Seasonal changes in entrainment cues for the circadian rhythm of the supratidal amphipod Talorchestia longicornis

The supratidal amphipod Talorchestia longicornis Say has a circadian rhythm in activity, in which it is active on the substrate surface at night and inactive in burrows during the day. The present study determined: (1) the circadian rhythms in individual versus groups of amphipods; (2) the range of temperature cycles that entrain the circadian rhythm; (3) entrainment by high-temperature cycles versus light : dark cycles, and (4) seasonal substrate temperature cycles. The circadian rhythm was determined by monitoring temporal changes in surface activity using a video system. Individual and groups of amphipods have similar circadian rhythms. Entrainment occurred only to temperature cycles that included temperatures below 20 degrees C (10-20, 15-20, 17-19, 15-25 degrees C) but not to temperatures above 20 degrees C (20-25, 20-30 degrees C), and required only a 2 degrees C temperature cycle (17-19 degrees C). Diel substrate temperatures were above 20 degrees C in the summer and below 20 degrees C during the winter. Upon simultaneous exposure to a diel high-temperature cycle (20-30 degrees C) and a light : dark cycle phased differently, amphipods entrained to the light : dark cycle. Past studies found that a temperature cycle below 20 degrees C overrode the light : dark cycle for entrainment. The functional significance of this change in entrainment cues may be that while buried during the winter, the activity rhythm remains in phase with the day : night cycle by the substrate temperature cycles. During the summer, T. longicornis switches to the light : dark cycle for entrainment, perhaps as a mechanism to phase activity precisely to the short summer nights.