Scientists at the University of New Hampshire (UNH) have found that blue mussels that call a vast swath of the New England shoreline home seem to have evolved a defense response to an invading species of crab in the very short time scale of 15 years:
Freeman looked at the inducible defense – shell thickening – of blue mussels (Mytlius edulis) in the presence of two invasive crab species in New England, the Asian shore crab Hemigrapsus sanguineus and the green crab Carcinus maenas. While Carcinus was introduced to New England from Europe between 150 and 200 years ago, Hemigrapsus is a relative newcomer, arriving from Asia to New Jersey in 1988. While previous research had established that mussels recognize Carcinus, it had not be determined if they recognize Hemigrapsus. And, crucial to the design of Freeman’s study, Hemigrapsus is not present north of mid-coast Maine.
“This set up a chance to look at populations that had been exposed to the predators for varying lengths of time,” says Freeman. “We wanted to know, how is it that these mollusks can recognize a crab that is historically not present in North America?”
Freeman exposed mussels native to the northern – above mid-coast Maine – and southern New England to both Carcinus and the Hemigrapsus. Both populations thickened their shells when exposed to waterborne cues of Carcinus, but only the southern mussels – Freeman describes them as “more worldly” – expressed inducible shell thickening in the presence of Hemigrapsus.
“The mussel’s inducible response to H. sanguineus reflects natural selection favoring the recognition of this novel predator through rapid evolution of cue specifity or thresholds,” Freeman and Byers write.
Findings were consistent in two experiments over two years, one in a laboratory setting in Nahant, Mass., and one in the field at Woods Hole, Mass. “The consistency over two years and two sites really suggests an underlying robust mechanism,” says Byers, who is Freeman’s dissertation advisor.
Sure it’s a small step to go from not recognizing to recognizing a new crab species as dangerous, but Evolution works in small steps. It’s not at all surprising to scientists that this change came about—indeed Evolution would predict such a change—it’s the speed in which the change happened that’s amazing.
While this sort of rapid evolutionary response to predators has been exhibited in some other species, all have been vertebrates. The blue mussel, which Freeman describes as the lab rat of marine biologists, is an invertebrate “that people assume is not very bright,” he says. Yet his findings indicate that within the brief span of 15 years, it has evolved an inducible response to a new predator.
How do mussels evolve so quickly? In southern New England, the scientists say, mussels are prey to many crabs as well as other marine species. “When Hemigrapsus came along the mussels’ wheels were well-greased to respond,” says Byers. “That’s our best guess.”
Byers helps put the impact of the research in context. Because extensive data does not exist on invasive ecology, “there’s a tendency to extrapolate any data you get on an invasive species. But here we show that the response from the prey differs over just a couple hundred kilometers.”
Natural selection at work. In environments with the new crabs the mussels evolved to deal with that threat. Of course this will do nothing to appears the IDiots who will demand to know why the mussels didn’t evolve into a tree or Volkswagen and then chuckle smugly to themselves thinking they said something frighteningly clever.
Found via Skepchick.