Evolution in time lapse
Rats in New Zealand, parrots in Europe, starlings in America - humans are masters at introducing species into foreign habitats - often with devastating consequences. With no natural enemies, they descend upon the defenseless locals in droves. But the victims also quickly defend themselves.
Actually, they have no place on the American coast - but man made it possible: In 1817 he carried the shore crab Carcinus maenas from its original habitat in European or North African coastal waters to America and in 1988 the Asian shore crab Hemigrapsus sanguineus followed. Since then, the two have been attacking small animals in their new homeland, greedy as locusts – an enormous competition for the native crabs. H. sanguineus can easily consume 170 small mussels per square meter in one day under laboratory conditions. A hard life for young mussels!
But the little mussels are not completely defenseless against the invaders, as Aaren Freeman and James Byers from the University of Hampshire found out.
Because mussels have a basic defense strategy against predators: If they see signs in the water that crabs are living near them, they thicken their shells so that they are more difficult to crack and thus make it more difficult for the crabs to work. The crux: The mussels must first realize that danger is imminent.
The mussels already recognize the beach crabs, which have been living on the American east coast for more than a hundred years, and therefore form thicker shells when they are present. Freeman and Byers now wanted to know whether they could do the same with the Asian shore crab, which was only introduced 15 years ago.
This provided an opportunity to study two populations exposed to the predators for different lengths of time
(Aaren Freeman) The scientists took advantage of the fact that both newcomers live on the southern east coast of America, but only C. maenas further north. So the mussels of the southern coastal areas are already familiar with both crab species, while those in the north are only familiar with C. maenas. "This provided an opportunity to study two populations exposed to the predators for different lengths of time," says Freeman.
Freeman and Byers collected young mussels from both coasts, placed them in tanks in the laboratory, added water-soluble substances derived from either C. maenas or H. sanguineus, and allowed the mussels to grow. After three months, the mollusks from the southern coastal areas that thought they were threatened by the beach crab or the Asian coast crab had significantly thicker shells than reference mussels that had grown up without predators. Among the mussels that came from the northern coastal areas, only those who were led to believe the presence of C. maenas armed themselves against the predator. However, they showed no reaction to the unknown crab species H. sanguineus.
In a second experiment in the field, in which the scientists placed mussels in a cage in the sea together with one type of crab separated by a grid, the mussels reacted in the same way: the northern lights only recognized C. maenas and thickened their shells, while specimens from the southern areas defended themselves against both crab species.
The mussels have learned to recognize a new predator and to use a suitable defense strategy against it in what is an incredibly short time for evolutionary processes - only fifteen years. In a kind of turbo evolution, the mollusks react to the new enemy and fight back.
