Sponges are a very cool life form. They may look and act like plants or algae, but they are invertebrate animals, animals without spines.
Too bad Montana doesn’t have any sponges you might say—but you’d be wrong! Surprise—Montana has five identified species of freshwater sponges.
Who knew? Even the joint Montana Fish, Wildlife & Parks and the Montana Natural Heritage Program Animal Field Guide lists only one species of freshwater sponge at this time. The state’s nongame animal surveys haven’t extended yet to these unusual wildlife species about which relatively little is known.
One expert on Montana’s fresh water sponges is John Addis, a professor of biology at Carroll College in Helena. He and his students have identified five species of freshwater sponges since the mid-1990s.
Addis said freshwater sponges are among the simplest of animals without nerve or muscle cells. There is no digestive or circulatory system either. Most sponges, marine and freshwater, have skeletons made from silica, so they are more glass-like than bone-like. These skeletal pieces, called spicules, are joined together by protein.
Freshwater sponges are filter feeders. Their bodies consist largely of a system of water-filled canals. An inner layer is lined with special feeder cells with flagella, threadlike appendages that provide motility. These feeder cells draw water into the sponge through many small pores, absorb any available nutrients and then expel the water.
Sponges have ancient roots. Marine sponge species have existed for at least 750 million years old. Freshwater sponge species are more recent, at an estimated 200 million years old.
“Freshwater sponges can be difficult to see because they are green from the algae trapped in the cells of the sponge,” Addis said. “Most of the freshwater sponge species in Montana benefit from this relationship with algae by absorbing nutrients the algae produces. In effect, sponges have discovered a way to domesticate some algae.”
Where should you look for this unusual Montana wildlife species?
“Freshwater sponges are found in the lakes, ponds, rivers and streams of Montana,” Addis said. “The best places to look are at the ends of lakes or ponds where the water flows into a river or stream.”
Sponges usually grow as mats on rocks and logs, and may look something like moss. Occasionally they grow on sand or mud and extend branches. Addis said he has not found sponges in water that doesn’t flow.
All of Montana’s sponges reproduce sexually in the early summer. Sperm enters the female sponge’s canal system to fertilize the eggs and when the larva form they are released.
Four of five freshwater sponge species in Montana also reproduce asexually by forming cyst-like “buds” in the fall. The remainder of the sponge dies back for winter and is then renewed the next spring when these buds grow to form a new skeletal system. The gemmules, or buds, are small, about the size of poppy seeds, and yellow-brown. Freshwater sponges can spread to new areas when these buds break free and are swept downriver or picked up and transported by birds.
If you set out to observe Montana’s freshwater sponges, remember to tread gently, as they are fragile. Addis said he and his students have noticed a decline in freshwater sponges in locations with increased recreation or other activities that disturb their shallow water habitat.