Locomotor activity rhythm in the Japanese eel Anguilla japonica elvers

Under artificial LD cycles (6, 12, 18 L), the elvers of Japanese eel, Anguilla japonica, showed a 24 h cycle of locomotor activity rhythm being most active at light transitions:the eels' activity rose to a primary peak after lights-off, followed by a quiescent period during which they buried into the shelters or lying motionlessly on sand for most of the time, and then reached a secondary peak before lights-on.Elvers could resynchronize their activity rhythm with a new photo cycle within 4 d.Moreover, their activity level at dark phase significantly increased as the light period was prolonged:higher activity levels during shorter dark period.However, the elvers did not display clearly the existence of a circadian rhythm under constant light or dark conditions.The timing of daily activity rhythm evidenced in the Japanese eels may occur through the action of the LD cycles with a weak participation of an endogenous circadian system.In all the LD cycles, over 99% of the activity occurred in the dark phase, indicating that the eels were always nocturnally active no matter what time of day it might be.Under 12 L conditions, the eels' activity level and the time outside sand were significantly elevated both at light and dark phases as temperature increased from 10~15 to 20~25℃.The activity rhythm pattern (i.e., two peaks occurring around light transitions) did not apparently change among temperatures.However, in contrast with the primary activity peaks immediately after lights-off at 20 and 25℃, the timing of the primary peaks at 10 and 15℃ showed a latency of a few hours following lights-off, indicating the inhibiting effect of low temperature on the eels' activity.