DRINKING RATES AND NA-CHANGE IN 2 SPECIES OF MARINE FLATFISH - DAB, LIMANDA-LIMANDA AND PLAICE, PLEURONECTES-PLATESSA( EFFLUXES IN RESPONSETO TEMPERATURE)

The relationship between fish mass and drinking rate in two species of flatfish, dab and plaice, weighing between 1 and 150 g was investigat ed. Both plaice and dab showed increased drinking rates with increasin g fish mass, although - when calculated on a weight-specific basis - t he increase was negligible. Fish were acclimated to winter and summer temperatures of 9 and 14 degrees C, respectively. In winter both speci es were acutely transferred to 5, 14 and 21 degrees C and in summer to 5, 9, 21 and 25 degrees C. Drinking rates, Na+ efflux and body ion co ntent were measured. Dab showed lower drinking rates than plaice (e.g. the weight-specific drinking rates of summer- and winter-acclimated d ab were 0.12+/-0.01 and 0.06+/- 0.006 ml.h(-1).100 g(-1), respectively , compared to that of plaice which were 0.25+/-0.02 ml.h(-1).100 g(-1) L in summer and 0.17+/-10.02 ml.h(-1).100g(-1) in winter). Summer dab exhibited decreased weight-specific drinking rates at 5, 9 and 25 deg rees C, while winter dab increased drinking at 21 degrees C. Winter pl aice also showed increased drinking at 21 degrees C and a decrease at 5 degrees C, but in contrast summer plaice did not increase drinking a t either 21 or 25 degrees C but showed a decrease at 5 and 9 degrees C . Winter dab and plaice showed similar Na+. efflux rates but summer da b showed higher efflux at all temperatures except 5 degrees C. The dat a indicates that (a) the osmoregulatory function of plaice is much wea ker than that of dab at higher temperatures (> 20 degrees C) and (b) m ass has a greater effect on drinking and Na+ efflux rates than tempera ture; although when calculated on a weight-specific basis neither drin king nor efflux showed any variation with fish mass suggesting that th ese functions occur at similar intensities across the entire weight ra nge.