OSMOREGULATION AND EPITHELIAL PERMEABILITY IN 2-EURYHALINE BIVALVE MOLLUSKS - MYA-ARENARIA AND GEUKENSIA-DEMISSA

Mya arenaria (subclass Heterodonta) survives for at least 5 wk in media of 100-1000 mOsm; Geukensia demissa (subclass Pteriomorphia) survives in media of 50-1000 mOsm. Tissue hydration does not increase either in G. demissa or in M. arenaria acclimated to diluted seawater (SW). The concentrations of Na+, K+, Mg2+, and Cl- in the blood of G. demissa conform to ambient over the entire salinity range. The concentration of Ca2+ in the blood of animals acclimated to media of less-than-or-equal-to 200 mOsm is almost-equal-to 2.5 mM. Blood concentrations of Na+, Mg2+, and Cl- largely conform to ambient in M. arenaria. In M. arenaria acclimated to SW of > 200 mOsm, blood Ca2+ conforms to ambient, but is significantly hyperionic in animals acclimated to 50 mOsm SW. The diffusional water, Na+, and Cl- permeabilities of isolated mantle tissue from Mya acclimated to 1000 mOsm SW are, respectively, 3.8 x 10(-4), 5.7 x 10(-5), and 2.7 x 10(-5) cm . s-1. The permeabilities of the mantle of G. demissa acclimated to 1000 mOsm SW are: H2O, 1.3 x 10(-4), Na-, 4.0 x 10(-5); Cl-, 2.9 x 10(-5) cm . s-1. There is no consistent trend of a decrease in epithelial permeability in animals acclimated to dilute SW. The hypercalcemia that occurs in Mya acclimated to dilute SW also occurs in other brackish water heterodontid clams. This response does not occur in brackish water pteriomorph bivalves. The relatively low water permeability of Geukensia is an adaptation to the high intertidal habitats preferred by this species. Mya, which has a higher permeability to water, is protected from severe desiccation stress by its deep burrowing habit.