TAYLOR, J. R. *; WHITTAMORE, J. M. ; WILSON, R. W. ; GROSELL, M. ; RSMAS, University of Miami; Exeter University, UK; Exeter University, UK; RSMAS, University of Miami: Postprandial acid-base balance in freshwater and seawater-acclimated European flounder
Intestinal HCO3– secretion and resultant carbonate precipitation are important in the hydromineral balance of all fishes sampled to date in hyperosmotic conditions. Furthermore, intestinal Cl–/HCO3– exchange plays an important role in maintaining postprandial hydromineral balance. Digestion is a nearly constant occurrence in the natural existence of most animals and greatly modifies physiology, but surprisingly its effects in fishes have been largely ignored by comparative physiologists. This study is the first to investigate the effects of feeding on acid-base balance in seawater and freshwater acclimated euryhaline fish. Using European flounder, Platichthys flesus, we found that base efflux is maximal 6-12 h post feeding regardless of salinity and is of greater magnitude in seawater-acclimated flounder than in freshwater flounder. In freshwater flounder, Cl– uptake was measured and does not match the magnitude of base efflux, suggesting that mechanisms additional to Cl–/HCO3– exchange contribute to branchial postprandial base output. Ammonia flux measurements revealed an ammonia efflux of similar magnitude in both salinities, but with maximal output occurring during the first 6 h post-feeding in seawater-acclimated fish and between 6 and 12 h post-feeding in freshwater flounder. This ammonia efflux counters the base efflux in seawater fish, yielding no distinct net acid-base disturbance. Based on our findings of similar ammonia efflux but reduced base output in freshwater flounder, the possibility of a postprandial alkaline tide was investigated at both salinities, but no evidence for disturbed acid-base balance was observed. (Supported by JEB/CoB and SEB travel awards to J. R. Taylor and by NSF-IBN 0416440 to M. Grosell).
