INFLUENCE OF ENVIRONMENTAL TEMPERATURE ON PLASMA IONIC AND OSMOTIC CONCENTRATIONS IN MUGIL CEPHALUS LIN FRANK G. NORDLIE Department of Zoology, University of Florida, Gainesville, FL 32611 U.S.A. zyxwvutsrqponmlkjihgfedcbaZYX (Received 1 March 1976) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLK Abstract-l. Plasma Na’ in Mugil cephulus ranged from a concn of 158.8 f 1.1 m-equiv/l from fish acclimated to 10°C to 178.0 k 2.2 m-equiv/l from fish at 3O”C, plasma Cl- ranged from 138.8 + 0.9 m-equiv/l at 10°C to 158.2 + 2.1 m-equiv/l at 30°C and plasma osmotic concentration ranged from 323 mOsm/kg to 378 mOsm/kg for the same acclimation temperature range. Plasma K+ remained roughly constant. 2. These patterns of response in inorganic ions and total osmotic concentrations to environmental temperature are opposite to what one would expect in fishes acclimated to sea water and would seem to involve an additional metabolic expenditure for regulation at lowered temperatures. INTRODUCTION In recent years the literature has contained several accounts of the influences of environmental tempera- ture on the pattern of osmotic and ionic regulation in euryhaline organisms, beginning with the early works of Panikkar (1940) on crustacea, and Wikgren (1953) on fishes. Most of these efforts have been con- cerned with the response in total plasma concen- trations at near freezing and freezing temperatures such as in the works of Scholander et al. (1957) Pearcy (1961) and DeVries & Wohlschlag (1969). More recently Houston et al. (1968) Prosser et al. (1970) and Umminger in a series of papers culminat- ing in a review in 1971, have evaluated response to temperature of plasma or serum ionic and osmotic components in several fresh water and marine fishes. Umminger (1971) presented 4 categories of compensa- tions in fresh water fishes and evaluated the signifi- cances of these adaptations. I have recently studied the pattern of osmotic and ionic regulation in the euryhaline striped mullet (Mugil cephalus Lin.) as a function of the range of acclimation temperatures (between 10 and 30”) that these fishes normally encounter throughout one year in Florida waters. This paper presents the results of this investigation. MATERIALS zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA AND METHODS The fishes used in these experiments were juvenile striped mullets (?r wt ca. 18 g) obtained in cast net collec- tions made during the fall and winter months in the Matanzas River near the C. V. Whitney Marine Labora- tory at Marineland FL. They were returned to the labora- tory in Gainesville, FL, and placed in aerated aquaria in a constant temperature room at ambient temperatures near those of the water from which collected. The ambient temp was raised or lowered at the rate of l”C/day until the desired acclimation temp was reached; the fish were main- tained at this temperature for a minimum of 14 days before blood samples were taken. All groups were kept in freshly collected filtered Atlantic ocean water (945 mOsm/kg) obtained from Marineland, FL. Purina catfish chow was provided as food during the acclimation period. Free- flowing blood samples were collected from the caudal artery following the technique described by Nordlie & LeflIer (1975). Na+ and K+ concns were measured using a Radiometer FLM 2 photometer, and Cl- was measured using a Radiometer chloride titrator. Plasma osmotic concns were measured using a Precision Systems osmo- meter. It was necessary to pool plasma samples in order to obtain the 0.2ml quantity necessary for this analysis. RESULTS Plasma Na+, K+, and Cl- concns from striped mullet acclimated to temps of 10, 15, 20, 25, and 30°C in full sea water are found in Table 1. It can be seen that both Na+ and Cl- concns rise or fall in a paral- lel fashion over the temp range investigated. Plasma K+ showed no significant difference in concn between the 10°C acclimated fish (10.7 m-equiv/l) and that of the 30°C fish (10.5 m-equiv/l). Results of several pre- vious studies have indicated that in some instances inorganic ions of the plasma and the total osmotic concn of the plasma are altered independently of one another+e.g. the total plasma concn is increased at low temps by the addition of organic molecules. This possibility was evaluated by comparing the total osmotic concn of plasma samples from the various acclimation groups with the sums of the inorganic ions measured for the same acclimation groups. Results of these determinations are found in Table 1. It can be seen from these data that the total osmo- tic concns of plasma taken from fish acclimated to the various temps followed the same pattern as did Na+ and Cl- with concns ranging from 323 mOsm/kg at 10°C to 378 mOsm/kg at 30°C. Cal- culations showed that the percentage of osmotic acti- vity accounted for by the sums of Naf, K+, and Cl- remained quite constant over the experimental temp range,. varying from 95% at 10°C to 92;b at 30°C. Thus there would seem to be no major alteration in the inorganic electrolyte concn to total concn ratio over this temp range. 379