As if Lake Michigan fish don’t have enough competition for resources. An Illinois-Indiana Sea Grant study has found that the diet of an invasive freshwater hydroid includes organisms that are an important food source for young-of-the-year and bottom-dwelling fish.

Cordylophora caspia typically eats larval zebra and quagga mussels,” said Nadine Folino-Rorem, Wheaton College biologist. “However, when those sources are not readily available, the hydroid can feed on other invertebrates, which potentially affects prey availability for fish.” Folino-Rorem, along with Martin Berg, a Loyola University Chicago biologist, studied the distribution and diet of C. caspia in Lake Michigan.

The hydroid lives in freshwater and brackish or slightly salty habitats. The freshwater colonial hydroid is native to the Caspian and Black Seas. C. caspia colonies consist of several polyps or individuals approximately one millimeter long that are interconnected by their gastrovascular cavities. Colonies grow on hard surfaces; in southern Lake Michigan, C. caspia can be found in harbors on rocks, piers, pilings, and on clusters of zebra and quagga mussels.

The researchers found C. caspia in all eight Chicago harbors sampled as well as at two offshore sites. In fact, the population of the freshwater hydroid is growing in Lake Michigan. Folino-Rorem speculates that this may be due in part to street salts washing into the lake and changing water quality. “C. caspia thrives in higher salinity,” she explained.

The researchers also found that the freshwater hydroid can eat organisms—chironomids– that are two to three times its size. “This was often accomplished by working together,” said Folino-Rorem. “When one polyp gets a hold of a chironomid, the organism can continue to thrash about until another polyp latches onto it too. The two polyps engulf the chironomid, sometimes meeting in the middle.”

C. caspia is limited in its range due to its need to colonize on hard surfaces—Lake Michigan’s muddy bottom does not provide a hospitable habitat. However, the recent spread of quagga mussels may increase the amount of available substrate for attachment. Unlike zebra mussels, quagga mussels can colonize the soft, muddy bottoms found in deeper areas. According to Tom Nalepa, a NOAA biologist at the Great Lakes Environmental Research Laboratory, 99 percent of what his team finds when sampling offshore in southern Lake Michigan waters is quagga mussels.

“They are more efficient than zebra mussels in using food resources,” he said. “And they tolerate cooler temperatures. We found that the number of quagga mussels in deep and in shallow waters far exceeds zebra mussel numbers even at their peak.”

For C. caspia, the spread of quagga mussels may prove beneficial in terms of expanding their range to offshore waters. For fish populations, this may prove to be more bad news.

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Four science educators from Illinois and Indiana have been selected for the 2025 Shipboard Science Immersion on Lake Michigan July 7-13. The educators will spend a full week alongside researchers aboard the EPA research vessel Lake Guardian. Afterwards, they will bring Great Lakes science back to their classroom. Learn more and meet the four incredible teachers representing Illinois and Indiana at the link in bio.
🌿 Educators—Explore Restoration in Action! Join us Thursday, July 31 at Purdue Northwest (Hammond, IN) for a FREE full-day workshop diving into the transformation of the Grand Calumet River Area of Concern.🚍 Tour restored sites🧠 Engage with VR curriculum📚 Earn 6 PD hours🥐 Breakfast & lunch included🔗 Register now at the link in bio and bring real-world science to your classroom.Register by July 21st.
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Big news from Illinois‑Indiana Sea Grant! We’re thrilled to announce Dr. Stuart Carlton, our Assistant Director since 2018, will step into the Director role on July 14, 2025. Join us in congratulating Stuart, and read on at the link in bio to find out more about the transition!
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