MEPS 294:249-256 (2005)  -  doi:10.3354/meps294249

ABSTRACT: In the teleost fish otolith, the alternate deposition of CaCO₃-rich and protein-rich layers results in the formation of daily increments. In order to clarify the mechanism of daily increment formation, a precise understanding of the relationship between ionic and organic controls of otolith growth is essential. In the present study, we studied diel variations in the aragonite saturation rate (S_a) of the endolymph and the mRNA expression of 2 major otolith matrix proteins, OMP-1 and otolin-1, in the saccular tissue. A new technique for simultaneously quantifying endolymph S_a and mRNA expression in the saccular tissue from an individual rainbow trout was developed. Endolymph Ca²⁺ levels, CO₂ partial pressure (P_CO2) and pH were simultaneously measured using an automatic pH/gas/electrolyte analyzer, and the S_a was calculated. Total RNA was isolated from sacculi of individual fish after the endolymph was obtained. cDNAs were synthesized and used to quantify OMP-1 and otolin-1 mRNA expression levels by real-time PCR. Significant diel variations were observed in endolymph pH and P_CO2 levels in an antiphasic manner. Endolymph S_a did not exhibit significant diel variations and was maintained at a value of more than 2.0, indicating that the endolymph was kept supersaturated with respect to aragonite during the day-night cycle. Expression of otolin-1 mRNA had apparent diel variations with high levels at night, whereas that of OMP-1 mRNA was almost constant. These data strongly suggest an organic control of daily increment formation in the otolith. The most probable candidate protein for daily increment formation is otolin-1.

KEY WORDS: Calcification · mRNA expression · Endolymph · Daily increment · Aragonite · Otolith · Rainbow trout