Possible natural circaseptan rhythm in the beach beetle Chaerodes trachyscelides White

Activity rhythms in groups of captive beach beetles (Chaerodes trachyscelid es White) have been recorded in an actograph over a period of 29 days. Unde r constant illumination and when no sand was provided for the beetles to bu rrow in abnormal behaviour occurred. With sand and in constant darkness a s trictly nocturnal activity period with apparently circaseptan components su perimposed was observed. The beetles in their natural habitat are confined to the debris zone where they feed on washed up seaweed and this debris zon e moves up and down depending on the heights of the tides. Circaseptan elem ents in the beetles' activity may act as adaptations to the weekly alternat ions between spring- and neap-tides.