. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 116 B. SCHIERWATER AND C. HAUENSCHILD O) JB X o> T3. 23456789 month 10 Figure 3. The annual lite-cycle of Sly/aria The external signaling factor daylength is given for 52°N. lat. The laboratory studies and field samples suggest that the switch from asexual paratomic fission to bisexual reproduction occures in mid September at this latitude. About three weeks later the first worms become sexually mature, and the first diapausing cocoons are produced during October. Further ex- planations are given in the text.
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 116 B. SCHIERWATER AND C. HAUENSCHILD O) JB X o> T3. 23456789 month 10 Figure 3. The annual lite-cycle of Sly/aria The external signaling factor daylength is given for 52°N. lat. The laboratory studies and field samples suggest that the switch from asexual paratomic fission to bisexual reproduction occures in mid September at this latitude. About three weeks later the first worms become sexually mature, and the first diapausing cocoons are produced during October. Further ex- planations are given in the text. not freeze before December. The genetic variation within and between populations would be the predicted basis for adapting 5". lacustris to different annual cycles with respect to temperature and photoperiod ( Sauer, 1977; Sauer el a/.. 1986; Groeters and Dingle, 1987). This could be tested by collecting 5. lacustris at different latitudes with similar annual temperature cycles, or at different altitudes with similar annual photoperiod cy- cles, and measuring the threshold for the photoperiodic reaction (c;/. Hairston and Olds, 1984, 1986). 'All or nothing'life-history strategy By using the vegetative mode of reproduction, S. la- custris can double its number at least every 5 days at 20 to 25°C (Table I). Data on the rates of population in- crease by vegetatively reproducing populations of S. la- custris given in the literature range from mean doubling times of to 12 days between 15°C and 20°C (Streit, 1978; McElhone, 1982; Finogenova, 1984). The data agree with Streit (1978), who calculated mdt of (T = 19°C) and McElhone (1982), who estimated values of 4-6 days (T == 20°C). In the latter case it is not clear whether these are mean values or maximal values. Dur- ing summer all efforts are invested in vegetative repro- duction, leading to the most rapid population increases, regardless of the actual physical environmental condi- tions (r-strategy); during
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Keywords: ., bookauthorlilliefrankrat, booksubjectbiology, booksubjectzoology
