Common Name: Freshwater sponge and bryozoans
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Freshwater Sponges

Although sponges are animals, they are included in this manual because they are sometimes mistaken for aquatic plants or algae. Sponges are multicellular animals consisting of masses of cells embedded in a gelatinous matrix. The matrix is bound together by minute, spine-like structures of calcium or silica called spicules and spongy organic fibers called spongin. Although most of the more than 5,000 known sponges are found in marine environments, 150 species live in freshwater. Freshwater sponges are pitted with pores and often are yellow, brown or greenish. Sponges filter large volumes of water through their pores, capturing tiny particles for food. Freshwater sponges vary in size from a few millimeters to more than a meter across. All species have a free-swimming, microscopic larval stage, but are attached (sessile) as adults. They are widely distributed in well-oxygenated ponds and streams where they grow on plant stems, pieces of wood, and other submersed objects. They will overwinter in a dormant state (called gemmules), but are most commonly seen in summer or fall. They may be lobed, composed of finger-like projections, or quite irregularly shaped and are robust enough to be picked up without falling apart, unlike many kinds of algae.


Bryozoans are tiny colonial animals that are fairly common in lakes and streams with suitable habitat. Different species form colonies that range in appearance from delicate wispy moss-like growths to basketball-size gelatinous masses. Each colony is made of many individual creatures called "zooids." Zooids are microscopic cylindrical creatures with a mouth, digestive tract, muscles, and nerve centers. The zooids are covered by a protective matrix which may be delicate, hard, or gelatinous depending on the species. They feed by filtering tiny algae and protozoa through a crown of tentacles (lophophore). Bryozoan colonies grow by budding from the adult zooids. New colonies will establish from a free-swimming, microscopic larval stage or by growth of dormant spore-like "statoblasts." Most Bryozoans live in salt water, and of the 20 or so freshwater species found in North America, most are found in warm-water regions attached to plants, logs, rocks and other firm substrates. The forms most likely encountered in the northwest are translucent, brownish-gray, jelly-like masses that look like they have little black dots embedded in them.

Photographs: Bryozoan

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