9 J. exp. Biol. 190, 9–21 (1994) Printed in Great Britain © The Company of Biologists Limited 1994 CIRCADIAN LOCOMOTOR ACTIVITY AND ITS ENTRAINMENT BY FOOD IN THE CRAYFISH PROCAMBARUS CLARKI FRANCISCO FERNÁNDEZ DE MIGUEL* AND HUGO ARÉCHIGA Departamento de Fisiología, Biofísica y Neurociencias, Centro de Investigación y Estudios Avanzados del IPN, Apartado postal 14-740, 07000, México, DF Accepted 18 January 1994 Summary The aim of our experiments has been to study the effect of light and food in the locomotor activity rhythm of the crayfish Procambarus clarki. Experiments were carried out under light:dark (LD) cycles of 12 h:12 h, under continuous darkness (DD) and under continuous light (LL). Under LD cycles, two peaks of activity were observed during the night phase of the cycle, while resting was characteristic of the day phase. Under DD or LL, it was possible to follow a free-running rhythm with a periodicity of 22.3±0.84 h in DD and 24.8±0.27 h in LL, typical of circadian rhythms of nocturnal species. A single delivery of food in the day phase of the LD cycle resulted in an outburst of locomotor activity that lasted for several hours. In the ensuing days, an activity peak appeared in phase with the time of food delivery. The food-related activity peak could be followed for up to 2 weeks without food reinforcement. Under DD and LL, food induced an activity rhythm in previously arrhythmic animals. Here the period was longer than 24 h in DD (26.2±0.12 h) and shorter in LL (22.5±0.46 h). Together, these results strongly suggest that light and food may play a role entraining a locomotor activity rhythm in crayfish. Introduction It is commonly accepted that overt biological rhythms result from the interplay of endogenous timing mechanisms and a complex host of environmental entraining agents (Pittendrigh, 1981). The characterization of entraining influences and the nature of their interactions with the intrinsic pacemaking systems are critical to the understanding of biological rhythms. There is ample evidence that food can entrain circadian rhythms (Boulos and Terman, 1980), and its interactions with endogenous rhythms and with light:dark cycles have been described in mammals (Stephan, 1986; Coleman and Francis, 1991; Hau and Gwinner, 1992; Jilge, 1992). In invertebrates, the interaction of food with the time sense of bees has been known for a long time (see Kolterman, 1971), although the information on food as an entraining agent is rather limited. *Present address: Departamento de Neurociencias, Instituto de Fisiología Celular, Universidad Nacional Autonóma de México, Apartado postal 70-253, 04510 Mexico, DF. Key words: circadian rhythms, locomotor activity, food, crayfish, Procambarus clarki, crustacean.