Zebra mussels are diecious (i.e., separate sexes), and the numbers of males and females in most populations are approximately equal. Hermaphrodism (i.e., both sexual organs concurrently present in the same zebra mussel) is encountered, but rarely.
Gonad (sexual organ) development commences during the winter (Gist et al. 1997), then accelerates as temperatures increase in the spring, leading to gamete maturation within 2 months at temperatures around 12 °C (Borcherding 1991). In temperate climates peak spawning often occurs at 15-17 °C during the early summer (Claudi and Mackie 1994), but there are numerous exceptions. Release of gametes by adults can sometimes be a highly synchronized event, focused over a 1-2 week period (Nichols 1996).
A female D. polymorpha releases eggs, visible as tiny white dots, into the water column via the exhalant siphon.
Female zebra mussels can spawn more than a million eggs, and males up to nearly 10 billion sperm, each contributing to more than 30 percent of their body weight prior to spawning (Sprung 1991). Internally, the paired gonads make up much of the visceral mass, releasing eggs or sperm after being stimulated by environmental factors including temperature, rates of temperature change, food availability, and the effects of neighboring mussels (Ram et al. 1996). Since fertilization occurs externally in the water column, release of eggs and sperm must be concurrent.
Mussels are almost always capable of reproducing within their initial 12 months of life. Mussels settling in late spring or early summer typically grow and mature quickly during the warm summer months. Adults of shell lengths exceeding 8 to 9 mm can start to spawn as early as May of the following year. In North American waters, spawning is generally completed by fall. In the southern states, where temperatures are warmer, and in thermally enhanced waters, the spawning period of populations may extend for a longer period during the year, with individual mussels possibly spawning twice per year.