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

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Branchiopods (BRAN-kee-oh-pods) come in a variety of forms and are difficult to define as a group. They have two pairs of antennae, both of which are uniramous (YU-neh-RAY-mus), or unbranched at their bases. They all have mouthparts that are either lacking or greatly reduced in size. The maxillipeds (mack-SIH-leh-pehds), or leglike structures that are associated with the mouth, are absent. A shieldlike carapace (CARE-eh-pes) covers the head and segments of the thorax or midbody. The number of segments and limbs varies. The thoracic (thuh-RAE-sik) or midbody limbs are leaflike and are used for swimming, filtering water, breathing, or gathering food. The abdomen or tail usually does not have any appendages underneath, but does have threadlike appendages on the tip. The Branchiopoda is divided here into different groups: fairy shrimps, tadpole shrimps, and clam shrimps and water fleas.

Fairy shrimps are medium-sized branchiopod crustaceans and usually measure from 0.39 to 1.18 inches (10 to 30 millimeters) in length. Some predatory species may reach 3.9 inches (100 millimeters). Their limbs are flat and leaflike, and they differ from other branchiopods because they have no carapace. Most fairy shrimps have 11 thoracic segments, but some species have as few as 10 or as many as 17 or 19. Nearly all species are translucent and lack any kind of coloration. However, the egg sacs carried by females are usually brilliant orange, red, or blue. The threadlike appendages at the tail in some species are red or orange. The males of some species have specialized antennae that are used to grasp the females during mating.

Tadpole shrimps range in length from 0.4 to 1.6 inches (10 to 40 millimeters), with some species measuring 4 inches (110 millimeters). Like fairy shrimps, they also have 11 thoracic segments. A large, flat carapace covers the head and midbody of tadpole shrimps. The carapace is attached only to the head and partially covers the abdomen. They have a pair of compound eyes located on the front margin of the carapace, each with multiple lenses. The broad carapace and narrow abdomen give these animals a tadpolelike appearance. The carapace ranges from silvery gray, yellowish, olive, to dark brown and is sometimes spotted. Their bodies are usually translucent, but may be pinkish or reddish due to the presence of hemoglobin (HE-meh-GLO-bihn) in their blood. Hemoglobin is a protein in blood that captures oxygen.

Clam shrimps and water fleas usually range in size from 0.008 to 0.7 inches (0.2 to 17 millimeters). They resemble clams and other bivalve mollusks that have hinged shells. These branchiopods are flattened from side to side and are protected by a large, hinged or folded carapace that mostly or completely covers their bodies and limbs. Their bodies are divided into two regions, the head and body trunk. In clam shrimps the halves of the carapace are hinged like a clam and nearly cover the entire body. The carapace even has growth rings, just like a clam. The bodies of water fleas are never completely covered by the carapace. Their carapaces fold over their backs like an upside down taco, leaving their heads exposed. The body trunk of clam shrimps are made up of 10 to 32 segments, each with a pair of flattened, leaflike limbs. The trunks of water fleas have only 4 to 6 limbs located toward the head. They use their larger pair of antennae for swimming, while the smaller pair is used to sense their environment.


Branchiopods are found on all continents.


Most fairy shrimps live in temporary rain pools, but some species prefer high mountain lakes, Arctic or Antarctic ponds, or saline lakes. They usually swim in open water and prefer habitats that are free of fish and other predators. Tadpole shrimps live only on the bottom of temporary rain pools, avoiding predation by fish entirely. Clam shrimps also prefer temporary pools and ponds, although some species live in permanent bodies of water covered with thick mats of algae (AL-jee), or plantlike growths that live in water. They are usually found burrowing in mud. Water fleas are sometimes common in lakes, ponds, slow-moving streams, and rivers. A few species are found in the sea. Depending on the species, they are found swimming in open water, on vegetation, or on the bottom. One species lives in water collected on mosses living up in the tree canopy of tropical cloud forests in Puerto Rico.


Most fairy shrimps use their specialized leg bases to filter bacteria and algae from the water and direct it to a groove under their bodies. The groove leads directly to the mouth. Some species eat microscopic animals, such as rotifers, crustacean larvae (LAR-vee) or young, and even other fairy shrimps. Some species are predatory and attack other crustaceans. Tadpole shrimps eat anything, living or dead, including bacteria, algae, microscopic animals, insect larvae, tadpoles, other crustaceans, plant roots and shoots. Clam shrimps are mostly filter feeders, eating whatever flows through their mouths in water, but they can also scrape and tear at their food and will scavenge almost any organism in their environment. Some species also scrape algae from rocks and eat bacteria or prey on microscopic animals.


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.


