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

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.