Diel activity cycles of freshwater gastropods under natural light: Patterns and ecological implications

Though much is known about freshwater snail ecology, their circadian rhythms remain poorly investigated. Well-fed, stress-free, mid-size adults of six species common in central Italian lakes were exposed to natural sunlight and photoperiod, and their activity status was recorded at 3-h intervals during a 9-d indoor experiment. All species exhibited evident diurnal habits despite high individual variability, with middayto- early-afternoon activity peaks. Activity was correlated with diel light conditions but not with short-term changes in albedo. The prosobranch Bithynia (=Codiella) leachii and the pulmonates Physa (=Physella) acuta and Planorbis planorbis were the most active species and exhibited the longest-lasting response to daytime food addition. The prosobranch Valvata piscinalis exhibited long periods retracted in its shell with the operculum shut, and the remaining taxa (the pulmonates Galba (=Lymnaea) truncatula and Radix (=Lymnaea) auricularia) exhibited an intermediate degree of activity. P. acuta was the most active species at night and exhibited the quicker response to nighttime food addition. Alertedness to (diurnal) predators may be highest for the highly active P. acuta and P. planorbis, whose antipredator defenses are mainly behavioral. Diel activity patterns and other ecological characteristics suggest that P. acuta may be favored in food-rich habitats, while V. piscinalis may not be able to fully exploit food resources, especially if in limiting quantities. All snail species – and P. acuta in particular – may stimulate periphyton metabolism while keeping its biomass low by grazing mainly during the time of maximum photosynthesis.