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Genetic differentiation in the Chilean blue mussel Mytilus chilensis (Hupé 1854) was investigated based on the variation in the allozyme frequencies of Pgm, Gpi, Icd, Me, Gsr, Lap and Pep in eight samples collected along 1800 km from Arauco (VIII Region) to Punta Arenas (XII Region). Despite the large geographic separations, values of Neis unbiased genetic distance, D (0.004-0.048) and standardised genetic variation among populations, Fst (0.011-0.055) were small. The levels of gene flow (Nm = 8) found in this study prevent the effect of differentiation among populations by genetic drift. This findings indicate that its long-lived planktotrophic larvae provides this species with considerable dispersal ability throughout its range which is favoured by the ocean currents along the chilean coast. In terms of management of the M. chilensis fishery, the results provide no evidence for discrete stocks, with the possible exception of the Punta Arenas population. Considering the intensive aquaculture activities with this species the present study provide preliminary data which can be used as a baseline for further characterization and /or monitoring these mussel populations.

The Chilean blue mussel Mytilus chilensis is an important commercial species. However, little has been published on the population genetics of this species, despite the need to implement management and conservation policies. Randomly amplified polymorphic DNA‐polymerase chain reaction analysis was used to estimate genetic variation within and between eight natural populations along the whole range of its Chilean natural distribution (ca. 1900 km from Arauco (VIII Region) to Punta Arenas (XII Region)). The values of Nei's unbiased genetic distance, D (0.030–0.107), among populations were small, despite the large geographic separation. A mantel test using 50 000 randomizations showed evidence for a significant correlation (r=0.74, P<0.05) between genetic and geographic (coastal) distance. Punta Arenas population was the most genetically differentiated from the others, although the scale of differentiation was not large (D=0.076–0.107). The levels of gene flow (Nm=1.55) found in this study prevent differentiation among populations by genetic drift. This is the result of the long‐lived planktotrophic larvae of M. chilensis, which provides this species with considerable dispersal ability throughout its range, which is favoured by the ocean currents along the Chilean coast. A restricted larval dispersal towards the north due to the Cape Horn Current derived from the West Wind Drift could be the cause of the higher genetic differentiation of Punta Arenas population from the northern populations. For management purposes of the M. chilensis fishery, the results provide no evidence for discrete stocks, with the possible exception of the Punta Arenas population. The present study provides the baseline data in order to continue further characterization of these mussel populations, considering the great increase in aquaculture of this species.

The heritability of larval size is estimated in Mytilus chilensis, based on a nested design, involving 95 full‐sib and 19 half‐sib families (19 males each mated to 5 separated females) grown under controlled laboratory conditions. The half‐sib heritability estimates at 10, 25 and 40 days for larval and spat shell height ranged between 0.38±0.33 and 0.84±0.45, suggesting that selection for shell size would be effective.

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A random sample of 150 individuals of a laboratory-produced cohort of Ostrea chllensis Philippi, 1845 was taken at 10 and 36 mo of age to estimate physiological variables and individual heterozygosity uslng 4 loci (Lap, Pgi, Pgm and Ca). Juveniles of 10 mo of age showed a mean Dvalue of 0.134 (p > 0.05) and a positive correlation between oyster size and multiple-locus heterozygosity (MLH) (p < 0.05). Also, there was a positive correlation (p < 0.05) between ingestion rate, absorption rate and MLH. A negative correlation between excretion rate (p > 0.05), oxygen consumption rate (p < 0.05) and MLH was found. The K2 value (standardized net growth efficiency) was positively correlated (p < 0.05) with MLH. At 36 mo a heterozygote deficiency was present with a mean value D = -0.431 (p < 0.05). No relationship between standard dry weight and MLH and also a negative correlation between the scope for growth and MLH were found. The oxygen consumption and excretion rates also showed an ~n -crease in large size individuals. The slopes for f~ltration and excretion rates against MLH were negative and not statistically slgnif~cant. However, ingestion and absorption rates showed significant (p < 0.05) decrease with an increase in heterozygosity. The results seem to indicate that within sexually immature individuals of 0. chilensls, a positive correlation between growth rate and MLH can be found, while in adults the higher energy allocation for reproduction precludes the detection of this relationship.

Oysters (Osfrea chilensis PHILIPPI 1845). from a 40-month-old cohort which came from the natural population of the Quempillen River Estuary (Southern Chile), were used as parental stock (n = 600) for a hatchery-produced cohort. At 12 months of age this cohort was analysed physiologically (filtration rate, organic ingestion rate, absorption rate, assimilation efficiency, excretion rate, respiration rate) and electrophoretically (multilocus heterozygosity (MLH) using four loci: LAP, GPI, PGM, CA). Most physiological variables were positively correlated with dry weight (P < 0.05). with the exception of assimilation efficiency, excretion size and net growth efficiency (P z 0.05). Also. there was a positive correlation between body size, ingestion rate, absorption rate and net growth efficiency with MLH. Respiration rate and consequently energy losses were negatively correlated with MLH. The energetic efficiency product of a lower metabolic cost in the heterozygous individuals in this cohort is reflected in a higher net growth efficiency and a positive correlation between growth and MLH. The genetic basis for the relationship between metabolic parameters and heterozygosity can be due to an overdominance in this 12-month-old cohort.

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