Endogenous circadian control of chromatophore pigment dispersion in the intertidal crab Neohelice granulata

Circadian (near 24 h) and circatidal (near 12.4 h) rhythms allow organisms to anticipate daily environmental cycles and optimize physiological responses. In the intertidal crab Neohelice granulata, chromatophore pigment dispersion follows a circadian pattern, with pigments dispersing during the day to darken the animal for UV protection and aggregating at night, causing a lighter phenotype. To investigate the endogenous control of this phenomenon, individual crabs were maintained in continuous darkness (DD) and constant water level for 24–113 h and photographed under IR light every 30 min to quantify carapace color changes. Carapace pigmentation exhibited a robust circadian rhythm in DD, though signal amplitude decreased after the first 24 h—reflecting a transition from initial light-cycle inertia (hourglass effect) to a true free-running rhythm. To account for this amplitude decay, non-linear regression modeling (dampened sine fit) was applied to estimate period (τ). Including the first 24 h of recording yielded estimated values clustered around 24 h. Conversely, excluding the first 24 h increased parameter dispersion, with free-running periods shifting toward longer values ranging from 24 h up to 28.9 h (with several individuals clustering around 27 h). These results confirm an endogenous circadian but not circatidal rhythm of pigment dispersion in N. granulata paving the way for future studies on the dynamics of endogenous clocks properties in decapods.