By controlling the inheritance of sex, the sex determination mechanism constrains sex allocation strategies and sex ratio adaptation; however, sex ratio selection also influences the evolution of sex determination mechanisms. Much of the sex determination literature focuses on how sex determination mechanisms transition between genetic and environmental factors (i.e. GSD vs. ESD), and if genetic sex factors are involved, how many (e.g. chromosomal vs polygenic systems). The study of sex allocation largely focuses on deviations in sex ratio from a theoretically 1:1 evolutionarily stable strategy, such as when sex ratios reflect ‘cost’ differences between the sexes. Tigriopus californicus is a tidepool copepod with polygenic sex determination, and shows wide variability in sex ratios in the field and lab that cannot be explained by genetic and stochastic processes alone, which suggests that an environmental variable might influence sex ratio. Females and their offspring were fed diets of different nutritional quality in a crossed design, and the sex ratio of each clutch was recorded for up to 8 clutches from a given female: this design allowed the influence of female diet vs. that of her offspring to be distinguished. The clutch sex ratio changed over the laying order according to the offspring’s diet, which is evidence for environmental sex determination in this species. Sex ratio also showed the influence of maternal diet, consistent with sex allocation theory. While dietary carotenoids showed no association with sex ratio or clutch size, long chain polyunsaturated fatty acids (particularly EPA and DHA) were implicated as the agent of sex ratio effect, providing a direction for future studies. The situation of T. californicus at the intersection of major themes in sex evolution makes this system an ideal model for selection studies. / Graduate / 2018-09-13