Invasive saltwater snails, including dreaded oyster drills, seem to be far more leery of predators than native snails under certain conditions, according to a new study by Emily Grason, whose research earned her a doctoral degree from the University of Washington.
Why non-native snails in Puget Sound would run and hide while native species stand their ground remains an open question, but the difference in behavior might provide an opportunity to better control the invasive species.
Of course, snails don’t actually run, but I was surprised to learn that they can move quite rapidly to find hiding places when they believe they are under attack.
Like many marine animals, snails use chemical clues to figure out what is happening in their environment. For her experiments, Emily created a flow-through system with two plastic shoeboxes. Chemical clues were provided in the upstream bin, while the reaction of the snails was observed in the downstream bin.
The most dramatic difference between native and non-native snails seemed to be when ground-up snails were deposited in the upstream bin, simulating a chemical release caused by a crab or other predator breaking open snail shells and consuming the tender morsels inside.
Confronted by what appears to be an attack on their own kind, the non-native snails moved to a refuge area in the box, while the native snails did nothing. The natives apparently were waiting for more clues of an actual attack. The same pattern was noted among multiple species of native and non-native snails. Why were the non-natives so skittish?
Emily, who I met while covering invasive species for the Encyclopedia of Puget Sound, points out that native snails have evolved among ever-present threats from a variety of predators and environmental conditions. They refuse to disrupt their essential feeding and reproduction behaviors unless they perceive the threat as high risk.
Invasive snails, on the other hand, don’t take as much risk when confronted by smells that lead them to think that snails of their own kind are being devoured by predators. The strategy seems to work for them, because these non-natives not only survive but thrive under conditions that are foreign to them — at least when they first arrive in Puget Sound.
One of the key questions is whether the invasive species arrive in their new territory with this wariness trait or develop it through natural selection after they arrive, Emily noted. The next step in the investigation would be to run Emily’s experiment in the home territory of the invaders. For oyster drills, that would be Japan or the East Coast of the United States.
“If they arrived here with this response pattern, could that be an indication that they are better invasive species?” Emily asked, suggesting that a test like this might be used to predict the success of a new invader when it first arrives or even before.
Non-native species become invasive species when they are able to out-compete natives in some way, particularly when they disrupt the local food web and cause economic damage. Often, it is because the invaders either have no major predators in the new place or because the invaders’ newfound prey have never been forced to develop defenses against them. Successful invaders may also be the best at escaping unknown predators in their new location.
“The non-native snails show up and they are just neurotic enough, and a bit wary, and that actually helps them survive in certain situations,” Emily said in a UW news release by Michelle Ma. “You end up with invasive snails that hide at the right time, even if they don’t know what the predator is. And that’s exactly what happens when snails show up in a new spot; they are surrounded by predators never encountered before. General wariness might keep them alive.”
Emily, who published her findings in “The American Naturalist (PDF 801 kb),” experimented with six different scenarios that provided different chemical clues coming from the first bin — including a hungry crab, a well-fed crab and so on. Results were somewhat mixed among the various native and non-native snails, but the smell of the crab alone did not elicit much response from either native or non-native snails, while the crab was a factor for some natives, especially when combined with the smell of injured snails.
I asked Emily if her research could be used to battle invasive oyster drills, which create a hole in an oyster shell and digest the living oyster inside. Oyster drills can have devastating effects and create serious economic losses in commercial oyster beds. One idea that could be tested, she said, is the use of crushed snails as a type of “scarecrow” to cause oyster drills to hide.
“We have some evidence that invasive snails not only hide more but they reduce their feeding rates,” she said, wondering if her findings could play out on a real-life beach with tides and wide-open spaces. Perhaps crushed oyster drills could be placed in bags located in strategic locations in an oyster bed.
Bill Dewey of Taylor Shellfish Farms told me that oyster growers who work in infested areas normally wait for oyster drills to congregate together in the spring for mating and forming egg masses. That’s when it is easiest to collect them by hand, he said. While one would not want to disrupt that pattern, perhaps the scarecrow idea could be used at other times of the year to disrupt predation by the snails.
Emily’s presentation on her doctoral dissertation is both informative and entertaining, as these things go. (See the video on this page.) Her talk helps explain the ecological context of invasive species and suggests why such invasions may or may not be successful.