Seasonal changes in ionoregulatory variables of the glass eel Anguilla anguilla following estuarine entry:: comparison with resident elvers

In the present study, glass eels Anguilla anguilla in the Minho River estuary (41.5 degrees N, 8.5 degrees W) decreased in size (standard length, L-S and mass, M) from the beginning (autumn) to the end of the sampling season (summer). On the other hand elvers increased in L-S and M from spring to summer and were significantly larger than glass eels in paired comparisons. Branchial Na+/K+-ATPase and vacuolar (V-type) proton ATPase (in vitro activities), two important ion transporting pumps, did not show significant seasonal changes in either glass eels or elvers although in glass eels Na+/K+-ATPase (activity) expression was significantly higher than in elvers. In a single month comparison Na+/K+-ATPase branchial mRNA expression was also higher in glass eels as was the protein level expression of both Na+/K+-ATPase and NKCC (Na+:K+:2Cl(-) co-transporter). Immunofluorescence microscopy indicated apical CFTR Cl- channel labelling in Na+/K+-ATPase positive chloride cell in glass eels which was absent in elvers. Whole body sodium concentration and percentage water did not show significant seasonal differences in either glass eels or elvers although there were significant differences between these two groups during some months.