Diel Activity Rhythms in the Nocturnal, Bottom-Dwelling Sharks, Heterodontus francisci and Cephaloscyllium ventriosum

Abstract

The horn shark, Heterodontus francisci, and the swell shark, Cephaloscyllium ventriosum, exhibited distinct nocturnal activity patterns when observed underwater in the natural environment. Swimming activity began shortly after dusk and continued until dawn, after which very little activity was seen. Laboratory records for the horn shark under fixed and shifting LD 12:12 (12 hr light-12 hr dark) regimes indicated an exogenous rhythm, with an activity onset directly controlled by the onset of darkness. Horn sharks showed a high level of aperiodic activity in constant darkness and a low level of aperiodic activity in constant light. Activity in a swell shark appeared initiated by the onset of darkness during fixed LD 12:12 regimes, but slightly preceded darkness during a shifting (1 hr later/day) LD regime. This swell shark exhibited an endogenous (i.e., circadian) rhythm of shortened period (advancing phase drift) in constant darkness, and lengthened period (delaying phase drift) in constant light.