Citizen Monitoring Research: Freshwater Sponges

Freshwater sponges are aquatic animals that grow in lakes, rivers, bogs, and streams attached to submerged rocks, sticks, logs, or aquatic vegetation. These sedentary animals feed by filtering small particles from the water, and so are thought to be sensitive indicators of pollution. A small number of scientists studied Wisconsin’s freshwater sponges extensively in the 1930s and found them growing in many lakes and major river systems. Since then, extensive studies have not been done, though some limited research seems to indicate that the range of some species is more restricted now than in the 1930s. This Citizen-based Monitoring study tries to shed more light on how abundant and widely distributed Wisconsin’s sponges are today.

This brochure [PDF, 2.7MB] provides additional information about freshwater sponges and includes directions for submitting observations of sponges in your local waterways.

What do freshwater sponges look like?

Freshwater sponges vary from marble-sized to elongated masses and can grow to be thin or thick encrusting layers. Their surfaces may be smooth, textured or wavy, or have finger-like projections. Their structure is supported by spicules, tiny needle-like structures made of silicon that are distributed throughout the sponge body. You can use a magnifying glass to see at least a hint of the needle-like spicules.

Sponges can grow as encrusting layers with smooth, textured, or wavy surfaces or they can grow with finger-like projections.

Unfortunately, color and shape are not particularly helpful in identifying sponges to the species level. Instead, biologists rely on the spicules, which are microscopically quite diverse in their size, shape, and number of prongs. Some have hooks or are dumbbell-shaped. They can be smooth or spined. Much of this variability is species-specific (i.e. each species has its own sizes and shapes).

Can we find them in our lake or nearby river?

Sponges grow in relatively shallow water and so can be found by wading and observing the surfaces where they might grow. You might find a rake useful for turning over debris. The sponges may be colored green by algae that live inside their cells or they may be beige to brown or pinkish in color. Sponges can be delicate to very firm feeling but are not slimy or filmy. Some sponges prefer the underside of logs and sticks; these are usually not green in color.

Gemmules (indicated with arrows above) contain cells from which new sponges grow. About the size of poppy seeds and tan in color, they can be clustered or scattered in the sponge.

Wisconsin’s sponges exhibit an annual life history in which they grow through the summer, die back in the winter, and begin a new growth cycle in spring. So, it’s best to look for them in late summer and early fall. In late summer, sponges form gemmules, small spherical protective structures that contain cells from which new sponges will grow in spring. The gemmules will appear about the size of poppy seeds, but are tan in color. They can be clustered or scattered in the sponge.

How common are freshwater sponges?

We don’t know. Since little modern survey research has been conducted, their conservation status remains unknown. Biologists have found sponges in only about half of Wisconsin’s counties. So there are many gaps in our knowledge.

You can help us learn more about Wisconsin sponges!

Have you seen sponges in your area? You can use the questionnaire in this brochure [PDF, 2.7MB] to submit observations of sponges in your local waterways to help biologists prioritize future survey efforts.

What has been written about freshwater sponges?

The following technical references provide additional information on freshwater sponges. You can usually obtain these documents from public or university libraries directly or through interlibrary loan services.