Tadpole Shrimps Fairy Shrimps and Clam Shrimps and Water Fleas: Branchiopoda

Both fairy shrimps and water fleas move up and down through the water on a daily cycle. They remain protected in deeper waters during the day and swim to the surface at night to feed. Fairy shrimps swim upside down in a rhythmic motion. Water fleas use their legs to produce a constant current of water that allows them to filter food particles. The food items are collected in a groove at the base of their legs and mixed with mucus to form a bolus or mass that is moved forward toward the mouth. Clam shrimps use their second antennae in addition to their legs for swimming, sometimes in an upside down position, performing spiral or staggered movement. They use their forefeet to collect food, while the hind appendages are modified as mandibles (MAN-dih-bulz) for biting and grinding large food particles.

Tadpole shrimps usually require males and females to reproduce. Some species can produce young without mating, a process called parthenogenesis (PAR-thih-no-JEH-nih-sus). Some species are hermaphroditic (her-MAE-fro-DIH-tik), with individuals having both male and female reproductive organs. Different populations of the same species may use different types of reproductive methods depending on circumstances, allowing them to survive and reproduce under all kinds of environmental conditions. Fertilized eggs are carried in a brood pouch for several hours before they are released into the water. The eggs of some species are incredibly tough and can survive without water and in freezing temperatures for up to one hundred years. The eggs hatch as larvae and develop rapidly. The bodies of larvae have only mouthparts and antennae as appendages. Under the right conditions, tadpole shrimps will molt, or shed their external skeletons, numerous times in just 24 hours. They gain new pairs of limbs with each molt. Adulthood is usually reached in about two weeks after hatching.

Most species of fairy shrimps reproduce by mating or by parthenogenesis and lay eggs. The antennae of the males are specially equipped to hold the female during mating. The eggs are fertilized inside the female's body. Depending on the species, up to 4,000 eggs are laid in a special pouch that is carried outside the body. Eventually, the eggs are released into the water, where they sink to the bottom or float on the surface and later wash up on shore.

Clam shrimps reproduce by mating or by parthenogenesis, or both. The female carries up to several hundred eggs attached to a specialized structure that are laid when she molts. In other species, the eggs are stored in a special pouch attached to the carapace. A few species lay eggs that are resistant to drying out and are distributed by wind or water.

Water fleas reproduce mostly by parthenogenesis but, depending on conditions, will also mate. Reproduction usually begins as temperatures warm in spring. Reproductive activity drops off in summer due to overcrowding and lack of food. In some species a second peak in the population may occur in fall. Eggs are carried in a special chamber between the body and carapace. Some eggs hatch right away, while others enter a resting state called diapause (DYE-uh-pawz). Eggs in diapause are capable of surviving without water and in extreme temperatures. Most eggs develop into females. The development of males is triggered by environmental conditions such as crowding, availability of food, or the shortening day length as fall approaches.