Abstract
Abstract: :
Purpose: To determine whether horseshoe crabs, Limulus polyphemus, exhibit circadian rhythms in locomotor activity, and if so, whether they are coordinated with the known rhythm of visual sensitivity. Methods: We assessed locomotor activity of juvenile crabs (carapace width < 10cm; age < 4yrs) with an automated, computerized video tracking system (EzVideo Multitrack, AccuScan, Columbus, OH). We simultaneously tracked movement of four juvenile crabs under conditions of constant light (LL) and constant dark (DD), using infrared illumination and an IR sensitive camera. In a separate experiment, we assessed the circadian changes in visual sensitivity of one animal by recording its ERG under DD conditions using an automated, computerized system and an LED light source. Results:Juvenile horseshoe crabs exhibit a free–running, endogenous circadian rhythm in locomotor activity. 10 out of 24 animals tested showed regular, cyclic locomotor activity when kept in LL (n = 3) and in DD (n = 7). Using circadian data analysis software, we found that the period lengths of the activity cycles fell into two groups. In the first group, periods varied from 12.3h to 15.8h (average = 13.7h ± 1.5h); in the second, they varied from 23.1h to 26.6h (average = 25.0h ± 1.2h). The cycles of locomotor activity in several animals drifted out of phase with the solar day. Delivering 2h light pulses during the subjective night in DD phase–shifted the free–running rhythms of 7 out of 12 animals by 0.7h to 7.7h. 5 out of 12 animals also responded with a change in period length of more than 1h; 4 animals added or dropped activity bouts. Finally, we found that the period and phase of the ERG were nearly synchronized with those of the preceding locomotor rhythm. Conclusions: Juvenile horseshoe crabs exhibit endogenous, cyclic locomotor activity. The cycles free–run under constant conditions and can be phase–shifted by light pulses. Both properties are indicative of endogenous biological oscillators. The two groupings of period lengths suggest the existence of crepuscular and/or circatidal oscillators as well as a circadian one. Correspondence between locomotor activity and retinal sensitivity rhythms suggests coupling between the controlling oscillators.
Keywords: circadian rhythms • retinal connections, networks, circuitry