SHELL GROWTH AND VIABILITY DIFFERENCES BETWEEN THE MARINE MUSSELS MYTILUS-EDULIS (L), MYTILUS-GALLOPROVINCIALIS (LMK), AND THEIR HYBRIDS FROM 2 SYMPATRIC POPULATIONS IN SW ENGLAND

Mussels were collected at high and low shore locations from two Mytilus edulis/Mytilus galloprovincialis populations, Croyde Bay and Whitsand Bay, in S.W. England. Genotype-dependent length-at-age values were determined. At high and low shore locations at both sites, M. edulis-like mussels had significantly smaller length-at-age values than M. galloprovincialis-like and putative F1 hybrid individuals. The putative F1 hybrids exhibited length-at-age values between those of the parental types, but much closer to those of M. galloprovincialis-like rather than M edulis-like individuals. Genotype frequencies as a function of age were determined and relative viability coefficients estimated from comparisons of genotype frequencies of young versus old mussels. At high and low shore locations at both sites, the relative viability coefficient of M. galloprovincialis-like individuals was greater than that of M. edulis-like mussels. Putative F1 hybrids at both sites had relative viability coefficients intermediate between those of the parental types. These data indicate that the length-dependent variation in allozyme frequencies that characterizes sympatric populations can be attributed to a small but significant genotype-dependent difference in length-at-age values, but mostly to large and highly significant differences in viability.