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Rationale: Otoliths are usually used to estimate the age of fish and the chemical composition such as nitrogen stable isotope ratios (δ 15 N values) may record environmental information and ecological role of the fish. However, the isotopic fractionation of δ 15 N values between diets and otoliths has rarely been investigated and remains unclear., Methods: Nitrogen isotopic fractionation between five different diets (δ 15 N diet values) and otoliths (δ 15 N oto values) were elucidated in tilapia Oreochromis mossambica reared in controlled feeding experiments. The otoliths were dissolved with hydrogen chloride and peroxodisulfate was used to oxidize the total organic materials to nitrate, which was further converted into N 2 O gas by denitrification bacteria before the measurement of δ 15 N oto values by isotope ratio mass spectrometry. The δ 15 N values of muscles, gills, scales and livers of the tilapias were also measured by isotope ratio mass spectrometry., Results: The peroxodisulfate oxidation-bacterial conversion method reduced the minimum mass of the otoliths required for analysis to as low as 2 mg, unlike past methods, which have required masses of 8-155 mg. The δ 15 N oto values were not significantly different from the δ 15 N diet values of the five diets. Furthermore, the somatic growth rate had no effect on the δ 15 N oto values. Nevertheless, the δ 15 N values of metabolically active tissues were significantly different from each other and higher than the δ 15 N diet values, due to the deamination of these tissues., Conclusions: These results suggest that diet was the main source of amino acids for the otolith organic matrix and there was no biochemical transamination during the assimilation of dietary amino acids to otoliths. The δ 15 N oto value can be used as a proxy of nitrogen sources of fishes and may have potential application in ecological studies such as the detection of diet shift, migration, trophic levels and environmental changes experienced by the fish population., (© 2018 John Wiley & Sons, Ltd.)

Rationale: Nitrogen and carbon stable isotope ratios (δ 15 N and δ 13 C values) of carbonate-bound organic materials in otoliths can provide information to address the biological and ecological functions of fish. Correct interpretation of otolith δ 15 N and δ 13 C profiles requires knowledge of the metabolic routes of nitrogen and carbon isotopes. However, the isotopic assimilation of δ 15 N and δ 13 C compositions from diets to otoliths has rarely been investigated., Methods: This study traced the daily nitrogen and carbon isotopic assimilation between diets and otoliths using nanoscale secondary ion mass spectrometry (NanoSIMS). Isotopically labeled algae (Tetraselmis chui) were fed to tilapia (Oreochromis niloticus) for 14-17 days. NanoSIMS and conventional isotope ratio mass spectrometry were used to measure δ 15 N and δ 13 C variations in the otoliths and fish muscle, respectively., Results: Otolith δ 15 N values abruptly surged from natural abundance levels by 1000-2300‰ after the fish ate 15 N-spiked algae with δ 15 N values of approximately 2200‰. However, the δ 15 N values of fish muscle increased to only approximately 500‰ at the end of the feeding experiment. Much higher δ 15 N values (3700-14 000‰) and moderate δ 13 C values (60-200‰) were detected in the otoliths after the tilapia ate 15 N- and 13 C-spiked algae with a δ 15 N value of 36667‰ and a δ 13 C value of 272‰. Mapping analysis showed sub-micrometer-scale distribution of 15 N embedded in the otolith growth increments with a low-to-high δ 15 N signal after the tilapia shifted diets from non-spiked to 15 N-labeled algae., Conclusions: These results suggest that otolith nitrogen and carbon isotopes from food were directly assimilated on the same day. Food is the major and in some cases only source of otolith nitrogen isotopes but makes only a partial contribution to otolith carbon isotopes. Therefore, the δ 15 N values recorded in the sclerochronological layers of the otoliths can be used to determine the trophic levels, food sources and diet changes of fish., (Copyright © 2018 John Wiley & Sons, Ltd.)

Thyroid-mediated otolith growth in tilapia was evaluated by the ontogenic triiodothyronine (T3) profile revealed by radioimmunoassay during the first month after hatching. Thyroid hormone receptor genes (TRalpha and TRbeta) were cloned and only the expression of TRalpha mRNA, quantified by real-time PCR, was similar to the T3 profile. Variations in otolith growth showed median correlation with the T3 profile and TRalpha mRNA expression pattern. Hypothyroidism and hyperthyroidism were induced in tilapia juveniles and larvae by administration of different concentrations of thiourea (TU) and T3, respectively, for 13 days. T3 and TU had little effect on otolith growth during the larval stage. However, T3 increased otolith growth and TU retarded, or stopped, otolith growth during the juvenile stage. Furthermore, TU treatment caused permanent changes in otolith shape in the ventral area. Otolith growth recovered slowly from hypothyroidism, requiring 2 days to form an increment during the first week. These results suggest that otolith growth, at least during the juvenile stage, is regulated by the thyroid hormones and the process may be mediated by TRalpha.