EPITHELIAL WATER PERMEABILITY IN THE EURYHALINE MUSSEL GEUKENSIA DEMISSA: DECREASE IN RESPONSE TO HYPOOSMOTIC MEDIA AND HORMONAL MODULATION.

The diffusional water permeability of isolated mantles from the mussel Geukensia demissa was reduced by incubation ofthe tissues in hypoosmotie media. The permeability of mantles from 1000 mOsm seawater (SW)-accimated animals was 6 x 10 cm/s. A four-hour incubation in 500 mOsm SW or 250 mOsm SW reduced the water permeability by 2 x l0 cm/s and 4 x 10 cm/s, respectively. A half-hour exposure to the hypoosmotic medium was sufficient to induce the decrease in permeability. The water permeability of mantles incubated in isosmotic SW containing acetone extracts of ganglia from 1000 mOsm SW-acclimated mussels or of mantle from 500 mOsm SW-acclimated mussels was significantly reduced. Extracts of gill had no effect. Ovine prolactin (50 mg/ml) decreased the water permeability of mantles in isosmotic seawater. Cortisol (10 M), arginine vasopressin (10 M), and the molluscan neuropeptide FMRFamide (10 M) had no effect. These results show that the epithelial water permeability of euryhaline bivalves varies with changes in the ambient salinity, and that these permeability changes may be modulated by factors of neural origin.