BIOLOGICAL AND PHYSICAL FACTORS INFLUENCING GENOTYPE-DEPENDENT MORTALITY IN HYBRID MUSSEL POPULATIONS

Biological (predation) and physical (wave action) factors influencing mortality of Mytilus edulis and M. galloprovincialis mussel types within hybrid and pure populations were investigated with the aim of identifying possible selective factors responsible for observed length- and age-dependent genotypic variation. In laboratory tests, shore crabs Carcinus maenas did not significantly prefer any type when choosing between mussels from a hybrid population. When presented with edulis and galloprovincialis from allopatric populations the crabs expressed no significant preference for either mussel type. Neither crab sex nor crab size significantly affected mussel type chosen. Dogwhelks also did not express a significant preference for mussel type when feeding upon mussels of a hybrid population, but did exhibit significant preference for edulis over galloprovincialis from pure populations. Mussel strength of attachment (SOA) to the substrate was tested in 2 hybrid mussel populations, Croyde and Whitsand in SW England. Shell length and genotype, but not population, explained significant variation in SOA. At all lengths, edulis-like mussels had a significantly lower SOA than galloprovincialis-like mussels. Overall, these results suggest that neither predator is likely to be responsible for the higher frequency of galloprovincialis in larger and older mussels of mixed populations. However, the SOA data indicate that this physical factor, which is related to site-specific wave action, is correlated with genotype-dependent mortality. It is concluded that the strong negative correlation between edulis frequency and length (and also age) in many hybrid populations results partly from a slight galloprovincialis growth advantage (measured by in situ growth experiments), but mainly from selective mortality of edulis individuals.