The daily rhythms of temperature preference are conserved in nocturnal and blind fish

Light and temperature are key environmental cues that synchronize circadian clocks, and in nature, these cycles are tightly linked. As ectotherms, fish cope with these daily cycles by adjusting their body temperature through behavioural strategies, as moving to warmer/colder areas depending on their internal state. While this pattern has been described in diurnal fish, it remains poorly explored whether nocturnal fish species show similar or opposite daily patterns, or whether blind cave-dwelling fish exhibit daily variations in thermal preference. To this end, we investigated potential circadian rhythms of thermal preference in two nocturnal species of commercial interest (Tinca tinca and Ameiurus melas) and two blind cavefish (blind Astyanax mexicanus and Phreatichthys andruzzii), both models for studying adaptive and regressive evolutionary traits. Using multi-chambered tanks with a horizontal thermal gradient, fish were allowed to choose preferred temperature over 27 days under different Light/Dark conditions (LD, DL), and constant darkness (DD). All species showed significant daily rhythms of thermal preference, moving to warmer temperatures during the day and cooler ones at night. After shifting from LD to DL, fish quickly resumed diurnal rhythms. Under DD, nocturnal species maintained significant behavioural rhythms of temperature selection, while both cavefish species became arrhythmic. These results reveal a conserved daily rhythm of thermal selection, regardless of the fish's daily activity pattern, pointing to a primarily circadian control. Our results may be applied to improve welfare of fish reared in captivity by providing different time-temperature niches that mimic natural daily light/temperature cycles.