Diapause induction in Daphnia requires two stimuli

Short-day photoperiods in conjunction with a second stimulus were required to induce sexual reproduction (and diapause) in an autumnally diapausing strain of Daphnia pulex. The second stimulus was associated with the unalkalized portion of a small lake and with density of the population in laboratory cultures. Diapause was induced at 12 degrees and 19 degrees C.