific name is Coralliophila violacea – on a single coral colony. But on degraded reefs where fishing was permitted, he found hundreds of the snails on some declining coral colonies, as much as 35 times more than colonies in the protected areas. To assess the damage, he devised an experiment to measure how the snails affected coral growth.

On the reefs near Votoa Village on Fiji’s Coral Coast, Clements isolated coral branches and attached snails to them. After a period of 24 days, he compared the growth of snail-infested coral branches to comparable branches that had no snails. During that three-week period, the predators reduced coral growth by approximately 18 to 43 percent, depending on snail size.

“A single snail can do a considerable amount of damage,” Clements said. “They are sucking the juice out of the coral. If you have a lot of snails feeding on a single coral colony, it can be very hard for the colony to thrive.”

In coral ecosystems, fish help keep many predators and seaweeds under control. For that reason, fishing i