Because male fitness improves more than female fitness with increased size, environmental sex determination is adaptive in this system by permitting males to experience a longer growing season than females. The branchiopod crustacean Daphnia magna parthenogenetically produces male progeny in response to a combination of three environmental factors, namely a reduced photoperiod in autumn, shortage of food and raised population density. Annelids Bonellia viridis, a marine worm, has location-dependent sex determination; sex depends on where the larvae land. Vertebrates The sex of most amniote vertebrates, such as mammals and birds, is determined genetically. However, some reptiles have temperature-dependent sex determination, where sex is permanently determined by thermal conditions experienced during the middle third of embryonic development. The sex of crocodilians and sphenodontians is exclusively determined by temperature. In contrast, squamates (lizards and snakes) and turtles exhibit both genotypic sex determination and temperature-dependent sex determination, although temperature dependence is much more common in turtles than in squamates. Ferns Most fern species (with a few exceptions, namely the Salvineales) are homosporous and lack sex chromosomes. Lacking genetic information coding for separate sexes, every fern spore has the capacity to become a male, female, or hermaphroditic gametophyte depending on the environment. In many fern species, including Ceratopteris richardii, environmental sex determination is linked to breeding systems. Fern gametophytes exhibit a wide variety of breed systems which can be divided into outcrossing and inbreeding. To promote outcrossing, female gametophytes release a chemical pheromone known as Antheridiogen which controls the sex of nearby developing gametophytes. Antheridiogen secreted by females promotes the development of nearby asexual gametophytes into males. This is adaptive because inducing maleness increases the probability of outcrossing as males provide sperm for the females rather than the females becoming hermaphroditic (or bisexual) and self-fertilizing. However, if no fertilization occurs, the female gametophyte can still become hermaphroditic and self-fertilize if the conditions are conducive to growth, ultimately resulting in inbreeding depression. Additionally, similar to crocodilians, homosporous fern gametophyte sex is determined by the abiotic environment in accordance with the size-advantage model. In stressful environments (crowding or nutrient stress), gametophytes are smaller and develop into males. While in more favorable growing conditions, gametophytes are larger and develop into females. Moss Moss gametophytes can be either asexual, female, male, or hermaphroditic like ferns. Unlike homosporous ferns, moss gametophytes can be either monoicous or dioicous (similar to monoecious and dioecious in vascular plants), with most studied dioicous