This study investigated the effects of temperature changes on the oxygen consumption rhythm in the Japanese eels, Anguilla japonica, using an automatic intermittent flow respirometer (AIFR). The experiments for the oxygen consumption rate (OCR) were carried out for two groups of the eels, i.e., adult (yellow eel) and juvenile stage (glass eel). Both of the freshly collected wild yellow and glass eels displayed a strong endogenous circatidal rhythm under constant temperature and darkness.
Especially, the glass eels which removed from their natural environment (estuary) and continuously maintained for six weeks under the condition of darkness, exhibited a clear endogenous circatidal rhythm in OCR. This results suggest that, yellow and glass eels collected from the brackish waters and the estuary likely exhibit responses that correspond to the tides in their original wild environments.
The magnitude of OCR of glass eels exhibited a highly variable range from 0.02 to 0.52 ㎖ O2 g-1 WW h-1 as the temperature increased from 12 to 25℃. According to the change of temperature, the OCR of glass eels coincided with the gradual increase of water temperature (1℃/24 h) in the experimental chamber. This results indicate that the glass eels were sensitively subject to environmental factors such as water temperature changes as little as 1℃.
The magnitude of the mean OCR (mOCR) of yellow eels showed a highly variable range from 13.5 to 237.7 ㎖ O2 kg-1 WW h-1 under constant conditions. In case of increasing temperature (0.5℃ 14 h-1) from 25 to 40℃, the mOCR of yellow eels exhibited a gradual increase showing the rhythmic pattern to 36℃. Above 36℃, the rhythms of the OCR dampened and then the mOCR decreased rapidly around 36 - 37℃. The OCR of yellow eels exhibited a maximum value at 38℃ and sharply decreased after that. This results suggest that the critical thermal maximum (CTM) of the yellow eels was around 38℃ when water temperature increased by 0.5℃/14 h following acclimation at 25℃.
In case of decreasing temperature (0.5℃ 14 h-1) from 25 to 10℃, the mOCR of yellow eels displayed gradually a decrease to 23℃. However, around 20 - 23℃, there was an agitation which showed a slight increase in the mOCR during one or two days. This results suggest that these changes of the metabolic activity in this species may be related to physiological processes in this temperature regime.