Cladocera

(Water fleas)

Phylum Arthropoda

Subphylum Crustacea

Class Branchiopoda

Order Cladocera

Number of families 15

Thumbnail description
Small transparent zooplankton found in bodies of freshwater throughout the world; a few species inhabit marine waters

Evolution and systematics

Cladocerans remain well-preserved in aquatic sediments because their shells are composed of chitin, a white semi-transparent horny substance. Fossil evidence suggests that the major evolutionary development of the cladocerans occurred by the late Paleozoic or early Mesozoic era, approximately 250 million years ago. The taxonomy of the cladocerans, however, is still controversial. Most scientists recognize four suborders, 10–15 families, about 80 genera, and over 400 species.

Physical characteristics

Cladocerans are small animals that are often shaped like flat disks. Most range in size from 0.008 to 0.1 in (0.2–3 mm), although one species, Leptodora kindtii can grow as large as 0.7 in (18 mm) in length. Their bodies are not clearly segmented like those of other crustaceans; however, three parts can be distinguished—head, thorax, and abdomen. The head is typically dome-shaped with large compound eyes and five pairs of appendages. Among the appendages are two pairs of antennae—a small pair that serves a sensory function and a larger pair that is used for swimming. The other three pairs of appendages function in securing food.

A thin transparent shell called the carapace encloses the thorax and abdomen of most cladocerans. The carapace may be patterned; some species have stiff spines protruding from their shells. The thorax holds four to six pairs of leaflike legs that are used for gathering food, filtering water, or grasping mates during copulation. The cladoceran digestive tract is contained within the thorax and abdomen. A pair of claws used for cleaning the thoracic legs may extend from the end of the carapace.

Distribution

Cladocerans are distributed worldwide.

Habitat

Most cladocerans are restricted to freshwater habitats, although eight species are found in marine waters. Cladocerans abound in lakes, ponds, slow-moving streams, and rivers. They are found from the Arctic to the Southern Ocean, from sea level to alpine ponds. Some cladocerans are benthic (found on the bottom of a lake, sea, or ocean); other species live on sediment or in vegetation virtually anywhere water is present, including swamps, puddles, ditches, and ground water. One species even lives in water trapped by mosses living on trees in a Puerto Rican cloud forest.

Behavior

Cladocerans migrate up and down in the water column on a daily cycle. They typically come to the surface at night and move to lower depths in the daytime. By seeking deeper waters during the day, cladocerans are protected by the darkness from such visual predators as fishes; they then maximize growth by moving to warmer surface waters at night. Occasionally, large swarms of cladocerans are observed at the surface during the day, typically as a response to light and the threat of predatory fishes. When cladocerans encounter predators, pelagic species often swim away rapidly. If caught by small

predators, some cladocerans fight to break free; if they escape, they stop moving and sink deeper into the water column.

Feeding ecology and diet

Cladocerans use their thoracic 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 towards the mouth. Because most cladocerans are filter feeders, they eat organic detritus of all kinds, including algae, protozoans, and bacteria. Although cladocerans cannot select individual food items, they can decide whether to accept or reject the mass of food collected in their mucus; they may reject the bolus if it contains toxic algae or other undesirable particles. A few genera of cladocerans are predatory and capable of seizing their prey, which consists primarily of other zooplankton. Because of their small size, cladocerans have a wide variety of predators, including fishes, amphibians, birds, predaceous zooplankton, and insects.

Reproductive biology

Cladocerans can reproduce both sexually and asexually; the mode of reproduction depends on environmental conditions, but most species reproduce asexually most of the time. Reproduction begins as water temperatures rise in the spring, but the population declines in the summer due to over-crowding and competition for food. A second peak of reproduction and population growth may occur in autumn.

Cladoceran eggs develop in about two days in the female's brood chamber, which is located between the body and the carapace. In most cladoceran species, the clutch size increases with body size, while the development time of the eggs is inversely related to temperature. Some eggs produced asexually develop into young immediately. If conditions are unfavorable, both sexually- and asexually-produced eggs may enter a resting state called diapause, during which they are resistant to heat, desiccation (drying out), and freezing. Most eggs develop into females. The production of male offspring is triggered by such environmental signals as crowding, changes in food concentrations, or decreasing day length.

During episodes of sexual reproduction, males use their first antennae to locate female mates. After finding a receptive female, the male grasps the edge of her carapace. The female continues to swim, carrying the male with her. After some time, the swimming ceases and both extend their post-abdominal region, presumably to allow the male to eject his sperm as close to the female's brood pouch as possible. Mating may take anywhere between 15 minutes and several hours to complete.

