Net fluxes of water in the isolated gills of Anguilla dieffenbachii
Journal of Experimental Biology
lished, though generally poorly accepted, fact for several species of teleost and may make a considerable contribution to the uptake of water in dilute media. In this paper are presented the results of direct measurements of net water fluxes in the isolated gills of the New Zealand long-finned eel, Anguilla dieffenbachii. The results are discussed with reference to the water balance of eels in fresh water and when adapted to sea water. METHOD Eels obtained from a local exporter were kept in a
... er were kept in a large covered concrete tank supplied with a continuous flow of de-chlorinated Dunedin tap-water. The eels weighed from°* 5 t o 3'5 kg with most weighing from 1-5 to 2*8 kg, and only immature yellow eels were used. Adaptation to sea water was accomplished by transferring eels to a similar tank containing aerated sea water obtained from Otago Harbour. A continuous flow of sea water could not be obtained so the water was renewed at frequent intervals. All experiments were carried out on eels obtained in the summer months (October-May). The technique used for measuring the net fluxes of water in the gill was modified from that of Bellamy (1961) and involves the use of an incubated, isolated gill preparation. The eels were killed by cutting through the body with a sharp knife at the level of the heart, immediately behind the pectoral fins. The head portion of the eel was pithed to reduce movements, and the operculum was removed. Individual gills were then dissected out as follows. A ligature of silk suture thread was tied around each end of the gill arch and the gill was removed by cutting through the arch outside the ligatures. The gill was then placed on a pad of cotton wool soaked in a Ringer solution (see Table 1 ). Ringer A was used for all gills removed from freshwater-adapted specimens and Ringer B was used for gills removed from eels adapted to sea water. Only the first three pairs of gills were used. At least two gills from each eel were used as controls. These control gills were rapidly rinsed in distilled water, gently blotted dry with filter paper and a sample of gill filament was trimmed off the arch. This was analysed for water content by weighing the wet tissue, drying the sample for at least 18 h in an oven at 110 °C, cooling in a desiccator and re-weighing. The remaining gills (excluding those from the fourth pair of gill arches) were rapidly rinsed in the appropriate external medium and then placed individually in tubes containing 75-100 ml of the desired experimental medium. The various external media used were either dilutions of the normal eel Ringer solution or an artificial 'sea water' solution (see Table 1 ), thus enabling net water flux to be determined in a series of external solutions with osmotic pressures ranging from those typical of most fresh waters (< 5 mOsm) to sea water (~ 1000 mOsm). The tubes containing the gills were placed in a water bath maintained at 17 °C, and the solutions were bubbled vigorously with air which provided a source of oxygen and ensured a rapid agitation of the medium over the surface of the gill, preventing the formation of any local osmotic gradients. The gills were left in these tubes for up to 1 h, depending on the osmotic concentration of the external medium (see later), after which they were removed, rapidly rinsed in distilled water and gently blotted dry with filter paper. A sample of gill filaments was then trimmed off the arch and analysed for water content as before.