Entrainment of the circadian rhythm of the supratidal amphipod Talorchestia longicornis by light and temperature: mechanisms of detection and hierarchical organization

The amphipod Talorchestia longicornis is active on the substrate surface during the night and inactive in its burrows during the day. The underlying circadian rhythm in activity can be entrained by the light : dark cycle. This study considered other aspects of entrainment, such as: whether the compound eyes or extraocular photoreceptors are used to detect entrainment cues, which visual pigment mediates the entrainment process, whether entrainment can also occur in response to diel temperature cycles and whether when presented with conflicting entrainment cues, the light : dark or temperature cycle dominates for entrainment. Entrainment was determined by monitoring temporal changes in surface activity using a video system, and assessed as the number of animals active on a sand substrate at 0.5 h intervals. The compound eyes and not extraocular photoreceptors are used for entrainment to the light : dark cycle. Although T. longicornis has two visual pigments with absorption maxima near 420 and 520 nm, entrainment occurred only by light that simulated the 520 nm absorbing pigment, perhaps because its absorption is matched to the spectrum of ambient light at twilight. The rhythm is also entrained by a diel temperature cycle. If amphipods are simultaneously exposed to a temperature and light : dark cycles phased differently, they entrain to the temperature cycle. These results suggest that for a burrowing amphipod, diel temperature cycles may be a more reliable indicator of environmental conditions than light cycles.