Conservation status

Cladocerans are common throughout the world, and distributions of most species are stable or expanding, often due to human activities that introduce non-native species to new areas. No species are considered threatened, and none are listed by the IUCN.

Significance to humans

Although cladocerans are not recognized by most humans, they are vital for sustaining functions that humans value in many aquatic ecosystems. In particular, cladocerans are a critical link in the food chain that enables aquatic habitats to support valuable species of fish.

Species accounts

List of Species

Fishhook water flea

Cercopagis pengoi

family

Cercopagidae

taxonomy

Cercopagis pengoi Ostroumov, 1891, Caspian Sea.

other common names

None known.

physical characteristics

C. pengoi lacks the ventral (lower surface) portion of the carapace; as a result, its limbs are uncovered and its body is very mobile. The dorsal (upper surface) portion of the carapace remains and protects the brood cavity. The head of this cladoceran is rounded and contains strong mandibles (jaws) that are hardened with chitin. C. pengoi has only four pairs of thoracic legs, which are covered with setae (bristles) and spines. It has a long caudal (tail-like) appendage extending from its abdomen. Females are 0.05–0.08 in (1.2–2 mm) in length, and males are 0.04–0.06 in (1.1–1.4 mm) long; the caudal appendage may be 5–7 times the length of the body.

distribution

Native to the Caspian, Black, Azov, and Aral Seas; introduced to the Baltic Sea and the North American Great Lakes.

habitat

Inhabits brackish water as well as freshwater seas, lakes, and reservoirs.

behavior

In its native range, this species migrates vertically on a daily basis; it lives at depths of 164–197 ft (50–60 m) during the day and rises to the surface layer at night. Young individuals are not found below 66–98 ft (20–30 m). Strong migration patterns are not observed in the Baltic Sea or Great Lakes.

feeding ecology and diet

C. pengoi is a predatory species; it actively catches prey and eats the tissues and soft body parts. Prey items include such other zooplankton as rotifers, copepods, protozoans, and other cladocerans. C. pengoi may be consumed by such larger planktivorous fish as herring and smelt.

reproductive biology

C. pengoi reproduces asexually for most of the summer. Sexual reproduction takes place in late autumn.

conservation status

Not listed by the IUCN.

significance to humans

The hook on the caudal appendage of C. pengoi often becomes tangled in fishing gear, fouling the equipment. In addition, C.

pengoi competes with fish for food, and high populations of this species may reduce the number of fishes in a water body. Further, the fishhook water flea may intensify the eutrophication (overgrowth of algae resulting from an increased supply of nutrients) of water bodies by eating zooplankton that would normally graze on the algae.

Common water flea

Daphnia pulex

family

Daphnidae

taxonomy

Daphnia pulex DeGeer, 1778.

other common names

French: Puce d'eau commune; German: Wasserfloh.

physical characteristics

D. pulex is a small laterally-flattened cladoceran. The body is covered by a carapace, but the head, large compound eye, and antennae remain open to view. The common water flea has two pairs of antennae and five pairs of leaf-like limbs on its body. The spine is located at the posterior end of the carapace. Individuals typically range in size from 0.008–0.1 in (0.2–3 mm).

distribution

Holarctic distribution across North America, Europe, and Asia; temperate zones of North America, South America, and Europe.

habitat

Freshwater lakes, ponds, rivers, and streams.

behavior

Water fleas migrate vertically in the water column on a diel (regular 24-hour) basis. When predatory fishes are present in the water, D. pulex forms defensive structures, particularly small protrusions in the neck region called neckteeth.

feeding ecology and diet

D. pulex eats algae and such small zooplankton as protozoans or rotifers. In turn, water fleas are consumed by small fishes and predatory insects.

reproductive biology

D. pulex reproduces asexually during the spring and early summer. Later in the season, some asexually-produced eggs become males, a change that allows sexual reproduction to begin. Eggs produced by sexual reproduction are covered in thick shells. Females have 3–9 young per brood, and these young mature in 6–8 days after leaving the brood pouch.

conservation status

Not listed by the IUCN.

significance to humans

The common water flea is commonly maintained in laboratories and often used to test for toxic substances in water.

Resources

Books

Thorp, J. H., and A. P. Covich, eds. Ecology and Classification of North American Freshwater Invertebrates. 2nd ed. New York: Academic Press, 2001.

Periodicals

Benoit, H. P., O. E. Johannsson, D. M. Warner, W. G. Sprules, and L. G. Rudstam. "Assessing the Impact of a Recent Predatory Invader: The Population Dynamics, Vertical Distribution, and Potential Prey of Cercopagis pengoi in Lake Ontario." Limnology and Oceanography 47, no. 3 (2002): 626–635.

Katherine E. Mills, MS