In general, female killifish are larger than the males (ranging from 50-155 mm, 2-6 in), although less brightly colored. As the days lengthen and temperatures rise during the spring/summer reproductive season, hormones cause bright colors to appear on males. These aggressive males fight with each other for the privilege of fertilizing a female, preferably a large one capable of laying up to 50,000 eggs per season. Deposition of eggs is closely tied to the tidal cycle for intertidal fishes like the mummichog. Preference is for placing the sticky mass of eggs above the strand line on stalks of marsh grass (Spartina alterniflora) or in empty mussel shells (Geulensia demissa) during a spring tide, allowing them to develop in air until the next spring tide washes them back into the water, stimulating them to hatch. Thus the eggs themselves are tolerant of a wide salinity range.
Young fish fry emerge from the eggs nine to 18 days after laying and are considered more developmentally advanced than most other fish species. Mortality among the larva and fry is quite high, despite their tendency to take refuge in the shallow intertidal areas beyond reach of the larger fishes and crabs. Given warm temperatures and adequate food resources, size increases throughout the season. While most species achieve reproductive maturity after the first season, few seem to live for more than two to three years. During the winter, many species take refuge in salt marsh pools which provide slightly warmer temperatures than intertidal creeks.
Killifish are generally carnivorous, but a few species also consume algae and other marine plants. Small invertebrates, especially brine shrimp, insect larvae, worms, and various zooplankton form the bulk of their diet. The killifishes in turn become food to many larger fish (such as flounders, perch, eels, and bass), and along the upper edges of the water, feed foraging birds and land crabs. Protection from predation varies with species, but includes cryptic coloring, small, elongated bodies to take advantage of hiding places and identification of potential danger by recognizing color contrasts from either above or below. The upturned placement of the mouth indicates their preference for swimming just under the surface and skimming for prey. Competition for a limited food resource between species of killifish may explain the distribution pattern that limits their cooccurrence in any given habitat.
Unlike the majority of killifishes, the small (30-75 mm, 1.17-3 in) pupfish have a more rounded shape and have omnivorous feeding habits. Its mouth is located on the terminus of the body. They also differ in their breeding strategy. Same age class schools forage together until the spring breeding season (temperatures above 20°C [68°F] from April-October), at which time the males establish territories, leaving females and juveniles in the group. When a female is ready to spawn, she leaves the school and ventures into a territory. Here she indicates her intention by biting a piece of the bottom and spitting it out. The male joins her and she begins laying a single egg at a time. Over a season, she may deposit from 50-300 eggs, depending on her size. High temperatures can speed up the life cycle, making it possible for up to 10 generations a year to occur in hot springs.