PHILADELPHIA - 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 defenses 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 hypothesize
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University of Pennsylvania