Marine medaka Otyzias melastigma at 4 months (young), 8 months (middle-aged) and 12 months old (senior) were employed to determine age-associated change of sex ratios, sex hormones, telomere length (TL), telomerase activity (TA), telomerase transcription (omTERT) and oxidative damage in the liver. Overall, O. melastigma exhibited gradual senescence, sex differences in longevity (F> M), TL (F> M) and oxidative damage (F < M) during aging. In females, the plasma E2 level was positively correlated with TL (TRF> 5 kb), TA and omTERT expression (p <= 5.0.01), and negatively correlated with liver DNA oxidation (p <= 0.05).The results suggest high levels of E2 in female O. melastigma may retard TL shortening by enhancing TA via TERT transcription and/or reducing oxidative DNA damage. The findings support TL shortening as a biomarker of aging and further development of accelerated TL shortening, abnormal suppression of TA and excessive oxidative DNA damage as early molecular endpoints, indicative of advanced/premature aging in marine medaka/fish. (C) 2017 Elsevier Ltd. All rights reserved.