Adaptations of Nymphs of a Marine Dragonfly, Erythrodiplax berenice, to Wide Variations in Salinity

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Nymphs of the dragonfly, Erythrodiplax berenice, are common on rocky mangrove flats in the lower Florida Keys at salinities of 36-48 ppt. Erythrodiplax berenice appears to be the only truly marine odonate, and the nymphs are able to tolerate wide variations in salinity. Hemolymph osmotic pressures for nymphs freshly captured in seawater or held in artificial seawater were 358-412 mOsm. There was little further variation at salinities between freshwater and 260% seawater (2,612 mOsm). In 300% seawater, hemolymph osmotic pressure rapidly increases to about 1,000 mOsm, and death ensues. The transition point between hyper- and hypoosmotic regulation occurred at approximately 350 mOsm seawater. Freshwater- and seawater-acclimated nymphs differ little in body water content (about 80%), but Na content is elevated from 50 to 89 Mmol/g wet wt, respectively. In 35 ppt seawater, Na influx and efflux were approximately balanced near 400 μmol/100g·h. In freshwater (1 mM Na), these fluxes decline to about 10 μmol/100g·h. The time for complete turnover of body Na increases from 0.9 day in seawater to 21 days in freshwater. On rapid transfer from water of 470 mM Na to water of 1 mM Na, the Na efflux in 1 mM was only slightly higher than expected for freshwater-acclimated nymphs. At a water Na level of 0.25 mM, Na efflux increases greatly, and there is a large net loss of Na. Water influxes of nymphs in distilled water or 35 ppt seawater were similar (33 and 40 μ1/g·h, respectively); complete turnover of body water occurs in about 0.8 day. Nymphs of E. berenice rival the better-known dipterans in osmoregulatory abilities and should be studied further to compare mechanisms of hypoosmotic regulation that have presumably evolved independently in these two orders.