In the late 1980s, zebra mussels invaded the Great Lakes and by 1989 quagga mussels were also found in the Great Lakes and began out competing zebra mussels. These mussels have caused many changes to the Great Lakes and the food web by filtering the water, creating waste, and creating multi-dimensional structures for macroinvertebrates to use as shelter. Previous studies have found that mussels become reproductively active between 5-12 mm in length, and their peak spawning season occurs during early July. Literature on the breeding habits of zebra mussels is extensive, however not as much research has been done on quagga mussels. This project aims to fill some of our gaps in knowledge by determining at what size quagga mussels reach sexual maturity. At five time points during the summer and one during the Fall, a quagga and zebra mussels were collected from the pier where Muskegon Lake connects to Lake Michigan. Mussels between the lengths of 5-25 mm were were induced to spawn by exposing them to serotonin and each mussel was scored based on how many gametes they released. The mussels were then dissected and their gamete maturity was assessed based on squash mounts of their gonads. Our results showed that there was no significant species differences in the mussels breeding behaviors or development under the conditions of this study. We found significant differences in mussels spawning over different lengths, with mussels not releasing gametes until they were about 7 mm in length and not releasing significant amounts of gametes until they were over 18 mm in length. It was discovered that early July had the highest mean gamete maturity, and October had the lowest. Invasive mussels have been wreaking havoc in the Great Lakes for decades, and researchers hope that by studying the reproductive capabilities of these invasive mussels, they will be able to predict their spread and population numbers and better anticipate future impacts.