Camp. Biochem. Physid Vol. 86A, No. 1, pp. 57-61, 1987 Printed in Great Britain ~3~-96~/87 $3.00 + 0.00 Pergamon Journals Ltd PLASMA OSMOTIC, Na+ AND Cl- REGULATION UNDER EURYHALINE CONDITIONS IN CYPRIN~~~N VARIEGATUS LACfiPfiDE FRANK G. NORDLIE Department of Zoology, University of Florida, Gainesville, FL 32611, USA. Telephone: (904) 392-l 107 (Received 25 March 1986) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQP Abstract-l. Plasma osmotic, Na+ and Cl- concentrations were evaluated in the teleost fish Cyprinodon variegators La&p&de following acclimation in a series of ambient salinities from 1% sea-water (SW) to 360% SW. 2. Plasma osmotic concentrations ranged from 337 f 13 to 635 + 39mOsm/kg while plasma Na+ ranged from 165 k 8 to 285 f 19 meq/l, and plasma Cl- from 129 2 12 to 270 f 19 meq/l over this ambient range. 3. Plasma Na+ was more tightly regulated than plasma Cl-, with a Na+:Cl- ratio of 1.28 in 1% SW and 1.05 in 360% SW. 4. Contributions of Na+ ~1~s Cl- to total osmotic concentration were not significantly altered over the ambient range. INTRODUCTION Coastal lagoons and smaller tidal basins and marshes may undergo changes in ambient salinities from near freshwater following heavy rains to salinities near saturation following extended dry periods, with the possibility of variations of 50%~~ or more within a day’s time. Cyprinodon variegatus is one of the few vertebrates (or animals of any kind) capable of existing under such conditions. Osmotic regulatory patterns of relatively few of such extremely eury- haline species of teleost fishes have been investi~ted over wide ranges of ambient salinities. This is, at least partially, because the majority of these forms are relatively small fishes with little or no direct economic importance. Even where there are data on the plasma osmotic concentrations at various ambient salinities (such as for C. variegatus Nordlie, 1985), in only rare cases have any plasma ionic concentrations been separately evaluated and reported. Perhaps the most notable exception was the evaluation of Na+ and Cl- regulation over a wide range of ambient salinities in Aphanius dispar (Lotan, 1971), a brackish water cyprinodontid of the Mediterranean-Indian Ocean areas. This paper contains an evaluation of the regulation of plasma Na+, Cl-, and osmotic concentrations in the cyprinodontid Cyprinodon variegatus Ladpede, over a range of ambient salinities from 1 to 360% sea-water (SW). This species has been found in nature, surviving at salinities in excess of 400% SW (Simpson and Gunter, 1956), as is also true of Aphanius dispar (Lotan, 1971). It seems especially appropriate to evaluate ionic regulation in C. varie- gatus as it and A. dispar are the most euryhaline teleosts known but show very different patterns of plasma osmotic regulation (Nordlie, 1985). MATERIALS AND METHODS Fish used in the experiments were collected from a single salt marsh area near Cedar Key, Florida. Ambient salinities ranged from 6 to 70% SW (median salinity, 35% SW) at times that collections were made. The f&h were transported to the laboratory in Gainesville in water taken from the site of collection. They were maintained in this water with aeration for a period of 24-48 h before being transferred to aerated and filtered 50% SW. in a room in which the temperature was controlled at 20 f 1°C and on a 12: 12 light: dark cycle. All experimental fish were fed TetraminR. Individuals were maintained in this environment for a period of 14 days before transfer to the next salinity in the series (either higher or lower), where they were again maintains for 14 days before transfer. This prorzdnre was repeated until the desired salinity was reached. Salinities of less than full sea-water were prepared by appropriate addi- tions of deionized water to filtered Atlantic Ocean water obtained from the C. V. Whitney Marine Laboratory at Marineland, Florida. Salinities above those of full sea-water were prepared by adding appropriate quantities of artificial sea salt (Instant OceanR) to the filtered sea-water. The salinity of 100% SW was 35%~ and all dilutions or concen- trations were made on this basis. Salinities were checked and adjusted daily using an American OpticaIR Hand Refrac- tometer for those of 50% SW (17.5960) or higher, and a Wescor osmometer, with appropriate calibration, for those of lower salinity. Higher salinities (2 50% SW) were main- tained within +3%, and lower ones (~50% SW) within +5% of the chosen level. An effort was made to use individuals as uniform in size as possible (56 cm) to minimize possible influences of size differences. Blood was taken by blind cardiac puncture using hepa- rinized microhematocrit tubes drawn out to fine points. Formed elements were separated from plasma by centrifu- gation for 10min in a hematocrit centrifuge. Deter- minations of total osmotic concentration were carried out on a Wescor 5 1OOB vapor pressure osmometer using 5 ~1 quantities of plasma. Plasma Na+ was evaluated on 10~1 quantities on a Radiometer FLM3 flame photometer that had been appropriately calibrated. Plasma Cl- was evalu- ated on a Radiometer chloride titrator on 5~1 samples. Because of the small sizes of individuals of Cyprimdon uariegatus it was frequently not possible to carry out all three analyses on the plasma from a single individual. Thus, Na+ values are not necessarily for the same total set of individuals as the CIl values and/or the plasma osmotic concentrations at any particular ambient salinity level. 57