Male ostracods have two penises and the females have two genital openings. The individual sperm are coiled up within the testis and, in some cases, when uncoiled the sperm can be up to six times the length of the male ostracod itself. Mating usually occurs during swarming, with large numbers of females swimming to the join the males.
In most species, eggs are either laid directly into the water or are attached to vegetation or another surface. In some species, the eggs are brooded inside the shell. The eggs hatch into nauplius larvae which already have a hard shell. Many freshwater ostracods reproduce asexually by cloning themsleves.
Ostracods can be carnivores, herbivores, scavengers or filter feeders. Some predatory Ostracods attack their prey in groups so that they can eat animals much larger than themselves. They have many predators especially small fish.
Some ostracods have a light organ which make light-producing chemicals. Most use the light as defence against predators and a few species use the light for mating.