Finicky snails provide new clues to evolution of coastal ecosystems

Findings lead Brown postdoctoral research associate to advocate for conservation of intact communities, not just certain species.

Adapted from news released issued by Stanford News Service

An animal that appears barely essential to the plant and animal community at one site along the ocean shore may play a vital role in the ecosystem just a few hundred miles down the coast, finds a new study led by a researcher who is now based at Brown.

The discovery of distinct populations of the same species along a coastline has significant implications for the design and management of marine reserves.

“These results are a strong argument for conserving and maintaining whole intact communities, rather than focusing on certain target species,” said Eric Sanford (left), who led the study while a postdoctoral fellow at Stanford. Sanford is now a postdoctoral research associate in Brown’s Department of Ecology and Evolutionary Biology.

“Scientists and resource managers have to be careful in assuming that local studies of species interactions apply everywhere that two species co-occur.”

Sanford led a study, published in the May 16 issue of Science, which focused on a species of mussels that California snails eat willingly but Oregon snails shun. The scientists discovered that this preference is likely inherited, resulting from generations of genetic and geographic isolation along the Pacific coast.

“Increasingly we are seeing that many populations of marine species are isolated from, or only weakly connected to, other populations, and this increases the odds that natural selection has acted to modify species and species interactions at the local level,” Sanford said.

“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, professor of marine science at Stanford and co-author of the study. “As a result, a species that is relatively unimportant in one habitat may turn out to be very important in another.”

The researchers analyzed the eating habits of the channeled whelk – an inch-long snail commonly found in coastal waters from Alaska to California, and along the East Coast from Massachusetts to northern Florida. Whelks are voracious consumers of mussels, despite being much smaller than their prey.

A whelk drills through the shell of a mussel using a file-like tongue and acid secretions. When the hole is drilled through, a tube called the proboscis is extended into the shell, and the soft mussel tissue is ingested.

In one laboratory experiment, researchers fed whelks a diet consisting exclusively of sea mussels – an abundant species along the Pacific coast. Although California whelks eagerly preyed on the sea mussels, the vast majority of whelks from Oregon and Washington refused to eat them. These 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 said. “But in Oregon, sea mussels are a meal that few whelks will touch, preferring instead to prey on bay mussels, 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.”

Whelk eggs sit atop a mussel

In a follow-up experiment, the scientists raised whelks in the lab using egg capsules that 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 said. “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 said, 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.

Important evolutionary differences may determine what role a species will play in a coastal ecosystem.

“If you studied whelks in Oregon, you would conclude that, although they certainly affect some species, such as bay mussels, they probably do not play an essential role in the overall community,” said Sanford. In contrast, 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 out-competing barnacles, algae and other species for limited space on the shore.”