Influence of the light-dark cycle in the diel activity rhythms of sea lamprey’s ammocoetes

Abstract

Ten sea lamprey ammocoetes, tagged with passive integrated transponders (PIT), were released in a tank (2 m3 capacity) and their position monitored twice a day, for a period of one month. Ammocoetes’ locomotor activity appeared to be conditioned by circadian rhythms. Differences in the proportion of detected activity versus apparent inactivity resulted mainly from the observations registered during the night. Ammocoetes were more active throughout the night period, particularly in the first and second weeks of trials. After an initial period of enhanced activity, differences between daily activity patterns tended to disappear. During the entire experimental period, ammocoetes spent most of the time stationary. There were no significant differences found when comparing the distance moved by the ammocoetes during the dark and light periods, through which they display a similar diel locomotor behaviour. On the other hand, since movements were more frequent at night, the total distance moved by the lampreys is clearly higher during the dark period. This method was found to be a valuable and effective technique to locate and identify individual ammocoetes burrowed up to 10 cm deep in the substrate, with acceptable accuracy and minimum disturbance.