A Photoperiod Determined Life-Cycle in an Oligochaete Worm.

For one common cosmopolitan naidid worm, Stylaria lacustris, we studied the effects of different environmental factors upon (1) the alternation of reproductive modes, (2) the rates of population increase, and (3) the combination of each of (1) and (2). While age, temperature, population density, or rate of feeding did not affect the mode of reproduction, photoperiod had a dominant effect. Under long-day conditions (LD > 12: 12), all worms reproduced exclusively by paratomic fission, theoretically ad infinitum. When transferred to short-day conditions (LD {le} 12:12) the worms ceased vegetative reproduction, and within 2 to 4 weeks developed the hermaphroditic genital apparatus and a clitellum. After an additional two weeks, the first cocoons were produced. The switch to the bisexual mode of reproduction was cum grano salis irreversible. These findings are consistent with observations of field samplings, and allow one to predict the annual life-cycle strategy of S. lacustris. This is the first example of a photoperiod determined life-cycle within the oligochaete worms. The vegetative mode of reproduction led to extremely high rates of population increase, whereas with the bisexual mode of reproduction the number of individuals was roughly stable. However, because S. lacustris could not withstand temperatures of 5{deg}C or lower, the switch to sexual reproduction and the formation of diapausing cocoons appear to be the only mechanism of overwintering. Nevertheless, some `asexual' clones never switch to sexual reproduction, whereas a loss of the asexual vegetative mode of reproduction did not occur. In contrast to some general predictions from life-history theories, the reproductive strategy of S. lacustris is highly preprogrammed and cannot respond to sudden and unexpected environmental changes.