Hydra hymanae: regulation of the life cycle by time and temperature.

Hydra hymanae, a hermaphroditic freshwater coelenterate, reproduces asexually at 24 degrees C and sexually at 15 degrees C. The appearance of gonads begins 12 days after transfer from 24 degrees to 15 degrees C and is complete 35 days after the temperature transition. Testes appear before eggs. Fifty percent of the mature embryos maintained at 15 degrees C hatch by day 61, but they have a low level of survival. Fifty percent of the mature embryos pretreated for from 5 to 25 days at 4 degrees C hatch by about day 45, and these have a high level of survival. Embryos maintained at 4 degrees C for longer periods (55 to 85 days) accumulate in a prehatching state and hatch with a high degree of synchrony approximately 7.5 days after return to 15 degrees C. Populations derived from newly hatched polyps are refractory to sex induction for approximately 120 days. The system is well adapted to ensure a regular alternation of reproductive modes in the natural environment.