Question:
My daughter and I love to swim and play in waters wherever we find them. While in French Gulch (Shasta County) last year, we decided to play around in Clear Creek. The creek was running pretty high, but when my daughter and I got out we had these black, worm-like things hanging off us. Our first thought was leeches, which got us out of the water quite quickly! Someone told me they were rock worms and wouldn't hurt us. We haven't returned there though because we're still too scared they were leeches.

We also stopped at Eagle Lake (Lassen County) to go swimming and ended up with these tiny little round slime balls on us. When picking up these slimy things in question, they flattened out on our hands and started slithering like a leech across our hands. This was another trip where my daughter and I ran screaming out of the water to rinse off under the faucet! There were lots of people swimming in the lake who either didn't seem to notice or else knew something we didn't.

Clear Creek was a very cold creek, but Eagle Lake was very warm, so I could understand Eagle Lake possibly having leeches. Do these leeches suck human blood? Are they harmful to humans in any way? I love the outdoors and swimming, but too many encounters with creepy leech-like things are making me leery about the safety of it. (Kim B.)

Answer:
Without pictures, it's tough to say, but it sounds like you encountered two different invertebrates. According to Department of Fish and Game (DFG) Associate Fish Pathologist Garry Kelley, Ph.D., the organism at Clear Creek was likely a free-living caddisfly larvae (Genus Rhyacophila), commonly known as a rock worm. This type of caddisfly crawls around rock bottoms in search of food and is commonly eaten by trout. Caddisflies are not at all harmful to humans.

The organism at Eagle Lake might be a leech based on the slithering swimming motion you described. There are many types of leeches and most are fluid feeders. Leeches are either scavengers or are parasitic (i.e., they feed on other organisms). Some species of leeches suck blood from vertebrates (humans, waterfowl, fish, etc.) while others feed on insects, mollusks, oligochaetes or dead animal matter. Kelley suspects the organism described at Eagle Lake was non-parasitic in nature because bloodsucking was not indicated.