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18Many studies into the responses of early life-stages to ocean acidification utilise offspring

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Significant deviations from the Hardy-Weinberg equilibrium in natural populations of bivalves may be due to a number of factors such as the Wahlund effect, null alleles and selection, and it is often difficult to decide which of these factors is operating. The use of laboratory controlled single crosses and mass matings, where the genotypes of the parents are known, can provide strong evidence for selection when deviations from expected genotype frequencies are observed in the offspring. Several cultures of offspring from a single cross and from a mass mating of the mussel, Mytilus edulis, were reared through the larval stage at either 32 or 25 parts per thousand salinity. At the early post-larval stage salinities were altered for some cultures while others remained unchanged. Samples were taken for electrophoresis as post-larvae and again as juveniles and were scored at the Lap, Pgm, Odh, Hex, Pgk and Gpi loci. Significant deviations from expected frequencies were observed at many loci in the offspring but these deviations did not appear to be related to salinity except at the Lap locus. The allele Lap94 was selected against at low salinity during the post-larval to juvenile stage, but no such selection occurred at low salinity during the larval stage. The data fit Koehn's (1985) model of post-larval selection at the Lap locus in mussels in Long Island Sound, USA, but suggest that there may be ontogenetic variation in fitness at this locus. Results at the Pgm and Gpi loci indicate that these loci are probably acting as markers for other loci on the same chromosome which have a strong effect on fitness. Density dependent factors may have caused some of the non-random mortality amongst offspring which was apparently independent of salinity.