Penn biologists demonstrate that size matters... in snail shells

A team of biologists at the University of Pennsylvania has completed a research study begun in 1915 and determined that a snail making its home in the northwest Atlantic Ocean around Mount Desert Island, Me., has experienced a dramatic increase in the size of its shell during less than a century, providing a clear illustration of how fast and effectively change can occur.

The study is published in the current issue of the Proceedings of the National Academy of Sciences.

The most striking finding, which has not been reported previously in Nucella lapillus, the Atlantic dogwhelk, is that shell length increased at all 19 sites where samples were taken. Shell lengths of N. lapillus increased by an average of 22.6 percent during the past century, with no evidence of changes in other shell characteristics. The Penn team's results demonstrate that monitoring changes in shell morphology requires careful accounting of variation in local conditions, such as wave exposure, which can affect not only shell shape but also size.

Why have snails gotten bigger? Within the last century, the Gulf of Maine has experienced reductions in the size and abundance of native predators of dogwhelks, increases in ocean temperatures and invasions of new predators, and all three factors could have played a role. Overfishing of native predators of dogwhelks, such as fish, and increases in temperatures could have lowered mortality and increased growth, both of which would cause an increase in size. Also, arrival of new predators as invasive species could have selected for larger body size.

Changes in the shell architecture of marine snails enhance defences and greatly improve survival against predators. Stouter and thicker shells have been reported for N. lapillus and several other species following the introduction of predatory Carcinus maenas crabs early in the 20th century. Indeed, researchers hypothesise that when the snails are exposed to crab cues, shells of small snails first thicken and then, once defended against shell-crushing predators, grow in length to a size beyond the abilities of the crab.

N. lapillus is an important member of the North Atlantic marine ecosystems, and the results suggest that the impacts of historical changes in species' key morphological traits on marine ecosystems remain underappreciated. The dramatic increase in size, the researchers hypothesise, may give the snail an edge when preying on large mussels and barnacles, while protecting them from predators like crabs.

Comparisons were based on archived shells collected from 1915 to 1922 from sites that were resampled in 2007. Finding samples from 1915 proved to be easy, thanks to the work of Harold Sellers Colton. Colton, a professor of biology at Penn until 1926 and a charter member of the Ecological Society of America, collected about 12,000 shells from 107 sites in Maine, depositing them for safe keeping at the Academy of Natural Sciences of Philadelphia.

The Penn team discovered the body size change simply by accident, as they were investigating the occurrence of a shell-boring worm using the shells in the collections at the Academy and noticed they were much smaller than those collected recently.

'This increase in size is a stunning observation, especially since dogwhelks are one of the best known and most widely studied organisms in the North Atlantic Ocean,' said Peter S. Petraitis, professor in the Department of Biology in Penn's School of Arts and Sciences. 'Imagine that much change in the height of humans occurring between 1915 and 2007. In 1915, the height of the average Ivy League man was 5 feet 9 inches. A 23 percent increase would now make the average height slightly over 7 feet. If Penn had a baseball squad that averaged 7 feet tall, I am sure Penn would be quite well known and perhaps doing very well in March Madness.'

Petraitis joined with graduate and undergraduate students, as well as a field associate from Portland, Me., to collect modern samples of the intertidal snail.

From the samples collected from 1915 to 1922, researchers measured shell length, shell lip thickness, aperture length and aperture width of N. lapillus shells from 19 lots and then resampled the same 19 sites in 2007 by using Colton's site descriptions and maps of sampling locations. The team also classified the relative wave exposure at each site as exposed coast, semiexposed shore or sheltered cove, allowing the research team to test the influences of site, time period and wave exposure on N. lapillus shell morphology.