|Many water fleas swimming together|
The water flea is a microscopic crustacean (around .2-5 mm long) that lives in lakes, ponds, swamps, and streams. The most common kind of water flea is the Daphnia. This little, translucent insect is named for the way it swims. It filter-feeds keeping waters pure and clean. They only have a life span of about 23-108 days depending on the weather conditions. It is considered threatened because it is a pretty major food source for fish considering its size and quantity.
Water fleas are very small kidney shaped creatures in the water.For sight they have a single compound eye. They also have two doubly branched antennae, and leaf like limbs which make a current inside the carapace that allows food and water to pass through it's gills. Their bodies are translucent, and under a microscope, you can see it's heart beating, and maybe even their last meal they are digesting.
The water flea's internal structures are protected by a shell-like carapace which is only one piece even though it looks like two valves. It is a double fold of the body wall stretching across the whole of its body like a little cape attached only to the neck. It's a large, thin, very flexible layer folded along the mid-line of the dorsal side of the water flea making two valves. The carapace is closed on its dorsal side, but at the posterior and ventral sides, the two valves are opened allowing water and food to pass through with the current. At the posterior end of the water flea there are paired claws used for getting rid of unwanted particles between the carapace.
Most of the rest of the water flea's body is in the thorax which has five pairs of biramous, setose and thoracopods (appendages). Two pairs of these appendages make the current through which water and food may run. It draws water through a setal filter where it exits through the spaces the limbs. Food particles are stopped at the setal filter and moved to the ventral food groove where they move from appendage to appendage until it reaches the mouth. The food will travel down in to a "C" shaped intestine where it is digested and goes directly from the mouth to the anus. 
The water flea's circulatory system consists of a heart, hemocoel (basically the body cavity), and blood. A short, oval heart is at the dorsal end and is surrounded by hemocoel. This heart has only a single pair of ostia (small openings) although it may not be evident, and no blood vessels are present. The heart forces blood to the hemocoel of the head. It then flows to the posterior end through three hemocoel chambers and then it will travel back to the heart. The Daphnia's carapace is the primary source of gas exchange.
ReproductionWater fleas can produce either sexually or asexually. Most of the water fleas in a colony are female. This is because, when conditions are right, a water flea will produce more females than males. When conditions are poor she will produce about an equal amount of males and females. The female water flea has a brood pouch in which eggs and embryos are held until birth.  There are different kinds of eggs in the brood pouch. One is Summer eggs which develops parthenogenetically (without fertilization), and has little yoke. Summer eggs are very numerous and are usually not released until they are sexually mature or maybe even have young of their own. Second are resting eggs which have a lot of yoke, a thick shell, and can only produce after being fertilized. There are only two resting eggs (one from each ovary) that are released into the brooding chamber as well. Third there are winter eggs which are released instantly either naked or in a protected cuticular ephippium. How they are released depends on their taxon. The eggs are held inside the female water flea for only a few days before a new generation of water fleas are born. 
Males are a lot smaller than females and have an appendage in their abdomen which is used to hold the female from behind, pry open her carapace, in injects the sperm inter her winter eggs. 
Water fleas live in fresh water lakes, streams, ponds, puddles, and swamps. Since water fleas are filter feeders, they are certainly not predators. The water flea makes a sort of jumping motion with it's appendages. If it didn't make this motion, it would sink right to the bottom. Water fleas will swim to upper levels of the water during the night and back down to lower levels during the day. This is probably a way they've developed to avoid predators. During the day, they will swim down into darkness to avoid the fish that hunt for food visually, and at night swim up to get to richer food like planktonic algae in the well lit upper levels.
Not only are water fleas and important source of food for fish and crustaceans, but they are used in the laboratory quite a bit because of their sensitivity to changes in the water. They are really easy to and cheap to raise and mature within a few days of birth. Water fleas are important and very useful for clearing pollution-caused algae blooms in lakes and ponds. The EPA uses water fleas to test toxicity in water since they are very sensitive to bleaching agents, metal ions, and many other broken-down toxins.
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- http://ei.cornell.edu Cornell University, November 2009
- http://www.environmentalleverage.com Environmental Leverage Inc. October 2008