The life history of a zebra mussel begins with the external fertilization of a mature oocyte (egg) by sperm in the water column.
A D. polymorpha sperm enters an egg released by a female, and fertilization occurs.
Temperature appears to be a major trigger in initiating gamete release, which can then ensue for 6 to 8 weeks. See the Water Temperature section for further details.
A female D. polymorpha releases eggs, visible as tiny white dots, into the water column via the exhalant siphon.
Exposure to ripe eggs and sperm in the water may also trigger the release of gametes by other zebra mussels (Nichols 1996). After fertilization, embryological development occurs, including spiral cleavage, blastulation, and gastrulation, ultimately resulting in the formation of a free-swimming ciliated larval form – the trochophore (»80-100 mm).
Following fertilization, embryological development occurs as a single cell divides by mitosis.
The trochophore stage is relatively brief and rarely seen outside of laboratory cultures. Nourishment is supplied by the yolk from the egg in this shell-less, non-filter-feeding larval stage (Nichols 1993).