Abstract
BackgroundAlthough 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.MethodsActivity 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.ResultsDugesia (= Girardia) tigrina Girard 1850 exhibited clear nocturnal behavior, Dugesia (= Schmidtea) polychroa Schmidt 1861 was predominantly but not exclusively nocturnal, and Polycelis tenuis Ijima 1884 was relatively more active from midnight through noon. Species-specific activity peaks were statistically similar, with peaks at dawn for P. tenuis and just before midnight for the two dugesiids; however, D. tigrina was comparatively more active in the early night hours, while D. polychroa was more active than D. tigrina during daytime. D. tigrina also responded less readily to daytime food addition. P. tenuis remained poorly active and unresponsive throughout the experiment. Individual variability in diel behavior was highest for D. polychroa and lowest for D. 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.ConclusionsThe 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 nonnative D. tigrina eventually displacing D. polychroa and P. tenuis in many European waters. The link between planarian diel rhythms and ecological characteristics are worth of further, detailed investigation.
Highlights
Most freshwater planarias are well known photonegative organisms, their diel rhythms have never been quantified
Though most planarian individuals were active at night, only D. tigrina exhibited evident nocturnal habits (Figure 1; Table 2)
The degree of activity for the two dugesiids was statistically similar at 0:00 and 3:00; D. tigrina was more active than D. polychroa at 21:00, and D. polychroa was more active than D. tigrina from 9:00 through 18:00 (Table 3)
Summary
Most freshwater planarias are well known photonegative organisms, their diel rhythms have never been quantified. The vast majority of published investigations on planarian phototaxis have employed observations of planarian response to abrupt, artificial exposure to light, often with simple light-vs.-dark conditions [e.g., [3,5,6]]. Such an “all-or-nothing” approach did not allow to ascertain planarian behavior in transitional light such as dawn or dusk. More recent findings of diel cycles in planarian melatonin production or storage [10,11] cast doubts on the validity of such artificial dark-vs.-light observations as evidence for nocturnal behavior. The aversion to light by planarias in the early studies cannot be ascribed positively to inherent nocturnal habits
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