Article: Mussels Evolve Quickly To Defend Against Invasive Crabs

Mussels Evolve Quickly To Defend Against Invasive Crabs

Scientists at the University of New Hampshire (UNH) have found that invasive
crab species may precipitate evolutionary change in blue mussels in as
little as 15 years. The study, by UNH graduate student Aaren Freeman with
associate professor of zoology James Byers and published in the Aug. 11
issue of the journal Science, indicates that such a response can evolve in
an evolutionary nanosecond compared to the thousands of years previously
assumed. The paper is called "Divergent induced responses to an invasive
predator in marine mussel populations."

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.

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."

And while its "real world" impact is not immediately obvious, Byers suggests
that perhaps northern Maine and Canadian shellfishers might consider
"beefing up the worldliness of their naïve mussel populations before the
Hemigrapsus arrives," he says, suggesting that this could be done by mixing
some of the responsive southern mussels into the naïve northern stocks.
"Although 15 years is fast to evolve better defenses to your predator, it
can be painfully long if you're a shellfisherman," Byers adds.

This paper is one chapter of Freeman's doctoral dissertation, which also
explores how mussels respond to sea stars and to multiple predators. He
anticipates completing his doctoral work by October 2006, when he will begin
a post-doctoral position with UNH research associate professor Fred Short.

Freeman notes that there's one predator mussels will not need to defend
themselves against: him. "I used to like them, before I started working with
them for my dissertation," he says. "Not anymore."

Source: University of New Hampshire
http://www.sciencedaily.com/releases/2006/08/060811091251.htm

Posted by
Robert Karl Stonjek


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