About Freshwater Jellyfish

Freshwater jellyfish belong to the phylum Cnidaria and class Hydrozoa, distinguishing them from “true jellyfish” (class Scyphozoa), which are exclusively marine. Despite their different environments, freshwater and marine jellyfish share many traits and perform similar ecological roles in their respective environments. There are thought to be up to 20 species of freshwater jellyfish globally, however only around 10 species are confirmed. The most well-known and widely distributed freshwater jellyfish is the species Craspedacusta sowerbii (sometimes spelled “sowerbyi”), or the peach blossom jellyfish. The common name “peach blossom jellyfish” comes from ancient Chinese records that compare the appearance of Craspedacusta to the timing of the blooming of peach blossoms. The earliest record of Craspedacusta is from 1609 in Chinese records.

Originally from the Yangtze river basin in China, C. sowerbii can now be found on every continent except Antarctica. Craspedacusta sowerbii live in still freshwater bodies such as ponds, lakes, reservoirs, and slow-moving rivers. They are not strong swimmers and can not tolerate swiftly moving water. Their dispersal throughout the world is largely attributed to the transport of aquatic plants, stocked fish, and possibly waterfowl migration. Their life cycle stages are critical for them to be transported around the world.

Craspedacusta sowerbii’s life cycle alternates between two main stages: a sessile polyp stage and a free-swimming medusa stage. The polyp is tiny, often only a few millimeters tall, and remains attached to substrates such as aquatic plants or rocks. These polyps are likely the key component of the transport of Craspedacusta between freshwater bodies. The polyps reproduce asexually through budding and can produce either more polyps or medusae. The medusa stage are either male or female and reproduce sexually, releasing gametes into the water, leading to fertilization and the formation of planula larvae. These larvae eventually settle and develop into new polyps. The polyps can last, even for decades, until ideal environmental conditions occur, then bud through a process called strobilation, in which they release medusae.

The medusae of this freshwater jellyfish are small, with a bell-shaped body measuring a maximum of about 2.5 cm in diameter. They are transparent or translucent and possess numerous tentacles used for feeding and movement. The medusa stage is relatively rare and unpredictable in appearance, often appearing only under favourable environmental conditions, such as warm temperatures. They may appear in one year, disappear the next year, and reappear several years later. As a result, freshwater jellyfish may go unnoticed for long periods, existing primarily in their microscopic polyp forms. Reports of C. sowerbii occurrences usually come between June and September during summer and autumn when water temperatures are high enough to trigger blooms and there is an abundance of food available.

Ecologically, freshwater jellyfish are predators that feed primarily on small zooplankton. Like their marine cousins, freshwater jellyfish have stinging tentacles to capture their tiny prey. They are opportunistic predators, catching small organisms that come within reach of their tentacles. Although they possess stinging cells like other cnidarians, their sting is too weak to affect human skin significantly. Few freshwater species of fish are thought to eat freshwater jellyfish, though crayfish are known to consume them. While the impact of freshwater jellyfish on freshwater ecosystems is still debated, they may influence zooplankton populations and food webs, especially during blooms, when their population rapidly increases. However, such blooms are typically temporary and irregular, and this species has yet to be determined to be highly invasive or harmful.

As climate change alters the distribution of the world’s biodiversity, it is likely that Craspedacusta will continue to expand their range and inhabit an increasing number of freshwater bodies around the world, making it important to research these organisms. The unpredictable and sporadic appearances make Craspedacusta sowerbii difficult to find, necessitating the use of methodologies such as citizen science to expand the geographical and temporal scale of research or technologies such as environmental DNA (eDNA) monitoring to find and identify freshwater jellyfish through their cast-off DNA.

Further reading:

https://www.nature.com/articles/s41598-021-02525-3

https://onlinelibrary.wiley.com/doi/full/10.1002/ece3.70350

https://besjournals.onlinelibrary.wiley.com/doi/10.1002/pan3.70344