MEPS 231:237-245 (2002)  -  doi:10.3354/meps231237

Otolith formation and endolymph chemistry: a strong correlation between the aragonite saturation state and pH in the endolymph of the trout otolith organ

Yasuaki Takagi*

Otsuchi Marine Research Center, Ocean Research Institute, The University of Tokyo, Akahama 2-106-1, Otsuchi, Iwate 028-1102, Japan
*Present address: Graduate School of Fisheries Science, Hokkaido University, Minato-cho 3-1-1, Hakodate, Hokkaido 041-8611, Japan. E-mail:

ABSTRACT: This is the first report on the aragonite saturation state of the endolymph in a single fish species, the rainbow trout Oncorhynchus mykiss, based on the direct quantification of electrolyte concentrations in the saccular endolymph. The Ca2+ level, CO2 partial pressure and pH of the saccular endolymph in 1+ and 2+ yr old trout were simultaneously determined using an automatic pH/blood gas/electrolyte analyzer. From the values of CO2 partial pressure and pH, HCO3- and CO32- levels were obtained using the Henderson-Hasselbalch equation. In addition, Na, K, Cl, Mg and inorganic P levels were measured in order to determine ionic strength of the endolymph. The aragonite supersaturation rate (Sa) was calculated from the Ca2+ and CO32- concentrations and the ionic strength. In both age groups, Ca2+ and CO32- concentrations were around 0.75 and 0.68 mmol l-1, respectively. Small differences in Na, P, and HCO3- concentrations were observed between the 2 age groups, but endolymph ionic strength was similar. The Sa ratio was 2.885 to 3.507 in 1+ yr old fish and 2.027 to 4.303 in 2+ yr old fish. Therefore, the endolymph is supersaturated with respect to aragonite. Sa was significantly correlated with CO32- levels, which were largely determined by pH. As a consequence, Sa was strongly dependent on pH, indicating that endolymph pH-regulation is important in the aragonite crystallization of the otolith.

KEY WORDS: Otolith · Endolymph · Chemical composition · Aragonite saturation state · Teleost

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