Biology Reference
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Scotia.This crab is a voracious predator on gastropod molluscs.The native
dog whelk ( Nucella lapillus ) was one of the common prey species encoun-
tered as the green crab spread north from the area of Cape Cod, Massa-
chusetts. Vermeij (1982) examined collections of dog whelks collected
before and after the arrival of the green crab from Massachusetts to Nova
Scotia. He noted that the frequency of repaired injuries to the shells of
whelks increased from 2.5% before the green crab's arrival to 4.4% after-
ward.Predation attempts on dog whelks had thus increased substantially
following appearance of the green crab. Furthermore,Vermeij found that
the thickness of the lip of the whelk shell and the height of the shell spire
were significantly greater in shell samples taken after the arrival of the
crab than before its arrival.Today, populations of dog whelks in areas with
green crabs possess more slender, heavier shells with a smaller aperture
and thicker outer lip than do populations in areas were green crabs are
absent. Shell shape in dog whelks has been shown to be under a strong
degree of genetic control (Funasaki et al. 1988).
Even more striking is the change in spire height and shell thickness of
Littorina obtusata in most locations in northern New England after the
arrival of the European green crab in about 1900 (Seeley 1986). Littorina
snails collected in 1982-1984 on the coast of Maine had much broader
shells and lower spires than those collected in the late 1800s. Shell thick-
ness was also much greater in the 1980s. Experiments in which snails with
shells of the two types were exposed to green crabs in natural habitats
showed that predation was much higher on the tall-spired, thin-shelled
animals. Laboratory tests also showed that green crabs could crush shells
of the vulnerable snails in less than a minute, but were successful in crush-
ing only a small percentage of the more resistant snails.
Seeley (1986) interpreted the change in shell morphology of Littorina
obtusata as direct natural selection on shell shape. More recently, Trussell
and Nicklin (2002) showed that change in shell shape in the presence of
the European green crab is an induced response. Littorina reared in the
presence of crabs show thickened shells, and when Littorina injured or
killed by green crabs are also present, induced thickening is accentuated.
Nevertheless, the strength of the induction response was apparently the
object of natural selection. In the northern Gulf of Maine, the green crab
has been present only about 50 yr, compared to about 100 yr in the
southern Gulf of Maine. Littorina in the southern Gulf of Maine produced
thicker and heavier shells than those in the north, even in the absence of
induction cues. Littorina in the northern Gulf of Maine showed variabil-
ity in the shell thickness response to simple presence of crabs two to three
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