Shell-shape variation in the Chilean blue mussel Mytilus chilensis was examined in eight populations covering the totality of its distribution range, which represents over 1800 km of latitudinal gradient in the southeastern Paci¢c. The shell outline shapes were assessed using elliptic Fourier analysis followed by canonical variates analysis. Fourier coe⁄cients showed a highly signi¢cant morphological variation between the populations studied. Canonical variates analysis showed a visual association of the First canonical with convexity of ventral edge and umbo shape, while the second axis was associated with shell elongation. Previously described genetic patterns were marginally congruent with our morphometric pattern, though geographic distance had a stronger e¡ect on morphology. Shape change (elongation) was correlated with latitudinal clinal variation. The morphometrical analysis proved to be an important tool for evaluating the components of shell-shape variation in mussels and to document association patterns with geographical and ecological variables. Such patterns are useful to establish awide ¢eld of work, including sexing, stock recognition, traceability studies and productive management in general.FONDECYT-Chile through project 1050848

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.

Selection, either natural or artificial, induces an inter‐generational response only when the trait exhibits additive genetic variation. An index of additive genetic variation is narrow‐sense heritability (h2): the ratio between additive genetic variance and phenotypic variance. Here, we present narrow‐sense heritabilities for mass and size in the Chilean blue mussel Mytilus chilensis at different ages and in three different populations. We found that h2 changed with age and that maternal‐non‐additive variance was high, generating relatively low estimates of h2 in most cases. Our results, together with previous studies, suggest that genotype‐by‐environment interaction does not appear to play a significant role. However, we found that non‐additive and common environmental variance are important determinants of phenotypic morphological variation in body size in M. chilensis. Further studies are needed to establish the precise contributions of these sources of variation, to assess the real potential for response to selection.

ABSTRACT. Adult specimens of M. galloprovincialis from Concepción Bay and M. chilensis from Yaldad Bay, Chile, were transferred to the laboratory to produce crosses of "pure" and "hybrid" species in order to evaluate early larval development and growth. These variables are important for understanding the dynamics of these two mussel species in this potential hybrid zone where they occur sympatrically. The study showed that fertilization occurred in all crosses and significant differences were not detected between pure lines and hybrids in terms of the percentage of eggs that developed into larvae. Hybrid larvae and spat from both reciprocal crosses grew significantly more than those from pure lines, although valve length values were within the ranges reported in the literature. Keywords: Mytilus chilensis, Mytilus galloprovincialis, larval development, hybrids, Chile. Producción y comportamiento de larvas de especies puras e híbridas entreMytilus chilensis y Mytilus galloprovincialis obtenidas en laboratorio RESUMEN. Ejemplares adultos de M. galloprovincialis de la bahía de Concepción y de M. chilensis de la bahía de Yaldad, Chile, se trasladaron al laboratorio para realizar cruzamientos puros de cada especie e híbridos, para evaluar el desarrollo larval temprano y su crecimiento. Estas variables son importantes para entender la dinámica de estas dos especies de mitílidos en esta potencial zona híbrida donde se encuentran en forma simpátrica. El estudio mostró que la fertilización ocurrió en todos los cruzamientos y no se detectó diferencias significativas entre líneas puras e híbridas en el porcentaje de huevos que se desarrollaron a larvas. Las larvas y juveniles híbridos de ambos cruzamientos recíprocos crecieron significativamente más que las larvas de los cruzamientos de especies puras, aunque los valores de longitud de la valva están dentro de los rangos reportados en la bibliografía. There is no evidence in the literature for the presence of unfit hybrid recombinants within Mytilus hybrid zones (Comesaña et al., 1999;Toro et al., 2002;Beaumont et al., 2004;Westfall & Gardner, 2010;Dias et al., 2011). As discussed by Comesaña et al. (1999), the degree of natural hybridization is generally higher in the Mytilus edulis-M. galloprovincialis hybrid zone in Europe than in the M. edulis-M. trossulus hybrid zone on the east coast of north America. According to Harrison (1993) the more genetically divergent the parental types, the lower will be the fitness of the hybrids produced. This difference in natural hybridization in these two mussel hybrid zones implies that M. edulis and M. galloprovincialis are more closely related (Gardner, 1994) than are M. edulis and M. trossulus (Rawson et al., 1996). This has been corroborated by successful interbreeding between M. edulis and M. galloprovincialis, the

Shell thickness in mollusks is generally considered adaptive because of their effects on fitness. However, little is known about the genetic basis of shell thickness. This is important, because the response to selection and the subsequent adaptive microevolution of a trait, such as thickness is only possible when that trait exhibits additive genetic variation. Here, we estimated the narrow-sense heritability (h 2 : ratio between additive genetic variance and phenotypic variance) for the traits 'shell thickness' and 'shell length growth' in a 34-month-old cohort of the mussel Mytilus chilensis obtained by using a half-full sib design and grown in the field. Also, phenotypic and genetic correlations were estimated between both traits. We found that h 2 showed significant values for shell thickness (0.294 AE 0.194) and length (0.731 AE 0.379). The phenotypic correlation between both traits was positive and significant; however, the genetic correlation between these traits was not. These results suggest both traits can evolve adaptively by selection, but because these traits did not show genetic correlation, it is possible that selection pressure affecting one trait may not affect the other.