Reproduction

NZMS reaches sexual maturity at approximately 3.5 mm, which is uasually after only 6-9 months and can produce an average of 230 juveniles per year (Lassen 1979). However fecundity rates tended to be a function of shell size and not a function of water temperature. Large NZM snails produced 0.1 to 1.3 daughters per day (Hall et al. In preparation).

Each snail can live up to two years. In the New Zealand mud snail's (NZMS) native habitat, the snail reproduces sexually, with the young being born every three years. However in the habitats it has invaded in the Western U.S. the vast majority of the specimen are females. This appears to be because the snails are asexually cloning themselves in a form of reproduction called parthenogenesis. Asexual females are capable of producing twice the number of daughters as sexual females, giving a full two-fold cost of sex (Lively , pers. comment). Genetic studies have shown that asexual lines are derived from sympatric sexual females; and that clonal diversity in mixed populations is very high (Dybdahl & Lively 1995).The NZMS independence from sexual reproduction also enables a single female to invade a new habitat and, through parthenogenesis, become the dominate mollusk in a few seasons.

 Females may be dominating the population because their native predator, a larval trematode parasite (genus Microphallus), is not present.  Natural predation like this will push the prey to keep mixing the gene pool in an attempt to prevent the predator from adapting to overcome all the prey’s defensive mechanisms (see the so-called “red queen hypothesis”). However, without the pressure from a natural predator the snails can save the energy of sexual reproduction and simply clone themselves because they have a genotype that works.  Evolution only happens when external pressures require adaptation. 

Being able to reproduce without sex appears to be allowing the snail to birth young year round, reaching its reproductive peak in summer and autumn.  In the western US, the snail reaches sexual maturity at 3.0 mm.   Number of young and brooding time: ranges from 20-120 embryos per female (Richards unpublished data, Winterbourn 1970); some sources suggest that the snail produces approximately 230 young per year. Reproduction occurs in spring and summer, and the life cycle is annual (Zaranko et al. 1997; Schreiber et al. 1998; Lively and Jokela 2002; Gerard et al. 2003; Hall et al. 2003). Othe sources suggest that reproduction peaks in spring and autumn (Richards, Kerans, Gustafson, Shinn, pers. comm.). 
Young snails are born every three months in New Zealand (Winterbourn 1970). New Zealand mud snails are live bearers (they release embryos and not eggs), and therefore, the presence of newly released young may indicate a possible population