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Marine Ecology Progress Series

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

Diel changes in endolymph aragonite saturation rate and mRNA expression of otolith matrix proteins in the trout otolith organ

Yasuaki Takagi1,*, Hidekazu Tohse1,2, Emi Murayama2,3, Tsuyoshi Ohira2,4, Hiromichi Nagasawa2

1Graduate School of Fisheries Sciences, Hokkaido University, Minato, Hakodate, Hokkaido 041-8611, Japan
2Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan
3Present address: Unité Macrophages et Développement de l’Immunité, Département de Biologie du Développement, Institut Pasteur, 25 rue du Dr Roux, 75724 Paris cedex 15, France
4Present address: Japan International Research Center for Agricultural Sciences, Owashi, Tsukuba, Ibaraki 305-8686, Japan

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 (Sa) 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 Sa and mRNA expression in the saccular tissue from an individual rainbow trout was developed. Endolymph Ca2+ levels, CO2 partial pressure (PCO2) and pH were simultaneously measured using an automatic pH/gas/electrolyte analyzer, and the Sa 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 PCO2 levels in an antiphasic manner. Endolymph Sa 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

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