hormones or hormone-inhibitors to the water in which they are reared. Aromatase, an estrogen-synthesizing enzyme which acts as a steroid hormone, plays a key role in sex determination in many non-mammalian vertebrates, including the Iberian ribbed newt. It is found in higher levels in the gonad–mesonephros complexes in ZW larvae than in their ZZ counterparts, although not in heat-treated ZW larvae. The increase occurs near the final stages of which their sex can be determined by temperature (stage 52). Conservation The IUCN has listed the Iberian ribbed newt as Near Threatened since its 2006 Red List. It received this listing because its wild populations appear to be in significant decline due to widespread habitat loss and the effects of invasive species, thus making the species close to qualifying for Vulnerable. Previously, in 2004, the species had been listed as Least Concern, the lowest ranking. This species is generally threatened through loss of aquatic habitats through drainage, agrochemical pollution, the impacts of livestock (in North African dayas), eutrophication, domestic and industrial contamination, golf courses, and infrastructure development. It has largely disappeared from coastal areas in Iberia and Morocco close to concentrations of tourism and highly populated areas such as Madrid's outskirts. Introduced fish such as the largemouth bass and crayfish (Procambarus clarkii) are known to prey on the eggs and larvae of this species, and are implicated in its decline. Mortality on roads has been reported to be a serious threat to some populations. Space experiments Pleurodeles waltl has been studied in space on at least six missions. The first Iberian ribbed newts in space may have been in 1985 on board Bion 7. The ten newts shared their journey with two rhesus macaques and ten rats, in an otherwise crewless Soviet Kosmos satellite. In 1992, Bion 10 also carried the newts on board, as did Bion 11 in 1996. Pleurodeles waltl research was continued later in 1996 by French-led experiments on the Mir space station (Mir Cassiopée expedition), with follow-up studies in 1998 (Mir Pégase expedition) and 1999 (Mir Perseus expedition). Foton-M2 also carried the Iberian ribbed newt in 2005. The newts were chosen because they are a good model organism for the study of microgravity. They are a good model organism because of the female's ability to retain live sperm in her cloaca for up to five months, allowing her to be inseminated on Earth, and later (in space)