The breeding patterns of Fundulus heteroclitus heteroclitus have been studied intensely. Fundulus heteroclitus heteroclitus have the ability to spawn up to eight times in one season (Rutherford, 1996). During spawning season, males become increasingly aggressive and they begin to display bright colors on their rear fins and bright spotting along the sides of their bodies. The spawning season begins in the spring and lasts until fall. Spawning takes place when the tides are highest during the new or full moon. This is because the eggs develop out of the water. They are laid on almost any surface around the spawning site. Common places for mummichog eggs are in empty mussel shells, on aquatic plants, in pits dug and covered by the female, and even directly on the bottom. The eggs are laid in the shallow area during high tide so when the tide goes out, they will be exposed to the air in which they develop. After the following monthly high tide, they are submerged in water again and begin to hatch (National Oceanic and Atmospheric Administration Coastal Services Center, 2001). This process takes approximately 24 days to complete. Females can release up to 460 eggs at one time and when the eggs are released, they affix themselves to whatever object they first contact (Government of Newfoundland and Labrador, date unknown).
When hatched, the larva of Fundulus heteroclitus heteroclitus are approximately seven millimeters long. They remain in the intertidal zone for six to eight weeks after hatching. Here they live on the outskirts of the marsh during high tides and in shallow pools during low tides. Once the larva are about 15-20 mm in length, they begin to move and swim with the adults in schools. When tides are low, these juveniles no longer stay in the shallow pools but move to subtidal marsh creeks and deep intertidal pools. Full physical maturity is reached in about two years (Rutherford, 1996).