The cost of brooding in an estuary: implications of declining salinity for gastropod females and their brooded embryos

Females of the gastropod Crepipatella dilatata brood their egg capsules in the pallial cavity under the shell for several weeks until the offspring hatch as juveniles-but at what cost? In estuaries, brooding females clamp tightly to the substrate during periods of low salinity (< 22 psu), isolating the pallial cavity from the outside environment and potentially limiting the availability of oxygen to developing embryos, as well as to themselves. In this study, non-brooding females maintained normoxic levels in the pallial cavity even after about 30 h of isolation from the surrounding environment, while brooding females showed levels of severe hypoxia in the pallial fluid in as little as 3 h. This oxygen restriction activated an anaerobic pathway identified through L-lactate production both in the female foot and the embryonic tissue. By 72 h, L-lactate levels had increased approximately 122% in both brooding and non-brooding females, and L-lactate concentrations in advanced embryos near to hatching had increased by approximately 200%. However, over time the concentration of lactate did not increase in early embryos. Moreover, prolonged isolation from the surrounding seawater produced a measureable 'oxygen debt' in brooding females, non-brooding females, and encapsulated embryos, with the debt being greater for brooding than non-brooding females. Thus, the isolation of the pallial cavity in response to low salinity surroundings generated a significant energy cost for females and their embryos, a cost that increased as embryonic development progressed.