Crown-of-thorns starfish are huge thorns in the sides of people trying to protect coral reefs. When populations of these coral predators get out of control, they can devastate reefs in dramatic fashion, especially if they are already struggling from pollution, overfishing and warming. Australia, for example, is spending many millions of dollars to control crown-of-thorns outbreaks on the Great Barrier Reef.
But these starfish aren’t the only animals that prey on corals. In fact, research is showing that scientists may be underestimating the role of much tinier predators – marine snails – on coral reef health.
If that’s true, though, it might be good news because it also means that controlling snail populations – and increasing populations of fish that eat snails – is one more way to boost the local health of coral ecosystems and help them survive climate change.
As reported in a study published last week in the journal “Ecological Applications,” one snail species, Coralliophila violacea, previously thought to be a “prudent” predator of corals – i.e. the snail’s appetite wasn’t enough to do much damage – turned out to be causing more havoc than anyone thought.
Coral researcher Mark Hay and a colleague, both at the Georgia Institute of Technology, studied these snails in Fiji as they preyed on a species of coral that is thought to be especially resilient to climate change and other stresses. Instead of being relatively benign, though, the snails were a bit like vampires, Hay said, or maybe like ticks.
Some predatory snails move around and chomp up corals, leaving visible scars and bites in their wake. But these snails were more subtle, attaching in one place and continuously sucked up the fluids in the coral polyps, which is why scientists have largely missed the harm they did. By conducting an experiment on corals with and without the snails attached, the researchers estimated that their presence reduced coral growth by a substantial amount: about 20 percent to 40 percent.
Like their more visible crown-of-thorns counterparts, there’s evidence these snail predators are part of a vicious cycle. The more overfished and degraded a coral ecosystem is, the more predatory snails there are – and the more snails there are, the worse the coral reef’s health becomes, until there’s not much left. “It’s kind of like a downward death spiral,” said Hay.
In Fiji, Cody Clements, a postdoctoral fellow in Hay’s lab, attached snails to corals in small marine protected areas and compared them to nearby areas that were heavily fished and covered in seaweed. When he later returned, he never found more than a few snails on a single coral colony in the protected area. But in fished areas, he found hundreds, likely because the fish that usually eat the snails were absent.
“Without the fish there to eat these [snails] when they’re little and keep them from building up on the coral, it makes it harder and harder for even the toughest coral,” Hay said. “We know that overfishing takes too many fish. We don’t adequately understand the indirect consequences.”
Duke University ecologist Brian Silliman, who has studied other predatory snail species in both coral reef and salt marsh ecosystems, said Hay’s work added further evidence that snails can have an outsized but under-appreciated effect on coral health.
“They are not big animals, but when they are superabundant, they can overwhelm their prey, just like insects can overwhelm plants,” he said. “It’s a thing that people tend to overlook.”
His research has also underscored this point. In a study published in June in the journal “Nature Ecology and Evolution,” Silliman found that when a common coral-eating snail was removed from corals in the Florida Keys as ocean temperatures were spiking in 2014, the corals were relatively protected from bleaching. Bleaching levels were only about 50 percent on corals where snails were manually removed, but on corals with high snail densities, nearly total bleaching occurred and recovery, months later, was far lower.
To both researchers, their experiments and observations offer hope that effective local actions can bolster one of the most imperiled ecosystems on the planet. That is a point that can be up for debate, Silliman noted, with some taking the view that local actions to save corals are futile unless greenhouse gas emissions are slashed to slow the rise in ocean temperatures that trigger catastrophic coral bleaching events.
Both suggested their research shows that small-scale efforts to cull snail populations on reefs may be effective. Already, reef managers around the world do snail removals, Silliman said.
Even more effective, though, is maintaining the populations of fish, such as triggerfish and puffer fish, that eat snails, he said.
In the area of Fiji where Hay has worked for more than a decade, marine protected areas (MPAs) where fishing is banned are tiny in size – but they are managed and fully enforced by the nearby villages. Residents who are found fishing in protected areas are publicly shamed at council meetings. Visitors are put on notice quickly, Hay said.
“For them, if you were going out in their water and started fishing without asking, it’s like you coming home and finding someone in your apartment eating your food. It’s no different,” he said.
The small, no-fishing areas have quickly made a big difference for the coral in them. Before fishing was banned in these areas, they were heavily degraded and now they’ve made huge comebacks, Silliman said, as the snail counts from his recent study show.
“At this particular place, MPAs are extraordinarily effective,” he said. “A lot of that is because they are so well-enforced. It’s easy to put out some buoys and say marine protected area. It’s really hard to stop people from fishing.”