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 Mytilu s edulis/Mytilus galloprovincialis populations, Croyde Bay and Whitsan d Bay, in S.W. England. Genotype-dependent length-at-age values were d etermined. At high and low shore locations at both sites, M. edulis-li ke mussels had significantly smaller length-at-age values than M. gall oprovincialis-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 th an M edulis-like individuals. Genotype frequencies as a function of ag e were determined and relative viability coefficients estimated from c omparisons of genotype frequencies of young versus old mussels. At hig h and low shore locations at both sites, the relative viability coeffi cient of M. galloprovincialis-like individuals was greater than that o f M. edulis-like mussels. Putative F1 hybrids at both sites had relati ve viability coefficients intermediate between those of the parental t ypes. These data indicate that the length-dependent variation in alloz yme frequencies that characterizes sympatric populations can be attrib uted to a small but significant genotype-dependent difference in lengt h-at-age values, but mostly to large and highly significant difference s in viability.