Abstract
Background: Although most freshwater planarias are well known photonegative organisms, their diel rhythms have never been quantified. Differences in daily activity rhythms may be particularly important for temperate-climate, freshwater planarias, which tend to overlap considerably in spatial distribution and trophic requirements.
Methods: Activity of stress-free, individually tested young adults of three common planarian species was recorded at 3-h intervals in a 10-d experiment under natural sunlight and photoperiod during autumnal equinox (D:L ~12:12). Individual activity status was averaged over the 10-d experiment, each tested individual thus serving as a true replicate. Twelve individuals per species were tested. Food was provided every 36 h, resulting in alternating day- and nighttime feeding events. Activity during the first post-feeding h was recorded and analyzed separately. Statistical procedures included ANOVAs, correlations, and second-order analyses of angles.
Results: Dugesia (= Girardia) tigrinaGirard 1850 exhibited clear nocturnal behavior,Dugesia (= Schmidtea) polychroaSchmidt 1861 was predominantly but not exclusively nocturnal, andPolycelis tenuisIjima 1884 was relatively more active from midnight through noon. Species-specific activity peaks were statistically similar, with peaks at dawn forP. tenuisand just before midnight for the two dugesiids; however,D. tigrinawas comparatively more active in the early night hours, whileD. polychroawas more active thanD. tigrinaduring daytime.D. tigrinaalso responded less readily to daytime food addition.P. tenuisremained poorly active and unresponsive throughout the experiment. Individual variability in diel behavior was highest forD. polychroaand lowest forD. tigrina. P. tenuis's general low degree of activity and late activity peak in the experiment may be related to a strong reliance on external stimuli.
Conclusions: The tested species are mainly nocturnal, consistent with their photonegative characteristics. The fine-scale differences in diel behavior among these three triclad species may not be sufficient to allow coexistence in the wild, with the nonnativeD. tigrinaeventually displacingD. polychroaandP. tenuisin many European waters. The link between planarian diel rhythms and ecological characteristics are worth of further, detailed investigation.