Yes, there are freshwater jellyfish, and the most widely known species is Craspedacusta sowerbii, a hydrozoan. These creatures are frequently overlooked because the recognizable jellyfish form, or medusa, is only one part of their life cycle and appears sporadically. Unlike their larger marine relatives, this freshwater species is relatively small and generally only makes a noticeable appearance under specific environmental conditions.
The Global Traveler: Identification and Appearance
The jellyfish form of Craspedacusta sowerbii is characterized by a translucent, bell-shaped body that typically measures between 5 and 25 millimeters in diameter, roughly the size of a dime or a quarter. Its body is composed of about 99% water, which contributes to its glassy, nearly invisible appearance. This transparency allows it to blend seamlessly with its aquatic environment, making it challenging to spot unless a large group is present.
The bell margin is lined with numerous fine tentacles, which can number anywhere from 50 to 500. These tentacles are armed with thousands of stinging cells, or nematocysts, which the jellyfish uses to capture its microscopic prey. Hanging down from the center of the bell is the manubrium, a structure that functions as the mouth and stomach. The overall appearance is delicate, often described as having a whitish or greenish tinge due to internal structures like the four canals and gonads.
Understanding the Habitat and Distribution
Craspedacusta sowerbii is native to the Yangtze River valley in China, but it has since spread across the globe and is now found on every continent except Antarctica. This wide distribution makes it an invasive species in most locations. The organisms are found in still or slow-moving freshwater bodies, such as lakes, reservoirs, ornamental ponds, and quarries.
The medusa stage often appears in the late summer or early autumn when water temperatures are at their warmest. The presence of the free-swimming form is correlated with warm conditions, typically appearing when the water temperature is between 19 and 30 degrees Celsius. This requirement triggers the production of the visible jellyfish stage, explaining why they can appear suddenly and then disappear for years.
The Hidden Life Cycle of the Freshwater Jellyfish
The life cycle of the freshwater jellyfish is complex, involving two distinct forms: a nearly invisible benthic polyp stage and the ephemeral, free-swimming medusa stage. For most of the year, the organism exists as a tiny, sessile polyp, less than two millimeters long, attached to submerged surfaces like rocks or aquatic plants. These polyps reproduce asexually by budding, which allows them to form small colonies or produce new polyps that settle nearby.
The sudden appearance of the jellyfish form is caused by the polyp stage asexually producing medusa buds. These buds detach and develop into the free-swimming jellyfish, a process often triggered by sustained warm water temperatures. This rapid asexual production results in occasional “blooms” where large numbers of jellyfish are observed. The medusae are the sexual stage, releasing eggs and sperm into the water. If fertilized, these develop into larvae that settle and become new polyps, completing the cycle.
Are They a Threat to Humans or Ecosystems?
Despite having stinging cells, the freshwater jellyfish poses no threat to humans. The nematocysts on their tentacles are small and weak, and cannot penetrate human skin. Any contact with the jellyfish would likely go unnoticed, unlike the painful stings inflicted by many marine species.
The jellyfish are opportunistic predators, feeding primarily on small zooplankton like copepods and water fleas. While they are an invasive species in many parts of the world, their overall impact on the freshwater ecosystem is considered minimal and localized. They do not cause widespread ecological havoc, and in some areas, they become a food source for native crustaceans.