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Influence of photoperiod and temperature on reproductive mode in the Brine shrimp, Artemia franciscana.

Brine shrimp, Artemia, exhibit two modes of reproduction: oviparity (diapause cyst production) and ovoviviparity (live larvae release). Environmental conditions determining these developmental routes are poorly understood, so we investigated the effects of photoperiod and temperature on reproductive mode. Nauplii of A. franciscana were hatched from cysts produced in the Great Salt Lake, Utah, and raised in 2% natural sea salt water under photoperiods of 24, 14, 12, or 10 h at 28 degrees or 20 degrees C. Mating pairs of mature shrimp were isolated and reared continuously under those conditions. The mode of reproduction shown by each pair was determined daily throughout their life span, and found to be greatly affected by photoperiod, and less influenced by temperature. The relative degree of oviparity increased as the photoperiod became shorter at both temperatures. In contrast, the degree of ovoviviparity was higher as the photoperiod became longer at both temperatures. The critical photoperiod appears to be between 12 and 14 h. For all photoperiods examined, the degree of oviparity was higher at 28 degrees C than at 20 degrees C, whereas the degree of ovoviviparity was greater at 20 degrees C than at 28 degrees C.