There are lots of ways to make a living as a clam, but probably one of the strangest is to be a “living drill bit.” Some species of clams are able to bore into solid rock or concrete—creating a burrow in a substance that is harder than their own shells.
Studying these clams, however, scientists noticed something odd about their evolutionary patterns. When an organism breaks into a new niche, it often results in a burst of new species; but even though “living drill bits” repeatedly evolve over the course of history, they never seem to flourish. Scientists suspect they aren’t the only example—and it may have implications for our understanding of evolution as a whole.
The study by David Jablonski, the William R. Kenan, Jr., Distinguished Service Professor of Geophysical Sciences at the University of Chicago, along with Stewart Edie at the Smithsonian and Katie Collins at the Natural History Museum in London, was published online in the Proceedings of the Royal Society B.
‘Boring but never tedious’
Scientists have catalogued about 200 species of clams that can bore into hard surfaces. Some are attracted to coral reefs or wood (causing problems for navies throughout history), but others head for solid rock.
Some clams release chemicals to burrow into wood, coral or soft limestone. But harder stone like granite requires a different approach. These clams generally start with a small crack or crevice as newly settled larvae, and slowly wedge their way in, bracing their bodies against one edge and levering their shell to chip off bits of rock as they grow. Some even trap bits of rock or hard minerals in the shell, increasing the abrasion they can bring to bear. The end result is a burrow that resists wave turbulence and most predators.
For years, Jablonski’s lab has studied bivalves – the category that includes all clams, such as scallops, mussels, and cockles -- as a way to understand the evolution of species over time, uncovering clues about the forces that shape bodies and lifestyles over time.
For this study, he worked with Edie and Collins to catalogue all the known species and fossils of these “boring” clams.