Environmental control of reproduction in a livebearing fish

Two populations of Poecilia latipinna were sampled in either Tampa or St. Petersburg, FL. P. latipinna has an annual reproductive cycle. Typically, reproduction begins before or around the time of the vernal equinox and ceases in September within weeks after the autumnal equinox. The reproductive cycle of the Tampa population was skewed annually, reproduction beginning and ceasing about a month after that of the St. Petersburg population. A suspected daylength element controlling the annual reproductive cycle was confirmed in laboratory studies. A short 8-hour photoperiod (8L:16D) caused cessation of female reproductive activity. A long 16-hour photoperiod (16L:8D) resulted in maintenance of or initiation of reproductive activity. Reproductive fish caught late in the summer responded to short photoperiods faster than reproductive fish caught in the spring. Male P. latipinna were continuously reproductive, but the gonadosomatic index essentially doubled in long, as compared to short, photoperiod fish. Low temperature could prevent the photoperiodic induction of reproductive activity. The prime environmental control of reproduction in P. latipinna is daylength. Temperature has a secondary effect.