PERIRECEPTOR MECHANISMS SUSTAINING OLFACTION AT LOW SALINITIES - INSIGHT FROM THE EURYHALINE BLUE-CRAB CALLINECTES-SAPIDUS

As the blue crab Callinectes sapidus moves from sea water to fresh wat er, the 'exposed' chemosensory dendrites in the olfactory sensilla (ae sthetascs) undergo changes in length that are positively correlated wi th environmental salinity, In this study, we demonstrate the following , (1) The responses of the olfactory receptor cells of freshwater-accl imated crabs are reduced relative to those of seawater-acclimated anim als, but increase with a time course comparable to the increase in den drite length when these crabs are transferred to sea water, (2) The ol factory response of seawater-acclimated crabs is lost and the chemosen sory dendrites osmotically ablated if the aesthetascs are acutely expo sed to low salinity, However, maintaining iso-osmotic conditions with mannitol preserves both the physiological response and the structural integrity of the dendrites, (3) The flux of [C-14]thiocyanate and Na-2 2 between the hemolymph and sensillar lymph of the aesthetascs indicat es continuity between these fluid compartments, (4) There is a net eff lux of Na+ from the hemolymph through the aesthetascs in freshwater-ac climated crabs, and measurements of electrical potential across the an tennules suggest that this efflux largely derives from passive diffusi on, (5) Dendrites in the aesthetascs of crabs acclimated to brackish w ater are intermediate in length between those of freshwater- and seawa ter-acclimated animals, Together, our findings suggest that, at low sa linities, the efflux of Na+ (and probably other ions) from the hemolym ph generates an ionic/osmotic microenvironment within the aesthetascs that sustains the structural and functional integrity of the olfactory dendrites, We propose that the length of these dendrites, and consequ ently the olfactory response, is constrained by the distance over whic h this microenvironment can be effectively maintained.