Activity rhythms of barnacles in relation to respiration and feeding

Records of cirral activity have been made over periods of 24–48 h for individual specimens of eight species of barnacles from three different localities. The effects of movement, gas content, and hydrogen-ion concentration of the water have been studied.In still water most species eventually settle down to a more or less steady rhythm (activity rhythm) of short bursts of cirral beating interspersed with inactive periods. Two exceptions are the surf- or current-loving species Balanus cariosus and Tetraclita squamosa which remain inactive or eventually cease activity. In moving water all species show practically continuous activity, the velocity of the flow needed to produce this response varying according to the species, habitat, and previous history of the specimen. Increased oxygen content has less effect on activity than other factors studied, but eventually appears to reduce activity. A gradual reduction in oxygen content first leads to increased activity and then signs of distress attributable to anoxia.