Growth-independent effects of temperature on age and size at maturity in Japanese medaka (Oryzias latipes)

In ectotherms, age, and length at maturity typically decrease with an increase in temperature; however, it is unclear whether this occurs in the absence of accelerated juvenile growth rates that are usually associated with,warmer thermal regimes. We investigated whether (1) life-history traits are affected by temperature in the absence of a growth effect, and (2) faster growth rates cause a reduction in age and size at maturity. Japanese Medaka were exposed to temperatures of 24, 27, 30, and 33 C and fed either ad libitum rations (at all temperatures) or limiting rations (at higher temperatures) to equalize growth rates across thermal regimes. Both age and size at maturity decreased as rearing temperature increased from 24 to 30 C, even when somatic growth rates were equivalent. However, age and length at maturity actually increased with a further temperature increase from 30 to 33 C. Medaka given ad libitum rations grew faster and matured earlier and at a larger size than those given limiting rations and reared at the same temperature. We conclude that temperature can affect the life-history traits of fishes independently of any effect of temperature on somatic growth. However, the direction of the effect may be reversed when the temperature approaches the maximum tolerance of the species and when food availability is low enough to impose severe limits on the amount of surplus energy available to the fish. In this case, the unexpected reversal of life-history effects at the highest temperature may have been caused by the disturbance of normal gonadal development in Medaka.