Preliminary Observations on the Effects of Temperature and Light upon Reproduction in Gambusia affinis

A NUMBER of workers have demonstrated that light plays a dominant role, and temperature a subordinate one in controlling the seasonal cycle in the gonads of certain animals. Excellent reviews of the subject have been made by Bissonnette (1936) and Rowan (1938). However, most of the work has been done on homoiothermic animals, and the data on poikilothermic animals are incomplete and seemingly nonconclusive. There is a limited number of reports regarding the experimental work on the effects of temperature and light in the seasonal cycles of fishes. In his investigation of the three-spined stickleback (Gasterosteus aculeatus), CraigBennett (1931) concluded that temperature was the important factor in controlling the sexual cycle and that light was unimportant. His conclusions were questioned by Rowan (1938) because the maximum amount of light used per day was of short duration. Tinbergen (cited from Rowan, 1938) reported that the stickleback was induced to breed at low temperature during the winter by means of light. Hoover (1937; Hoover and Hubbard, 1937) induced Brook Trout (Salvelinus fontinalis) and Rainbow Trout (Salmo gairdneri irideus) to spawn about four months in advance of normal spawning time by means of extended illumination. He suggested a classification of animals, as Garner and Allard (1920) did for plants, according to their sexual photoperiodicity, namely: those reproducing (1) as the day became shorter, (2) as the day became longer, and (3) without demonstrated relation to the length of day. Hoover (cited from Burger, 1939) also reported that light was ineffective in