Seasonal changes in activity/rest rhythms evaluated in LD and DD conditions in the estuarine crab Neohelice granulata

The estuarine crab Neohelice granulata is an excellent model for chronobiology due to its natural exposure to dual environmental synchronizers (zeitgebers): the solar and tidal cycles. Here, we show changes in the activity/rest rhythms and resting architecture of N. granulata between reproductive (spring/summer) and non-reproductive (autumn/winter) periods of the year and evaluate the role of endogenous control. Activity was recorded under natural light-dark (LD) and constant darkness (DD) conditions. Under LD, activity showed a bimodal pattern with higher nocturnal locomotion. While total daily resting time was conserved across seasons, rest architecture exhibited marked seasonal plasticity. Autumn rest was consolidated into a few long events, whereas spring rest was highly fragmented. Under free-running conditions (DD), crabs showed robust endogenous circadian rhythms with similar periods between sexes. Some animals showed endogenous circatidal rhythms despite constant laboratory water levels. Although some seasonal differences in endogenous rhythmicity were observed, these changes were less pronounced than the striking behavioral plasticity seen under LD conditions. Additionally, non-reproductive crabs displayed crepuscular activity peaks in LD that were absent under DD. These findings demonstrate that N. granulata rhythms are driven by internal clocks and dynamically modulated by seasonal factors to meet shifting ecological demands.