Shifting phase of circadian locomotor activity by light in the freshwater crab Pseudothelphusa americana

Abstract

The locomotor activity of the crab Pseudothelphusa americana displays a circadian rhythm consisting of two main bouts, a morning (lights-on related) and an evening (lights-off related) bout. The aim of the present work is to elucidate whether each bout of activity displays similar phase response shifts when exposed to a 2 h light pulse (150 lux). Phase shifts were evaluated in the free running rhythm under constant darkness from animals previously entrained to a light-dark regimen. Results obtained show that phase shifts are observed for both morning and evening peaks during the subjective day, while during the subjective night, only the evening peak of activity displays significant phase shifts. The aforementioned indicates that a multi-oscillatory system may drive the locomotor activity circadian rhythm in the fresh water crab Pseudothelphusa americana, and the differences in PRC shapes for evening and morning peaks could represent the light sensitivity of two different oscillators.