Biol. Bull. 207: 152. (October 2004)
© 2004 Marine Biological Laboratory
Circadian Rhythms in the Locomotor Activity of Juvenile Horseshoe Crabs
Mira Guo1,
Frederick Dodge2 and
Robert Barlow2
1 Princeton University, Princeton, New Jersey
2 SUNY Upstate Medical University, Syracuse, New York
The vision of horseshoe crabs, Limulus polyphemus, exhibits a remarkable circadian rhythm. Visual sensitivity increases at night in conjunction with an increase in mating activity. We investigated whether the animal exhibits other endogenous rhythms, and if so, whether they are coordinated with the rhythm of visual sensitivity. We found that juvenile animals exhibit a free-running, endogenous circadian rhythm in locomotor activity. Of animals tested, 10 out of 24 showed regular, cyclic locomotor activity when kept in constant conditions, both in constant light (n = 3) and in constant dark (n = 7). Using an automated, computerized video tracking system and existing circadian data analysis software, we found that cycles fell into two groups divided by period length. In the first group, periods varied from 12.3 h to 15.8 h (average = 13.7 h, standard deviation = 1.5 h); in the second, they varied from 23.1 h to 26.6 h (average = 25.0 h, standard deviation = 1.2 h). The locomotor activity of several animals with endogenous periods in the range of 24 h drifted over the course of 3 to 4 days. It is not clear whether the locomotor activity has a crepuscular component as well as a circadian one. We tested the effectiveness of brief light pulses to shift the phase of free-running activity rhythms. Exposing animals to a 2-h light pulse during their subjective night shifted the phases of 8 out of 12 animals by 0.4 h to 6.2 h. We conclude that juvenile horseshoe crabs possess cycles of locomotor activity. The rhythmic activity cycles free-run under constant conditions and can be shifted by light pulses. Both properties are indicative of endogenous biological oscillators. It remains to be determined whether the clock controlling circadian locomotor activity is linked with the clock modulating visual sensitivity.
Supported by NSF, NEI, NIMH, RPB and the Lions of Central New York.