Some fairy shrimp eggs are called cysts (cists). Cysts are resistant to drought and extreme temperatures and remain dormant or inactive until conditions are more favorable for development. In 1960 the cysts of Artemia were first sold through comic books as Sea Monkeys. The cysts hatch just hours after adding water. Today, people around the world still buy kits promising animals that develop quickly, swim upside down, breathe through their feet, and reproduce with or without sex.

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.


Eggs of fairy shrimp are collected, cleaned, dried, packed, and sold to pet stores and fish farms as food for fish. They also provide people living in parts of Libya with their major source of animal protein. Two species from the hills of northeastern Thailand are fished by local people and used in a variety of dishes. The tadpole shrimps are sometimes considered pests of rice fields. In large numbers, these animals expose and eat the roots of rice seedlings as they roam the bottom in search of food. However, in Japan, Triops longicaudatus is used to help control weeds in rice fields. Some species of Triops are sold in kits for rearing as pets. Water fleas are a critical link in the food webs of many aquatic habitats and ensure the survival of fish populations.


Twenty-eight species of fairy shrimps are listed by the World Conservation Union (IUCN). Six are considered Critically Endangered or facing an extremely high risk of extinction in the wild; nine are Endangered or facing a very high risk of extinction in the wild. Ten are Vulnerable or facing a high risk of extinction in the wild; one is Lower Risk/Conservation Dependent, meaning if the conservation program were to end, the animal would be placed in one of the threatened categories. One is Lower Risk/Near Threatened or at risk of becoming threatened with extinction in the future, and one is listed as Data Deficient, which means there is not enough information to make a judgment about the threat of extinction. Five of the Endangered species live in the United States, three of which live in California. They are threatened by the development of land for farming, home, and business interests. Efforts are underway in California to protect the habitats of these species.

The IUCN also lists a tadpole shrimp in California, Lepidurus packardi, as Endangered. It is threatened by habitat destruction. No species of clam shrimps or water fleas are considered endangered or threatened.


Physical characteristics: The longtail tadpole shrimp is a large species that reaches up to 1.5 inches (40 millimeters) in length. The second maxilla is absent.

Geographic range: The species is found in North America (including Hawaii but not Alaska), Central America, South America, Japan, the West Indies, the Galápagos Islands, and New Caledonia.

Habitat: The longtail is the most widely distributed of all tadpole shrimps. It is found in a wide variety of temporary waters, including rice fields.

Diet: Longtail tadpole shrimps scavenge both living and dead plant and animal materials. They also prey on microscopic animals, insect larvae, other small crustaceans, and even each other.

Behavior and reproduction: They root about on the bottom of temporary pools in search of food. When oxygen levels are low, they swim upside down near the water surface.

They reproduce by mating or parthenogenesis.

Longtail tadpole shrimps and people: The longtail tadpole shrimp is sometimes considered a pest in rice fields mainly in the United States and Spain, where it damages the roots and leaves of rice plant seedlings. In large numbers this species stirs up the muddy bottom, blocking out sunlight needed by developing plants. In Japan, rice plants are too big to be eaten. Instead, the tadpole shrimps attack unwanted weeds. As a result, they are considered to be beneficial. Their dried eggs are sold in breeding kits to schools and to people interested in raising unusual pets.

Conservation status: This species is not considered endangered or threatened. ∎


Physical characteristics: Common water fleas are small, measuring 0.008 to 0.1 inches (0.2 to 3 mm) in length. They are flattened from side to side. A large, folded carapace covers all but the head. They have two pairs of antennae and five pairs of leaflike limbs.

Geographic range: Common water fleas are found in North America, Europe, and the cooler regions of South America.

Habitat: Common water fleas live in freshwater lakes, ponds, rivers, and streams.

Diet: They eat algae and microscopic animals such as rotifers.

Behavior and reproduction: Water fleas migrate up and down through the water on a daily cycle. They reproduce by parthenogenesis in spring and early summer. Late in the season, some of these eggs develop into males, marking the beginning of a period of reproduction by mating. Eggs produced by mating, or sexual reproduction, have thick shells. Females produce three to nine eggs at a time. The young take six to eight days after hatching to reach adulthood.

Common water fleas and people: Common water fleas are kept in laboratories as living test subjects for the detection of water pollutants.

Conservation status: Common water fleas are not considered endangered or threatened. ∎



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Web sites:

Cladocera.http://www.cladocera.uoguelph.ca/ (accessed on February 8, 2005).

Fairy Shrimp.http://www.vernalpool.org/inf_fs.htm (accessed on February 8, 2005).

Introduction to the Branchiopoda.http://www.ucmp.berkeley.edu/arthropoda/crustacea/branchiopoda.html (accessed on February 8, 2005).

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

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