We know what you're thinking . . . freshwater jellyfish? Must be an aquatic myth, along the lines of the Loch Ness Monster and the Land Shark, right? Wrong. Freshwater jellyfish do exist -- and they've been known to appear in our local lakes and ponds.

Scientifically speaking, freshwater jellyfish (Craspedacusta sowerbii) are a bit of a mystery. The species has been spotted in warm regions all over the world, in a variety of freshwater environments, but the occurrence of the species is relatively rare and unpredictable. The tiny, transparent polyps that develop into freshwater jellyfish dwell in microscopic colonies on the bottoms of many freshwater lakes, ponds, and streams, but not all polyps reach the jellyfish (or "medusa") stage of the life cycle. In fact, a water body may host underwater polyps for years without any appearance of jellyfish. The exact conditions required for the development of freshwater jellyfish from the polyps, and the means by which the species is transported between water bodies, have not been conclusively determined. Observational evidence suggests that freshwater jellyfish are most likely to be found in calm, man-made impoundments and reservoirs and in water temperatures of 80°F and above.

Freshwater jellyfish are similar in appearance and function to marine jellyfish, but are smaller and far less treacherous for swimmers. A full-grown freshwater jellyfish is typically half an inch in diameter. The species is carnivorous, using stinging tentacles to prey on microscopic zooplankton, but the stinger cells are too small to pierce or significantly irritate our skin. The presence of jellyfish is not hazardous for a water body, and poses no threat to fish populations.

During the warm, late summer months, freshwater jellyfish tend to swim just below the surface of the water, especially on sunny days. Large groups (jellyfish "blooms") have been known to pop up, making for a startling sight. Look for the elusive freshwater jellyfish in a lake near you!