MOLLUSCS IN EVAPORITE SETTINGS: DESIGNS FOR SURVIVAL IN HYPERSALINE AND OLIGOTROPHIC CONDITIONS

Few macroscopic metazoan organisms are able to survive the extreme hypersaline and oligotrophic challenges of evaporite environments. In the Mollusca a number of bivalve and gastropod taxa independently have evolved remarkable abilities to thrive, sometimes at unusually high densities, under these conditions. Halophillic mollusks are potential models for understanding how complex life can arise and persist in extreme environments on Earth and in extraterrestrial systems. Environmental correlates of high salinity (>50 ppt) that pose significant physiological challenges to metazoan life include high temperatures (particularly at low latitudes) and oligotrophic conditions. Ecological correlates include low species diversity, high standing crop and biomass of the dominant species, and short life spans coupled with high rates of population turnover. Anatomical correlates include dwarfing, osmoregulatory ability to deal with constant high salinity or with rapid salinity fluctuations, and novel nutritional strategies.

Three ideal modern settings for analysis of mollusks under stress are the Laguna Madre of Texas, the Sivash on the western margin of the Sea of Azov, and Shark Bay, Western Australia. In each of these settings there is a dominant bivalve adapted to salinities >50 ppt. The photosymbiotic coadaptation in the cardiid bivalve Fragum erugatum is developed as an example of extraordinary evolutionary and ecological success in a hypersaline, oligotrophic environment. Adaptations such as this provide insight into how to design a metazoan for life in extreme environments. The ability of some organisms to thrive in harsh hypersaline conditions further suggests a possible refugial role for evaporite systems during mass extinction and recovery from mass extinction.

Although geochemically charged fluids and hypersaline brines at seeps and vents are genetically distinct from brines produced by evaporation, the microbial partnership between metazoans and chemoautotrophic bacteria in these settings underscores a more pervasive pattern of evolutionary success via symbiosis in extreme environments.