Evidence for endogenous rhythmicity in cirral activity of the intertidal barnacle Fistulobalanus albicostatus

Most intertidal invertebrates align feeding and reproduction with the tidal cycle. However, it is unknown to what extent biological clocks are involved in these activities. We focused on the internal and external regulation of cirral activity in Fistulobalanus albicostatus, an intertidal barnacle. Barnacles were subjected to a light-dark cycle (LD) for 14 days, constant darkness (DD) for 14 days, or constant light (LL) for 13 days, while monitoring the changes in cirral activity under laboratory conditions. LD individuals showed a major peak in activity repeating in 24 h, while DD and LL caused two peaks per day repeating at approximately 12 h. The 24 h day-active or day-inactive rhythm under LD often disappeared or showed no clear circadian component after transition to DD. Rayleigh's test showed that activity onset was significantly clustered around specific tidal phases in most experiments, indicating a stable phase-locked circatidal rhythm. Additionally, the timing of collection from the field appeared to influence the response of the cirral activity to the light treatments. These differences in activity may be linked to tidal seasonality. We conclude that F. albicostatus possesses a plastic circatidal rhythm, and that this behavioural flexibility aids adaptation to the demanding intertidal environment.