Brood chamber isolation during salinity stress in two estuarine mollusk species: from a protective nursery to a dangerous prison

Faced with environmental stress, the females of many marine invertebrate species can isolate their brood chambers from the external environment for prolonged periods. Although brood chambers have therefore typically been assumed to protect the brooded embryos, the effects of continued isolation on conditions within brood chambers have not been examined. In the present research, we investigated the effects of external stress on conditions in the brood chambers for 2 sympatric species, the gastropod Crepipatella dilatata and the bivalve Ostrea chilensis. Both species incubate their embryos in the pallial cavity for at least 4 wk before the young emerge as larvae (oyster) or juveniles (gastropod). Brooding and non-brooding females were stimulated to isolate the pallial cavity from the external environment by exposing them to water of reduced salinity (<22 to 24 psu). We then recorded subsequent changes in salinity, dissolved oxygen, pH, and ammonium ion concentration within the pallial fluid. Although salinity within the pallial cavity remained high for both species, oxygen availability for brooding females of C. dilatata and O. chilensis dropped to hypoxic levels (<1.5 mg O2 l–1) within 12 h and 20 min, respectively. In addition, pH of the intrapallial fluid dropped substantially with prolonged maternal isolation, suggesting a possible impact on shell formation for brooded veligers. Indeed, in the case of C. dilatata, the pH of pallial fluid initially fell to 6.4, but then tended to become more basic after 6 h of maternal isolation, a possible consequence of shell dissolution. On the other hand, ammonium ion concentration increased progressively with prolonged maternal isolation. Our data show that, whereas initially the isolation mechanism protected incubated embryos from exposure to low salinity stress, continued isolation from the exterior transformed the brood cavity from a protective environment to an oppressive one, i.e. from a protective nursery to more of a prison.