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The reason for this little critters’ super-high gene count comes down to its rapid rate of gene multiplication. “We estimate a rate that is three times greater than those of other invertebrates and 30 percent greater than that of humans,” project leader and CGB genomics director John Colbourne said in a press release.

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The order Cladocera is included in the class Branchiopoda, and forms a monophyletic group, which is currently divided into four suborders. Around 620 species have been described, but many more species remain undescribed.[1] The genus Daphnia alone contains around 150 species.[4]

Most cladoceran species live in fresh water and other inland water bodies, with only eight species being truly oceanic.[3] The marine species are all in the family Podonidae, except for the genus Penilia.[3]

Daphnia spp. are a popular live food in tropical and marine fish keeping.[13] They are often fed to tadpoles or small species of amphibians such as the African dwarf frog (Hymenochirus boettgeri).

2. R.Ritter, R.Manuel, Collins Field Guide to Freshwater Life, Collins 1986

Means of spread: They can spread by attaching to fishing lines, downriggers, anchor ropes, and fishing nets. While female waterfleas die out of water, under certain conditions they produce eggs that resist drying and freezing, and can establish a new infestation. They also can be unintentionally transported in bilge water, bait buckets, or livewells.

The translucent water flea is a Daphnia pulex, and lives in ponds and lakes throughout North America, Europe and Australia. It can also reproduce without sex, is the most commonly found species of water flea and is a “model organism”, meaning it’s studied extensively and provides insight into other, rarer species.

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The findings were reported in the journal Science, by members of the Center for Genomics and Bioinformatics (CGB) at Indiana University Bloomington and the U.S. Department of Energy’s Joint Genome Institute.

Because of their thin membrane, which allows drugs to be absorbed, they are used to monitor affects of certain drugs, such as adrenaline or capsaicin, on the heart.

Image: Daphnia pulex, commonly known as the water flea./Jan Michels, Christian-Albrechts-Universitaet zu Kiel.

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Daphnia species are normally r-selected, meaning that they invest in early reproduction and so have short lifespans. An individual Daphnia life-span depends on factors such as temperature and the abundance of predators, but can be 13–14 months in some cold, oligotrophic fish-free lakes.[9] In typical conditions, however, the life cycle is much shorter, not usually exceeding 5–6 months.[9]

Editor's footnote: The classification of the cladocera has been subject to review recently by taxonomists. See, for example, the Cladoceran web site run by the University of Guelph which summarises the current status of their taxonomy.

Regulatory Classification: Spiny waterfleas are a regulated invasive species in Minnesota (DNR), which means introduction into another waterbody is prohibited.

To collect Water Fleas: First, put water from their habitat into a wide-mouthed container. Gently sweep a small, fine aquarium net through their habitat. (The fine nets are usually white, not green.) Put the net into the container of water and turn it inside-out to release the Water Fleas. You will probably find other species of aquatic critters that look like those in vernal pools. See how many you can identify using the Critter Catalog in your classroom or online at www.sacsplash.org.

The word "Cladocera" derives via New Latin from the Ancient Greek κλάδος (kládos, "branch") and κέρας (kéras, "horn").[8]

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The mouthparts are small, and consist of an unpaired labrum, a pair of mandibles, a pair of maxillae, and an unpaired labium.[2] They are used to eat "organic detritus of all kinds" and bacteria.[2]

Several Daphnia species are considered threatened. The following are listed as vulnerable by IUCN: Daphnia nivalis, Daphnia coronata, Daphnia occidentalis, and Daphnia jollyi. Some species are halophiles, and can be found in hypersaline lake environments, an example of which is the Makgadikgadi Pan.[12]

As well as having a massive number of genes, more than a third of them have never been seen before in other animals. “In other words, they are completely new to science,” says Don Gilbert, coauthor and Department of Biology scientist at IU Bloomington. Those previously unknown genes are due to the nature of the flea’s environment.

4. G.E.Newell, R.C.Newell, Marine Plankton, Hutchinson Educational Ltd 1973

Impacts: Spiny waterfleas eat small animals (zooplankton), including Daphnia, which are an important food for native fishes. In some lakes, they caused the decline or elimination of some species of native zooplankton. They can clog eyelets of fishing rods and prevent fish from being landed.

Daphnia may be used in certain environments to test the effects of toxins on an ecosystem, which makes them an indicator genus, particularly useful because of its short lifespan and reproductive capabilities. Because they are nearly transparent, their internal organs are easy to study in live specimens (e.g. to study the effect of temperature on the heart rate of these ectothermic organisms).

Daphnia, a genus of small planktonic crustaceans, are 0.2–5 millimetres (0.01–0.20 in) in length. Daphnia are members of the order Cladocera, and are one of the several small aquatic crustaceans commonly called water fleas because their saltatory (Wiktionary) swimming style resembles the movements of fleas. Daphnia live in various aquatic environments ranging from acidic swamps to freshwater lakes, ponds, streams and rivers.

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Species and Origin: Spiny waterfleas are zooplankton (microscopic animals). Native to Europe and Asia, they were introduced into the Great Lakes by ballast water discharged from ocean-going ships. They were first discovered in Lake Ontario in 1982 and spread to Lake Superior in 1987. Adults range from 1/4 to 5/8 inch long. Spiny waterflea have a single long tail with small spines along its length.

This tiny, near-microscopic water flea has more genes than you. In fact, this freshwater zooplankton is the first crustacean to have its genome sequenced, and its 31,000 genes crowns it the animal with the most genes so far. For those keeping count at home, the average human has about 20,000 to 25,000 genes.

Status: They have spread throughout the Great Lakes, and are established in some inland lakes and rivers in Minnesota. See US map .

Water Flea

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The thorax bears five or six pairs of lobed, leaf-like appendages, each with numerous hairs or setae.[2] Carbon dioxide is lost, and oxygen taken up, through the body surface.[2]

They are mostly 0.2–6.0 mm (0.01–0.24 in) long, with the exception of Leptodora, which can be up to 18 mm (0.71 in) long.[1] The body is not obviously segmented and bears a folded carapace which covers the thorax and abdomen.[2]

Doing genomic research on animals, even as tiny as this zooplankton, holds importance for humans too. “The Daphnia system is an exquisite aquatic sensor, a potential high-tech and modern version of the mineshaft canary,” James E. Klaunig of Indiana University said in a press release. By looking at how changing environmental agents affect the celluar and molecular processes of this flea, we can extrapolate that to the genes shared in humans.

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Swimming is powered mainly by the second set of antennae, which are larger in size than the first set.[11] The action of this second set of antennae is responsible for the jumping motion.[11]

Where to look: They collect in gelatinous globs on fishing lines and downrigger cables. They prefer deep lakes, but can be found in shallow lakes and rivers.