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Finicky snails provide new clues to the evolution of coastal ecosystems
Whether sautéed in wine or steamed in the shell, mussels have long been a favorite of seafood lovers. For most people, the type of mussel served isn't important -- as long as there are plenty of them on the plate.
But in the wild, it's a different matter. According to a new study in the journal Science, when it comes to preying on mussels, marine snails are often pickier than people.
The study, led by scientists from Stanford University, focused on a species of mussel that California snails love to eat but Oregon snails won't come near. The scientists discovered that this culinary preference is probably an inherited trait -- the result of generations of genetic and geographic isolation along the shores of the Pacific.
The discovery of finicky snail populations on the Oregon and Washington coast could have profound implications for managing marine ecosystems worldwide, the researchers added.
"If you go down the coast from Canada to Mexico, you will find species that individually look the same but actually have undergone genetic adaptations to local conditions," said George N. Somero, the David and Lucile Packard Professor in Marine Science at Stanford and co-author of the Science study. "As a result, a species that's relatively unimportant in one habitat may turn out to be very important in another."
The new findings, published in the May 16 issue of Science, were based on experiments conducted at Stanford's Hopkins Marine Station, where researchers analyzed the eating habits of the channeled whelk (Nucella canaliculata) -- an inch-long snail commonly found in coastal waters from Alaska to California. Whelks are voracious consumers of mussels, despite being much smaller than their prey.
"A whelk drills through the shell of a mussel using its file-like tongue -- the radula -- and acid secretions," said Eric Sanford, lead author of the Science study. "When the hole is drilled through, a tube called the proboscis is extended into the shell, and the soft mussel tissue is ingested."
Sanford, now a research associate at Brown University, led the study while he was a postdoctoral fellow in Somero's laboratory at Hopkins.
In one experiment, researchers fed whelks a diet consisting exclusively of sea mussels (Mytilis californianus) -- a species that is abundant along the Pacific coast. The reaction was decidedly mixed: The vast majority of whelks from Oregon and Washington refused to eat the sea mussels, while California whelks eagerly preyed on them.
These laboratory results mirrored what the research team had already discovered in the wild.
"We found that, along the California coast, channeled whelks feed intensely on sea mussels," Sanford explained. "But in Oregon, sea mussels are a meal that few whelks will touch, preferring instead to prey on bay mussels (Mytilis trossulus), which are less common in California. Remarkably, in our experiment, we have found that these differences in predatory behavior appear to have a genetic basis."
Unlike many marine invertebrates, which release offspring into the open sea, channeled whelks attach their egg capsules to nearby rocks. This breeding behavior led Sanford to speculate that newborn whelks probably hatch and grow up within a few yards of where their parents lived. By staying close to home, whelk populations eventually would become isolated from one another, he proposed, resulting in dozens of genetically distinct communities of whelks up and down the West Coast.
To test this hypothesis, Sanford collected whelks from 13 sites along a 900-mile stretch of the Pacific -- from Southern California to the northern tip of Washington State. He then brought the whelks back to the lab, where DNA analysis confirmed that very little interbreeding had taken place among the 13 populations sampled.
"When we did the genetics, we found that the snails are a single species that is reproductively isolated into separate populations," he said. "Generation after generation has lived and died, isolated on their own particular stretch of rocky coastline."
If all channeled whelks belong to the same species, then why is there such a sharp difference in the feeding behaviors of northern and southern populations? Do whelks born and raised in the Pacific Northwest inherit their dislike for sea mussels, or is it a learned behavior?
To find out, the scientists decided to raise baby whelks in the lab using egg capsules Sanford had collected from eight sites in California, Oregon and Washington.
"Once the juvenile snails hatched from their capsules, we raised them on a common diet of bay mussels until they reached adult size about 10 months later," he recalled. "We then tested whether these lab-reared whelks would drill sea mussels -- a species that they had never encountered before."
The results were dramatic and virtually identical to what had been observed in the field: 75 percent of snails raised from California eggs drilled into the shells of sea mussels, compared to only 7 percent of the Oregon and Washington hatchlings.
"Since the snails were raised under identical laboratory conditions, this strongly suggests that the differences in drilling behavior have a genetic basis," Sanford concluded, noting that these behavioral differences may have evolved in response to regional differences in prey availability. Bay mussels are scarce in California, but sea mussels are plentiful. Therefore, natural selection would favor California snails that feed on sea mussels.
The discovery of distinct whelk populations along the Pacific coast has significant implications for the design and management of marine reserves, Sanford noted.
"Is a snail just a snail wherever it is found, or are there important evolutionary differences that determine what role it will play in a coastal ecosystem?" he asked. "If you studied whelks in Oregon, you would conclude that, although they certainly affect some species, such as bay mussels, they probably don't play an essential role in the overall community."
In contrast, he noted, the California whelk is a predator on sea mussels -- a vital member of the rocky shore community: "If not held in check, sea mussels often are the dominant competitor -- overgrowing and outcompeting barnacles, algae and other species for limited space on the shore."
Earlier studies have shown that sea mussel populations are kept under control by another rapacious predator -- the ochre sea star (Pisaster ochraceus).
"The sea star is a keystone species," Sanford explained. "As long as it's there, you'll have diversity. But if you take the sea star out, then the sea mussels can move downshore and crowd out other species living on the rock."
If sea star populations were suddenly reduced by disease or some other disturbance, would the channeled whelk fill the void and help keep sea mussels in check?
To find out, the research team placed sea mussels in mesh cages specially designed to exclude sea stars, then attached the cages to intertidal sites in California and Oregon. After nine months, whelks had discovered and drilled nearly 20 percent of the caged mussels in California.
"In contrast," the authors wrote, "not a single mussel was eaten in Oregon, despite mean whelk densities that were seven times greater than in California." Given the slow growth and low supply of young sea mussels in California, the authors concluded that whelks could partially or completely fill the sea star's predatory role in California -- but not in Oregon.
"A species that seems only marginally important in one community may turn out to be an essential player in a community just a few hundred miles down the coast," Sanford concluded. "Thus, scientists and resource managers have to be careful in assuming that local studies of species interactions apply everywhere that the species co-occur. These results are a strong argument for conserving and maintaining whole, intact marine communities, rather than focusing on certain target species."
Other coauthors of the Science study are Melissa S. Roth, a former Stanford undergraduate now with the U.S. Fish and Wildlife Service; Glenn C. Johns, a Stanford postdoctoral fellow; and John P. Wares, a postdoctoral fellow at the University of California-Davis.
This study was part of the Partnership for Interdisciplinary Studies of Coastal Oceans (PISCO) -- a consortium of marine scientists from Stanford, Oregon State University, the University of California-Santa Barbara and the University of California-Santa Cruz funded by the David and Lucile Packard Foundation. Additional support was provided by the Andrew W. Mellon Foundation, the National Science Foundation and the Undergraduate Research Opportunities program at Stanford.
By Mark Shwartz