The oyster Ostrea chilensis develops in estuaries and incubates its embryos inside the pallial cavity. Reduced external salinity triggers valve closure and isolation of the brood chamber, which in turn alters conditions in the pallial fluid, including pH reductions ranging from 7.65 (SD = 0.05) to 6.97 (SD = 0.02) in non-brooding females and 6.96 (SD = 0.10) in brooding females after 12 h of isolation. Exposing veliger larvae to acidic pH's (5 and 3) decreased the thickness of the embryonic shell valves. Calcium, sodium, chlorine, tin, sulphur, and magnesium were the most important (>99%) elements constituting the veliger shells. Calcium content in the pallial cavity fluid increased with continued isolation in both brooding and non-brooding females, but calcium remained as the primary component (about 94%) of embryonic shells and maintained its normal proportion with respect to the rest of the elements at all tested pH's. The increased calcium in the pallial fluid was identified mainly as coming from the shells of brooding females. If any neutralizing elements were coming from embryonic shell valves, all elements in those shells must have been solubilising to the same degree. Veligers ceased all further shell growth while isolated within the female's brood chamber. However, veliger shell growth resumed once females were returned to high-salinity seawater. Our study shows that female isolation of the mantle cavity, often considered an adaptive response to the periodically reduced salinities of estuarine waters, can in fact generate adverse effects on brooded embryos. (C) 2009 Elsevier B.V. All rights reserved.