Environmental sex determination is the establishment of sex by a non-genetic cue, such as nutrient availability, experienced within a discrete period after fertilization. Environmental factors which often influence sex determination during development or sexual maturation include light intensity and photoperiod, temperature, nutrient availability, and pheromones emitted by surrounding plants or animals. This is in contrast to genotypic sex determination, which establishes sex at fertilization by genetic factors such as sex chromosomes. Under true environmental sex determination, once sex is determined, it is fixed and cannot be switched again. Environmental sex determination is different from some forms of sequential hermaphroditism in which the sex is determined flexibly after fertilization throughout the organism’s life. Adaptive Significance Environmental sex determination is similar to certain forms of sexual selection in that there are oftentimes different and opposing selective pressures on males and females because of the costs of reproduction. Sexual selection is common throughout the tree of life (most known in birds); often resulting in sexual dimorphism, or size and appearance differences between sexes in the same species. In environmental sex determination, selective pressures over evolutionary time have selected for flexibility in sex determination to optimize fitness in a heterogenous environment because of the different costs of sex in males and females. Certain environmental conditions differentially affect each sex such that it would be beneficial to become one sex and not the other. This is especially pertinent for sessile organisms that cannot move to a different environment. In plants, for example, female sexual function is often more energetically expensive because once fertilized they must use significant stored energy to produce fruits, seeds, or sporophytes whereas males must only produce sperm (and sperm-containing structure; antheridium in seedless plants, and pollen in seed plants). Mechanisms Lacking genetic information coding for separate sexes such as sex chromosomes, individuals that exhibit environmental sex determination contain genetic information coding for both sexes on autosomes. In general, once exposed to certain environmental cues, epigenetic changes cause developing individuals to become either male or female. Environmental cues that often trigger the development of males or females include temperature, nutrient (or food in the case of animals) and water availability, photoperiod, competitive stress, and pheromones from conspecific individuals. Specific mechanisms and cues vary between species. Taxonomic range Crustaceans The amphipod crustacean Gammarus duebeni produces males early in the mating season, and females later, in response to the length of daylight, the photoperiod.