Forest Preserve District of Cook County, Illinois
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Water Fleas and their Kin
Nature Bulletin No. 369-A   February 14, 1970
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

Disgruntled fishermen who wonder why an artificial body of water, like an old quarry, does not produce more fish and bigger fish, might well investigate its population of "microcrustacea" -- a fancy name for Water Fleas and their kin -- and the possibility of increasing their numbers. They are the principal food of nearly all little fish as well as many big ones. They are widely grown to feed tropical fish in aquaria.

Almost every natural body of water, fresh or salt, contains some crustaceans. Most of us are familiar with the larger ones found in the oceans -- lobsters, crayfish, crabs and shrimps -- and the freshwater crayfish, or "crawdads", and shrimp so common in our ponds and streams, but there is a host of very small species classed under that one name: "microcrustacea".

Although, like some of their larger relatives, many types of them live in the sea, they also exist in great variety and abundance in freshwater lakes, ponds and other quiet waters. By using a net or strainer of silk bolting cloth, from 20 to 50 kinds of them usually can be caught from such a body of water in a few hours. Some have common names and some do not but they all fall into four main types called: Fairy shrimps, Water Fleas, Copepods, and Ostracods.

The fairy shrimps are largest, commonly between a quarter-inch and one inch in length, and differ widely among themselves. Some are almost worm-like and glitter with iridescent greens and pinks as they swim about, on their backs, with their many leaf-like feet upwards. Some are nicknamed "mussel bugs" because they are enclosed in a tiny pair of shells, like a mussel or clam. Others have a broad flat shell like a miniature king crab. And -- note this -- over most of the United States, fairy shrimp are a better sign of spring than wild geese or the first robin because they appear and thrive best in ponds that may lie dry during much of the year but are filled in early spring.

When the first warm spring rains come, the thick-shelled "winter eggs" of the fairy shrimp hatch and produce young females which quickly mature and produce thin-shelled eggs that likewise yield other females, and so it goes on and on until the pond or pool begins to dry up. Then, and only then, both males and females appear and winter eggs are dropped. These can be carried from place to place on the feet of birds, or blown about by winds, or survive -- bone dry -- for years, Curiously enough, multitudes of a strange fairy shrimp, the Brine Shrimp, constitute the only visible animal life in the Great Salt Lake of Utah.

Water fleas are more tiny; the different kinds ranging from one- hundredth to one-tenth of an inch in length when fully grown. Their bodies are enclosed in a shell open on the front and underside but their heads stick out, Although equipped with 5 or 6 pairs of legs, they do their swimming with a branched pair of antennae. They are so transparent that, under a microscope, one can see the rapidly beating heart, the feet sweeping food to the mouth, the food-filled intestine, and the brood pouch with its eggs and young. Water fleas, like fairy shrimps, produce females and nothing but females, without fertilization, during most of the year and, believe it or not, in some species a male never has been found.

The copepods and ostracods, in spite of their big names, are still smaller than water fleas and, like them, have bewilderingly complicated life histories. However, with the water fleas, they form the most important link in the food chain connecting algae and other microscopic life to our large edible fish.

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