The Great Lakes system—Superior, Michigan, Huron, Erie, and Ontario—is a vast network of freshwater bodies containing over one-fifth of the world’s surface freshwater. Cnidarians, the phylum that includes jellyfish, are generally expected to require high saline content to survive, as oceans are their typical home. This unique ecology often leads to the question of whether a classic saltwater creature could exist in such an immense freshwater environment.
The Definitive Answer and Species Identification
Jellyfish are present in the Great Lakes basin, though they are not the large, stinging creatures people often envision. The species found here is the non-native Craspedacusta sowerbii, commonly known as the freshwater jellyfish or peach blossom jellyfish. This organism belongs to the class Hydrozoa, which is distinct from the larger Scyphozoa class that includes most marine jellyfish.
The medusa, or adult jellyfish form, is minute and highly transparent, making it nearly invisible in the water. These delicate creatures typically measure less than an inch (5-25 millimeters) in diameter when fully grown, about the size of a United States penny. Due to their small size and sporadic appearance, most people swimming or boating in the Great Lakes never encounter them. They have been recorded in Lake Erie, Lake Huron, and Lake St. Clair, as well as many surrounding inland lakes and streams.
Origin and Mechanism of Introduction
Craspedacusta sowerbii is not native to North America and is considered an invasive species in the Great Lakes region. It is indigenous to the Yangtze River valley in China, where it is adapted to variable water conditions. It was first documented outside of Asia in 1880 in a water lily tank in London, suggesting a pathway for global spread.
The most likely vector for the jellyfish’s introduction was the trade and shipment of aquatic ornamental plants, such as water lilies. The organism’s dormant stage, called a podocyst, is a tiny, resting body that easily adheres to submerged plants or other objects. The first record of the species in the Great Lakes region was in the Huron River near Ann Arbor, Michigan, in 1933, followed shortly after by a sighting in Lake Erie.
Other potential dispersal mechanisms include movement with fish stocking activities or in the ballast water of ships. However, movement on aquatic plants is the most widely supported theory for its initial global spread. Once established, the hardy, microscopic life stages can be further transported between water bodies on fishing gear, boats, or aquatic animals.
Life Cycle, Appearance, and Impact
The medusa stage of the freshwater jellyfish is characterized by a translucent, bell-shaped body that is flatter than a hemisphere, sometimes showing a whitish or greenish tint. It possesses a distinct ring of numerous tentacles, ranging from 50 to 500, packed around the bell margin. The bell margin also features a muscular, shelf-like structure called a velum, which aids in propulsion and distinguishes it from marine jellyfish.
The life cycle of Craspedacusta sowerbii is complex, including two main phases: the sedentary polyp and the mobile medusa. The polyp is the dominant, year-round life stage, existing as tiny, often colonial organisms attached to the substrate, such as rocks or submerged wood. These polyps are microscopic, measuring only a few millimeters, and reproduce asexually by budding, making them largely unseen.
The medusa stage is produced when polyps asexually bud off the free-swimming adult, usually in late summer or early fall when water temperatures are sufficiently warm, often above 77°F (25°C). The appearance of these medusae is sporadic and unpredictable from year to year, which explains why many people are unaware of their existence.
The medusae are opportunistic predators that feed primarily on small organisms like zooplankton, which they capture using stinging cells, or nematocysts, on their tentacles. Despite having stinging cells, the freshwater jellyfish poses virtually no threat to humans or pets because its nematocysts are too small and weak to penetrate human skin.
The ecological impact is generally limited, although the jellyfish can significantly reduce local zooplankton populations during a bloom. Since they are typically found at low densities, their overall effect on the Great Lakes food web is usually localized.