Overfishing Caused An Increase of 'Coral Ticks' That Harm Coral Reefs

A small, overlooked snail could be contributing to the demise of coral reefs.

Georgia Institute of Technology scientists studied how the Coralliophila violacea snail might further weaken already damaged reefs. The study, published in Ecological Applications on Thursday, found that the snail could reduce the growth of an important coral species, Porites cylindrica. The Porites coral can provide a foundation for reefs, as it's less likely to be damaged by seaweed and other threats.

"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," Mark Hay, a professor at Georgia Tech and author on the paper, said in a statement. "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."

The coral reefs are already weakened by pollution, rising ocean temperatures, invasive species and other factors. This snail, which ranges from about half an inch long to nearly 2 inches long, sucks fluid from the coral, hence why they're referred to as "coral ticks."

The scientists found a distinct link between the number of fish in an area and the number of coral in Fiji's Coral Coast. On a single colony in areas where fishing is not permitted, an author on the paper, Cody Clements, never found more than five snails. In areas where fishing is allowed, he found 35 times more snails on the coral.

Overfishing affects coral reefs because fish are known to keep seaweed and predators under control. To see how the larger amounts of snails affected the coral, Clements attached snails to isolated coral branches. He compared these branches to branches with no snails after 24 days. Depending on the snail size, the coral's growth reduced by 18 to 43 percent.

Turtle and Coral Reef
Tourists snorkel near a turtle as it looks for food amongst the coral in the lagoon at Lady Elliot Island north-east of the town of Bundaberg in Queensland, Australia. The coral reefs are weakened by pollution, rising ocean temperatures, invasive species, and other factors. DAVID GRAY/REUTERS

Clements, who used needle-nose pliers to remove more than 2,000 of the snails while conducting his research, confirmed that the lack of fish was causing an increase in snails by tethering snails to reefs in areas that allow fishing and those that don't. The snails in the areas that don't allow fishing were eaten, likely by triggerfish and other fish with teeth, while those in areas that allow fishing were not eaten. In areas that are protected from fishing, 220 percent more snails were preyed upon than in areas that allow fishing.

"A single snail can do a considerable amount of damage," Clements said in a statement. "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."