Osmotic and Ionic Regulation of Two Brine Fly Species (Diptera: Ephydridae) from a Saline Hot Spring

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Abstract
The brine fly Paracoenia calida Mathis is known only from the hot (source temperature = 54 C) saline (22 ppt) waters of Wilbur Hot Springs, Colusa Co., California. To investigate whether the high salinity is responsible for the endemic distribution of P. calida, I measured its hemolymph osmotic and ionic (sodium, chloride, potassium) concentrations and salinity tolerance (50% lethal exposure) of larvae exposed to a range of salinities (0-102 ppt) and water temperatures (15-40 C). Results were compared to similar measurements made on a co-inhabitant of P. calida, the geographically widespread brine fly Ephydra goedeni Wirth. In hypertonic waters (380-1,900 mM sodium chloride; 22-102 ppt) both species maintained their hemolymph hypoosmotic at concentrations between 150 and 350 mM NaCl regardless of water temperature. In hypotonic waters (0 and 25 mM NaC1), both species regulated their hemolymph hyperosmotic at 100 mM NaC1 regardless of water temperature. In contrast to their similar patterns ofosmoregulation, larvae of P. calida had an extremely low survivorship in deionized (0 mM NaC1) and diluted Wilbur Hot Springs water (7 mM NaCl; 1 ppt), whereas E. goedeni larvae exhibited their highest survivorship in deionized water. Paracoenia calida larvae in deionized water had high concentrations of K in their hemolymph. Perhaps this excess K leaked out from muscle and nervous tissues in response to declining hemolymph levels of Na and Cl; or the loss of K from these tissues may be an effect of cell death. Regardless, I suggest that P. calida is restricted to Wilbur Hot Springs because it cannot tolerate dilute salinities. In contrast, E. goedeni occurs throughout western North America in part because it can tolerate dilute salinities.