Recently introduced Asian shore crab Hemigrapsus sanguineus (left) and the previously-introduced green crab Carcinus maenas (right). Photo: Aaren Freeman/University of New Hampshire.

Researchers at the University of New Hampshire (UNH) have found that native New England blue mussels (Mytlius edulis) have evolved to respond to a recently-introduced predatory crab species in as little as 15 years.

Crabs prey on mussels by crushing their shells, so blue mussels along the New England coast have evolved the ability to thicken their shells in response to the presence of green crabs (Carcinus maenas), which were introduced to the waters 150-200 years ago. The Asian shore crab (Hemigrapsus sanguineus), however, reached New Jersey shores only in 1988, and it has not yet extended its range 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,” said Aaren Freeman, a doctoral candidate and lead author of the study, in a UNH press release. “We wanted to know, how is it that these mollusks can recognize a crab that is historically not present in North America?”

The scientists found that southern mussels exhibit shell thickening in the presence of Asian shore crabs. Northern mussels do not react to Asian shore crabs.

The study extended over two years and was coducted separately in both in the field and the laboratory for confirmation. “The consistency over two years and two sites really suggests an underlying robust mechanism,” said James Byers, an associate professor of zoology and Freeman’s advisor.

This study has important implications for fishermen, who could perhaps protect the northern mussel populations from Asian shore crabs by mixing in some of the adapted southern mussels.

REFERENCES AND FURTHER READING:

Freeman, A.S., and Byers, J.E. 11 August 2006. Divergent Induced Responses to an Invasive Predator in Marine Mussel Populations, in Science, vol. 313, p. 831-833. [full-text, $]

Potier, B. 10 August 2006. Mussels Evolve Quickly To Defend Against Invasive Crabs. University of New Hampshire. [press release]