Freshwater Jellyfish

by Sandy Engel / from Wisconsin Natural Resources Magazine

It’s a good thing they are considered harmless and “friendly.” The Department of Homeland Security would be no match in keeping out tiny exotic plants and animals that periodically find a foothold in waters across Wisconsin. One of the small ones is a freshwater jellyfish that is less than one-inch long at full size. It has an interesting, if sporadic, history here.

Freshwater jellyfish (Craspedacusta sowerbii) are thought to be native to China. They were first reported in North America as early as 1884 and the first sightings in Wisconsin date to 1969. The freshwater jellyfish exhibit a varied life cycle comprising three stages: egg, polyp and medusa. Two kinds of larvae and a cyst stage also form, so open up those biology books for a short remedial lesson.

The jellies start as eggs produced by female medusae. If fertilized, each egg hatches into a tiny, flat larva called a planula that is hard to see with the naked eye. It swims for a few days among zooplankton before settling down on an underwater plant, log, rock or piece of sediment. The planula then becomes a polyp that looks like a miniature hydra, sort of a bitty little bud without tentacles that has stinging cells to stun equally small prey. The tiny polyp soon forms a bud near its base that stays attached and develops into a second polyp. These two polyps are identical twins since they formed asexually. Soon, the twins form yet more fixed buds and expand into a polyp colony of perhaps 2-12 buds. Don’t expect some massive colony here. You’d still need a hand lens or microscope to study them.

Now and then the jellyfish polyps form a second kind of detachable bud that develops into a tiny, cigar-shaped larva. This larva frees itself from its parent polyp and either crawls a few inches away or is carried off by flowing water before it settles down to start forming polyps of its own.

So far, our life cycle involves asexual reproduction: polyps forming buds that either attach or float before settling down to form new polyps. In some years, especially during hot summers in Wisconsin, the polyp colony produces medusa buds. Each of these top buds becomes either a male or female medusa...

You may recall the mythical Greek maiden Gorgon Medusa whose long hairs became writhing serpents and who petrified anyone foolish enough to glance at her. (How’s that for a “perm!”). When polyps develop into medusae they develop a mass of really tiny wriggling hairlike tubes on top.

After a week or two, and still quite small, the medusa leaves home to become free swimming. In another five weeks, the medusa matures into a nearly transparent body called a bell, that dangles with long, hairlike tentacles we all associate with jellyfish.

Sometimes just female medusae form, sometimes just males form. Only rarely in North America and Europe do both male and female medusae appear together. Why jellyfish produce swarms of same-sex medusae, a seeming waste of energy, still remains a mystery. If female medusae produce eggs that are fertilized by a male, they hatch into planula larvae and the whole jellyfish life cycle starts over again.

The medusae live but a few weeks, release eggs and die. The polyps can live from spring until fall, when they into cysts that are covered with a chitinous “skin” enclosing fairly dry cells. The cysts are able to survive drought and cold. In Wisconsin, the cysts survive on the bottoms of ice-covered ponds, lakes, and quiet river pools where the water is slightly above freezing. But the cysts are more than a winter resting stage. They are a vehicle for jellyfish to spread north of their home range and invade new waters.

The freshwater jellyfish found here is one of two species of Craspedacusta believed to be native to China. Both species (C. sowerbii and C. sinensis) live in the Yangtze River, the world’s third longest river that is so vast it makes the Wisconsin River seem like a trout stream! Here, male and female medusae form from spring until fall and congregate in quiet reaches of the river.

Freshwater jellyfish were unknown outside of China until June 1880 when William Sowerby (1827-1906) found jellyfish medusae swimming in a large, water-lily tank at the Royal Botanic Gardens in Regent’s Park just outside London, England. Sowerby was the director of these private botanical gardens and in charge of their indoor and outdoor exhibits. He found the medusae, all males, among sediment and pickerelweeds only three weeks after filling the water tank. What a surprise to find the first freshwater jellyfish known to science! Thinking they came in from South America on the plants, he dubbed them “Amazon jellyfish.” The discovery came with much publicity and fanfare. Then, six weeks later, the curator’s worst nightmare struck: all the jellyfish medusae vanished!

In 1884, mature jellyfish polyps were again found in a water tank at Regent’s Park. That same year, immature polyps were also found in a stream in Pennsylvania. More than 40 years would pass before the two polyps—the bigger ones from England and the newly budded ones from America—would be classified as one species.

How did these freshwater jellyfish get to London and Pennsylvania? The jellyfish probably landed at both locations as polyps or cysts attached to sediments, water plants or fishes. The 1880s were the heyday of water gardens and carp stocking. Garden clubs and aquarium societies in this Victorian Era were busy gathering the world’s exotic plants and fishes for proud display and study.

Soon the jellyfish made their way to other botanical societies, as well as to public and private aquariums in England, Europe, North America, South America and Australia. Once again, plants and fishes likely provided a vehicle to move these polyps and cysts. Other jellyfish were flushed into lakes and rivers when aquariums were emptied, perhaps for cleaning or restocking with fish. Polyps and cysts, attached to river sediments, were swept downstream to new waters. Others may have arrived on the backs of turtles or the feet of water birds. Perhaps they even stuck to boats and boat motors...