the costs involved in finding mates. However, asexual offspring are clonal, so lack variation. This makes them susceptible to parasites, as the entire clonal population has the same resistance mechanisms. Once a strain of parasite has overcome these mechanisms, it is able to infect any member of the population. Sexual reproduction mixes up resistance genes through crossing over and the random assortment of gametes in meiosis, meaning the members of a sexual population will all have subtly different combinations of resistance genes. This variation in resistance genes means no one parasite strain is able to sweep through the whole population. New Zealand mudsnails are commonly infected with trematode parasites, which are particularly abundant in shallow water, but scarce in deeper water. As predicted, sexual reproduction dominates in shallow water, due to its advantages in parasite resistance. Asexual reproduction is dominant in the deeper water of lakes, as the scarcity of parasites means that the advantages of resistance are outweighed by the costs of sexual reproduction. Each female can produce between 20 and 120 embryos. The snail produces approximately 230 young per year. Reproduction occurs in spring and summer, and the life cycle is annual. The rapid reproduction rate of the snail has caused the numbers of individuals to increase rapidly in new environments. The highest concentration of New Zealand mudsnails ever reported was in Lake Zurich, Switzerland, where the species colonized the entire lake within seven years to a density of 800,000 per m². Parasites The parasites of this species include at least 11 species of Trematoda. Common parasites of this snail include trematodes of the genus Microphallus. In their native habitat, these parasites sterilize many snails, keeping the populations to a manageable size. However, elsewhere in the world in the absence of these parasites, they have become an invasive pest species. Other interspecific relationship Potamopyrgus antipodarum can survive passage through the guts of fish and birds and may be transported by these animals. It can also float by itself or on mats of Cladophora spp., and move 60 m upstream in 3 months through positive rheotactic behavior. It can respond to chemical stimuli in the water, including the odor of predatory fish, which causes it to migrate to the undersides of rocks to avoid predation. See also Invasive species of New Zealand origin References Further reading Kerans, B. L, M. F. Dybdahl, M. M. Gangloff and J. E. Jannot.