BRANCHIOPOD

(class Branchiopoda), any of more than 800 species of aquatic crustaceans that include water fleas, tadpole shrimp, fairy shrimp, and other small, chiefly freshwater forms. Branchiopods range in size from 0.25 mm (0.01 inch) for Alonella to 10 cm (4 inches) for Branchinecta gigas. They have few common features, although all forms have compound eyes, paddlelike limbs, simple mouthparts, and primitive nervous systems. Members of the order Anostraca, for example, have no carapace (a hard, horny shield covering the head and thoracic region), while those of the order Notostraca have only reduced antennae, if the antennae exist at all. Branchiopods are generally considered to be the most primitive crustaceans, with a fossil record that dates back to the Devonian Period (408 to 360 million years ago). Branchiopods are found worldwide in fresh waters. Members of the order Ctenopoda consume the algae and bacteria in lakes and ponds and are in turn an important source of food for fish. The life cycle of a typical branchiopod begins in the form of a nauplius (free-swimming larva) with a simple undivided body and three pairs of appendagesantennae, antennules (smaller antennae), and mandiblesalthough some branchiopods develop directly into miniatures of the adult, skipping the larval stage completely. As the nauplius changes into adult form, the body becomes segmented and additional limbs appear. The reproductive process varies from order to order, but all branchiopods mature rapidly, some being able to reproduce when only two days old. The sexes are separate in some branchiopods, while the females of others are modified so that eggs laid in a brood pouch under the carapace develop directly into females without being fertilized by males. Under unfavourable conditions, however, such as when food is scarce or there is a sudden temperature change, some of the eggs develop as males, and the females then begin producing some eggs that must be fertilized by these males. These fertilized eggs are better able to withstand harsh environmental conditions and will hatch when the conditions turn favourable. Many branchiopods can alter their life cycles to coincide with favourable conditions and produce resting eggs that can remain dormant for periods of up to several years. Branchiopods have anywhere from 4 pairs of trunk limbs to more than 70 pairs, depending on the order, but all use the trunk limbs to gather food. Fine hairs, or setae, serve as a filter to separate food particles, which can be as small as bacteria, from the water. Trunk limbs are used to propel notostracans and anostracans by beating in regular rhythms; the antennae, on the other hand, are used for locomotion by many branchiopods. The eyes of the four orders differ drastically, though all are able to perform rapid trembling motions, a process that seems to enable them to gain more information about their surroundings. While most branchiopods have both eyes joined together, anostracans have eyes on movable stalks and turn their ventral surface to any light source. The anomopod Daphnia alters its swimming pattern to search wider areas when the absence of such food as green algae results in less absorption of blue light in the water. The blood of branchiopods contains the pigment hemoglobin dissolved in plasma, in contrast to the blood of other crustaceans (which lacks hemoglobin) and to that of other animals whose hemoglobin is contained in blood cells. When branchiopods are in water that has little oxygen their blood is rich in hemoglobin, but when they are transferred to water that is well aerated their colour becomes pale in about 10 days. any of the roughly 900 species of the class Branchiopoda (subphylum Crustacea). They are aquatic animals that include brine shrimp, fairy shrimp, tadpole shrimp, water fleas, and other small, chiefly freshwater forms. Branchiopods are generally regarded as primitive crustaceans. Their long fossil record dates back to the Devonian period (408 to 360 million years ago). Although certain members of the group, such as the order Anostraca, are mainly confined to temporary pools, the water flea, order Anomopoda, is so successful that there are few fresh waters in the world without one or more species of anomopod. Additional reading H.G. Cannon and F.M.C. Leak, The Feeding Mechanism of the Branchiopoda, Philosophical Transactions of the Royal Society of London , Series B, 222:267352 (1933), provides a classical, beautifully illustrated account. A.R. Longhurst, A Review of the Notostraca, Bulletin of the British Museum (Natural History), 3:157 (1955), is a revision of the systematics of the Notostraca and a standard work on the subject. See also G. Fryer, A New Classification of the Branchiopod Crustacea, Zoological Journal of the Linnean Society, 91(4):357383 (1987).