Daily variations of the saccular endolymph and plasma compositions in the turbot Psetta maxima: relationship with the diurnal rhythm in otolith formation

We studied the circadian variations of the ionic composition ([Na+], [K+], [Cl-]), total CO2 ([tot CO2]), and protein concentration ([protein]) in the plasma and endolymph in the distal and proximal sides of the sacculus in t he turbot Psetta maxima. Daily variations in total plasma calcium ([tot Ga] ) were also recorded. Significant diurnal fluctuations occurred in most par ameters and the spatial heterogeneity of the endolymph composition was main tained throughout a 24 h period. The main parameters implicated in otolith calcification, endolymph [tot CO2] and endolymph [protein], fluctuated diur nally, [tot CO2] was highest and [protein] lowest at night; the relative co ncentrations were reversed during the day. The inverse, alternating relatio nship of maximal and minimal [tot CO2] and [protein] implies that calcium c arbonate and organic matrix rates of deposition vary in antiphase. We there fore propose that in the turbot P. maxima, in autumn, the incremental zone (L-zone) is formed at night whereas the discontinuous zone (D-zone) is form ed during the daytime.