Craspedacusta sowerbii, commonly known as the freshwater jellyfish or peach blossom jellyfish, inhabits freshwater ecosystems. This organism belongs to the phylum Cnidaria, a group that includes marine relatives like sea anemones and corals. Unlike most jellyfish, C. sowerbii has adapted to non-saline environments, making it a rare sight in lakes, ponds, and quarries worldwide. Its sporadic appearance makes it elusive; it can be present in a water body one year and seemingly vanish the next.
Physical Characteristics and Identification
The medusa, or jellyfish stage, of Craspedacusta sowerbii measures 5 to 25 millimeters in diameter when fully grown. Its body is bell-shaped and transparent or translucent, sometimes with a whitish or greenish tint. Around the edge of its bell, there is a dense fringe of up to 400 tentacles, which vary in length.
Four radial canals extend from the central stomach, the manubrium, connecting to a ring canal near the bell’s margin, facilitating nutrient circulation. These canals are often opaque white, along with the four large, flat reproductive organs (gonads) attached to them. Its small size helps distinguish it from other aquatic organisms.
Habitat and Global Distribution
Craspedacusta sowerbii thrives in calm, freshwater environments like lakes, ponds, reservoirs, and quarries, and in slow-moving sections of rivers, avoiding faster currents. This species is believed native to the Yangtze River basin in China, inhabiting shallow pools.
Its global spread beyond its native range is largely attributed to human activities. Transport of aquatic plants, fish, or other materials through waterways has introduced it to nearly every continent, with Antarctica being the sole exception. It has been recorded in many parts of the United States, including the Great Lakes region, and across Europe and Australia. It prefers warmer water temperatures, with medusa blooms occurring in late summer to early fall when temperatures rise above 25°C.
Life Cycle and Behavior
The life cycle of Craspedacusta sowerbii involves two stages: a sessile polyp stage and a free-swimming medusa stage. The polyp stage is microscopic, about 1 millimeter in height, and often goes unnoticed as it attaches to submerged surfaces like rocks, vegetation, or tree stumps. These polyps reproduce asexually by budding, forming colonies of two to four individuals, or by producing frustules, small, rod-like larvae that disperse and develop into new polyps.
Medusae bud off from these polyps, appearing in large numbers during warmer months. These immature medusae are 0.5 to 1.0 millimeter in diameter and mature over five weeks after detaching. As predators, both polyps and medusae use stinging cells called nematocysts on their tentacles to capture prey, primarily zooplankton such as daphnia and copepods. Sexual reproduction in the medusa stage, involving the release of eggs and sperm, is rare in many non-native populations, as most are either all male or all female.
Ecological Impact and Human Interaction
A common concern regarding Craspedacusta sowerbii is its sting, but its nematocysts are too small to penetrate human skin effectively. Contact typically results in an undetectable or mild, localized irritation, posing no significant threat to swimmers. In the aquatic ecosystem, C. sowerbii consumes zooplankton, impacting their populations. Studies show that medusae can consume many zooplankton daily, preferring larger prey like copepods.
Despite its widespread distribution, this freshwater jellyfish is not considered to have a significant negative ecological impact on native fish populations or the ecosystem. While polyps have been observed to kill and feed on striped bass larvae under laboratory conditions, and there is speculation about them consuming fish eggs, they are not considered significant predators of fish in natural settings. Observing these creatures is safe, and reporting sightings to local aquatic monitoring programs can contribute to understanding their distribution and ecology.