KIDDER, G.W.*; GOLDSMITH, C.E.; BALDWIN, J.L.; PETERSEN, C.W.; PRESTON, R.L.; Mt. Desert Island Biological Lab.; Illinois State Univ.; Illinois State Univ.; College of the Atlantic; Illinois State Univ.: Osmotic and osmoregulatory water fluxes in Fundulus
In fish, osmotic water flow is balanced by active salt and water transport, and fish weight can be used to assess net water flux. We developed a method for weighing Fundulus heteroclitus once per hour for many days without damaging its integument, suspending the animal by a thread through the lower jaw and raising it free of the water for weighing. When kept in the water and temperature to which they were acclimated, fish weight is stable for days. When cooled to 4oC, fish lose water in salt water (SW) and gain water in fresh water (FW). In both cases, the initial (~ 1 day) rate is higher than the final (1 – 6 day) rate, which implies an abrupt change in effective permeability (Pe), since the osmotic gradient is essentially constant. Returning to 16oC activates osmoregulatory transport and the weight returns to normal. Fish previously acclimated to the water in which they are tested start osmoregulatory compensation immediately, while fish for which the direction of transport must be changed require ~ 18 hours before osmoregulation begins, presumably to synthesize the necessary molecular machinery. We assume that most of the water flow occurs through the gill, where Pe is determined by membrane permeability and area, ventilation rate and blood flow rate, all of which are critical to oxygen uptake, and are thought to be controlled by oxygen demand. The Pe for the rapid phase at 16oC is 4 fold higher than that at 4oC, consistent with the increase in respiration between these two temperatures. F. diaphanus, which cannot osmoregulate in SW, shows a similar initial outflux to F. heteroclitus. We believe that the change in Pe is due to a change in ventilation and blood flow, and is probably accompanied by a change in oxygen uptake. (Sup. NSF C-RUI 0111860)
