Frontiers

Re-examining a Snail's Pace

Biologist Peter Petraitis and team discover rapid increases in shell size in the Atlantic dogwhelk.
August 2009

Nucella lapillus, the Atlantic dogwhelk, is one of the best known and most widely studied organisms in the North Atlantic Ocean. Yet until now, no one had noticed that over the past century shell lengths of these sea snails have increased in size by an average of 22.6 percent. That discovery was published this year in the Proceedings of the National Academy of Sciences by a team of biologists at Penn led by Professor of Biology Peter Petraitis and Jonathan Fisher, Gr’07. 

“Imagine that much change in the height of humans occurring between 1915 and 2007,” Petraitis says. “In 1915, the height of the average Ivy League man was five feet nine inches. A 23 percent increase would now make the average height slightly over seven feet.”

Petraitis explains that one possible reason this change went undiscovered for so long is that present-day researchers rely on online databases to search scientific literature, and most of these resources do not go back further than the 1950s. “There are plenty of references from prior to 1920 or so with data on the size of dogwhelks,” Petraitis says, “but you have to dig through the books and old journals to find it. Additionally, museums all over the world have old collections of all sorts of plants and animals, and these tend to be a resource that is overlooked by many modern ecologists.”

Thanks to a happy accident, Petraitis and his team were drawn to a particularly extensive set of dogwhelk samples collected by another Penn researcher nearly 100 years ago. Fisher had been investigating a marine worm that bores into the shells of N. lapillus when he found the collection at Philadelphia’s Academy of Natural Sciences. It had been compiled by Harold Sellers Colton, who was both a graduate of Penn and an instructor at the University until 1926. The collection contains about 12,000 shells that Colton gathered between 1915 and 1922 from 107 sites in Maine.

"Museums all over the world have old collections of all sorts of plants and animals, and these tend to be a resource that is overlooked by many modern ecologists." - Peter Petraitis

Fisher noticed that Colton’s samples were shorter than the present-day specimens he was used to. He and Petraitis—along with Penn undergraduates and a field associate from Maine—measured shell length, shell lip thickness, and aperture length and width of dogwhelk shells from 19 of Colton’s sites. They then collected modern samples from the same locations using Colton’s site descriptions and maps of sampling locations. The team also classified each site as exposed coast, semi-exposed shore or sheltered cove to test the influences of site factors such as wave exposure on the snails’ shell morphology.

Although it is currently unclear what is causing the increase in dogwhelk shell size, the researchers have a few theories. One possible reason is a faster rate of growth or a lower rate of mortality for N. lapillus now than when Colton’s samples were collected. “Mortality could be lower now,” Petraitis says, “because predators that feed on dogwhelks have been wiped out. This includes lobsters, cod and other fish that used to be very common in the intertidal zone but now have been fished out.”

Another possible cause is the presence of tributyl tin (TBT) in waters now but not in the early 1900s. This anti-fouling compound, used in bottom paint on boats, causes a kind of sterility in dogwhelks. Energy that the snail would have used to produce eggs could be going into growth, thereby making bigger snails. “The problem with this theory,” Petraitis says, “is that TBT has been banned for a while, and the effect should be markedly sex-specific to females.”

Shell length could also be increasing because of natural selection for faster growth due to the introduction of green crabs—a predator of small dogwhelks—to the snails’ environment.

This research shows just how quickly a species can change in a relatively short amount of time. The dogwhelk is an important member of North Atlantic marine ecosystems, and the researchers suggest that the impact on marine ecosystems of historical changes in species’ key morphological traits remains underappreciated.