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Freshwater Sponges
Nature Bulletin No. 448-A   March 11, 1972
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

FRESHWATER SPONGES
To most people the word sponge means a chunk of porous material used for mopping and washing -- remarkable for its power to absorb water and to become soft and pliable when wet without losing its toughness. Actually, such a sponge is the skeleton of a low form of marine animal that lives rooted to one spot like a vegetable. This skeleton is a fine fibrous network made of a substance similar to horn or fingernails. Among about 3000 species of sponges found in the sea, only three have much commercial value -- the bath sponge, the horse sponge and the sheep's wool sponge. Divers or dredges tear these from the rocky floors of warm shallow seas such as the Mediterranean and the Gulf of Mexico. After they have been allowed to die and rot on a ship's deck, they are trampled, beaten and washed until the jelly-like flesh is all removed. Other kinds are too small or too fragile to be used, or else have skeletons made of brittle scratchy substance like lime or silica.

Once thought to be plants, sponges were not clearly recognized as members of the animal kingdom until the 19th century. They have such irregular shapes, even within one species, that they look like some spreading underwater mold or fungus. This is particularly true of the freshwater sponges of which about thirty kinds are known in North America. In Illinois a half dozen species of these are widespread in clean ponds, lakes, streams and rivers where they form mat-like, encrusting growths on submerged rocks, pebbles, logs, twigs, and the undersides of lily pads. The different kinds when full-grown range in size from a tiny slimy patch the size of a postage stamp to rough, inch- thick mats with finger-like outgrowths covering many square yards. Usually brown, gray or yellowish in color, some may become bright green in sunlight due to an alga that grows in their jelly-like innards.

The sponge's body is very simply constructed. It has no brain, nerves, muscles, eyes, heart, gills or other organs. Examined under a microscope, you see large numbers of very small pores scattered over the surface and a few large openings the size of pin heads, usually at the tips of bumps. Inside the body these openings are connected by a maze of channels and chambers. The latter are lined with special cells with tiny whips that lash the water and cause a flow in through the small openings and out through the larger ones. The entire animal is a sort of sieve to catch the microscopic life and debris which nourish it. In the course of a day, a sponge the size of your hand may strain many gallons of water.

With the exception of a thin skin covering the outside of the body and the inside of the channels, most of the body is a watery jelly containing many creeping shapeless cells that perform a variety of tasks. They manufacture the minute spines and fibers that make up the skeleton; they digest food, carry oxygen and can even change into eggs and sperms. Like many other lower animals, sponges have remarkable powers of regeneration. Even when ground up and squeezed through a cloth the minute fragments creep together in a dish of water and rebuild a new sponge. In autumn freshwater sponges die after they form little thick-shelled, seed-like bodies called gemmules. Immune to freezing and drying, these can be carried by wind, by water or on the feet of birds to other ponds and streams to produce next year's sponges.

Sponges got stuck in an evolutionary blind alley.


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