Japanese Medaka, Oryzias latipes, is a widely used organism in biological investigations because of its high fecundity, small adult size, and ease of husbandry. However, many aspects of its life history have not been fully described. In this study we determine patterns of somatic growth, frequency of egg production, and competency of eggs. We also describe temporal aspects of reproductive output. We modeled the length-at-age relationship with a von Bertalanffy growth function and found mean parameter values: L  =  49.5 mm TL (total length), k  =  0.010 dph−1 (days post hatch), and t0  =  3.3 mm TL. These estimates are used to predict longevity of 347 to 485 days. The weight-at-length relationship was described using a power curve and resulted in mean parameter estimates, a  =  2.16 × 10−5 and b  =  2.79. The mean proportion of eggs that hatched was 73% (95% confidence interval: 53 to 93%). We observed that 50% of larvae emerge from eggs at 3.5 days and 99% of larvae emerge at 7.5 days in incubation. Daily egg production increased to a maximum number (range: 8 to 48) of eggs at 92 days post hatch. Total egg production by individual females ranged from 38 to 141 eggs. We found a significant negative linear relationship (F  =  34.7, P < 0.001, R2  =  0.28) between the number of eggs collected from an individual during a single egg deposition event (n  =  91) and mean egg volume. However, there were large variations in the sizes of individual eggs within a clutch (median CV  =  14.8). Our work provides previously unreported life-history information that can be used to effectively develop experiments in the laboratory and field, and increased understanding of this species.

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