GENOTYPE-DEPENDENT FECUNDITY AND TEMPORAL VARIATION OF SPAWNING IN HYBRID MUSSEL (MYTILUS) POPULATIONS

Mussel samples were collected at 4 to 6 wk intervals throughout 1987 from two hybrid Mytilus edulis/M. galloprovincialis populations, at Croyde Bay and Whitsand Bay, in southwest England. These were analyzed at two polymorphic loci which are diagnostic for allozyme differences which typify M. edulis and M. galloprovincialis. Dry mantle weight as a function of shell length was determined for all individuals of each sample. Size-frequency data for the two populations was obtained in September 1987 and March 1988. For all genotypes at both sites, fecundity was a function of shell length, and in both populations the frequency of M. galloprovincialis alleles was positively correlated with shell length. At both sites, allozyme genotype explained a significant amount of variation in mantle weight either when assessed as a main effect or when assessed as an interaction with shell length or time of collection. At Croyde, M. galloprovincialis mussels had greater estimated fecundity per unit length than the M. edulis mussels. Differences in the timing of spawning activity between the M. edulis and the M. galloprovincialis mussels were inferred, and these differences might act to reduce the amount of interbreeding at Croyde. At Whitsand, a reduced level of variability in the timing of spawning activity and fecundity between the genotypes was observed and explained by a higher degree of genetic mixing. Because the M. galloprovincialis mussels had (1) a greater estimated fecundity at any length, and (2) a greater mean length than the M. edulis mussels, the mean genotypic annual fecundity per M. galloprovincialis mussel was 2.8 times greater than an individual M. edulis mussel at Croyde, and 2.2 times greater than an individual M. edulis mussel at Whitsand. This evidence that M. galloprovincialis mussels have an advantage in fecundity, and thus perhaps in fertility, taken together with the evidence that M. galloprovincialis also has a higher viability, indicates directional selection in favour of the M. galloprovincialis phenotype. Because of the observed temporal stability of the population it seems likely that this selection is counterbalanced by a massive imigration of M. edulis spat from neighbouring populations. © 1990 Springer-Verlag.