Life cycle patterns of rotifers in Lake Peipsi

Life cycle strategies of rotifers in Lake Peipsi (Estonia) were examined. Bisexual (mictic) reproduction was detected in 26 species. Life cycle patterns were determined for 17 species. All three basic life cycle patterns were represented, with some evidence of intraspecific variability. The midcycle pattern, with mixis occurring near the population maximum, prevailed. Extended mixis was observed in littoral populations of Polyarthra luminosa and P. remata. Most species probably are monocyclic. Polyarthra dolichoptera and Synchaeta oblonga may be dicyclic. Mictic periods were not detected in Conochilus hippocrepis and C. unicornis, which probably are acyclic in L. Peipsi. Mictic ratios and egg ratios were calculated for some of the dominant species. The highest amictic egg ratios were observed slightly before or at the population maxima. High mictic ratios (1.0) indicated very intense bisexual reproduction in populations of A. fissa, P. dolichoptera and S. verrucosa. Definite periods of bisexual reproduction could not be distinguished in the rotifer community. During May–October, the spread of mictic reproduction merely reflects the general seasonal distribution of rotifers. Their life cycle strategies represent specific adaptations to unpredictability of their habitat. The results of the study confirm that mixis is an anticipatory event, and not a response to environment deterioration, or an ending of a rotifer population cycle.