Craspedacusta sowerbii

Jellyfish in Fresh Water?

As someone who once unwittingly swam with marine jellyfish on a New Jersey beach many years ago, my first question after learning there was such a thing as a freshwater jellyfish was "Do they sting?!" The answer is, yes and no. Though craspedacustathey have stinging cells on their tentacles and can paralyze tiny prey, the stingers apparently aren't long enough to penetrate human skin. With that fear answered in a reassuring way, my next question was, "Why have I never heard of them?" The answer to that question is a little more complicated.

The scientific name of the freshwater jellyfish found in North America is Craspedacusta sowerbii, and the few people who know of its existence tend to call it Craspedacusta. That's not very short for a nickname, but it does roll off the tongue once you get it started. Craspedacusta is a member of the Cnidaria (nI-'dar-E-a), an ancient phylum of animals that includes hydras, marine jellyfish, corals, and sea anemones. There are two basic body types among Cnidarians, the attached polyp form (think of the familiar hydra from high school biology, or, in a larger version, a sea anemone) and the free-swimming medusa (jellyfish) form. Some Cnidarians, including most marine jellyfish, have both a polyp and a medusa form. Craspedacusta, though it is more closely related to hydras than to marine jellyfish, also has both a polyp and medusa stage.

The polyp stage of Craspedacusta is so nondescript that few people would ever see it unless they were specifically looking for it -- and knew exactly what to look for. It lives underwater, attached to rocks, plants, and dead wood, and is only a millimeter high. If you laid them end to end, you could fit about 15 Craspedacusta polyps across a penny. If you know how to find Craspedacusta polyps, however, you don't have to look far. They can be found in all kinds of fresh water in North America, from ponds and lakes to large rivers and small streams. Doug Smith, a professor of invertebrate biology at the University of Massachusetts in Amherst, says he finds them just about everywhere he looks.

The medusa stage occurs as part of Craspedacusta's complex reproductive cycles. Craspedacusta is capable of reproducing in at least three ways. A polyp can grow a bud that matures to full size, and as many as two dozen polyps can become attached this way to form a colony. A polyp can also create a bud that becomes a "frustule," a tiny crawling larva that eventually finds a good spot, attaches itself at one end, and develops into a polyp. Finally, the polyp can bud off a medusa, a miniature jellyfish less than a millimeter in diameter. An individual medusa, when mature, will produce either sperm or eggs which are released into the water and combine to form another type of larva called a planula larva. This larva is free-swimming until, like the frustule, it finds a good spot and attaches itself to form a polyp.

A mature Craspedacusta jellyfish is about the size of a quarter, perfectly round, ringed with stinging tentacles, and nearly transparent. One researcher calculated that they are 99.26% water. Despite their water in water nature, people do see them, and when they do, it's an event not soon forgotten. Dr. Boyd Kynard, a fish behaviorist in the Natural Resource Conservation Department at the University of Massachusetts, saw a "bloom" of Craspedacusta medusae in a man-made lake in Arizona 25 years ago. He still remembers it as "One of the most amazing things I've ever seen in freshwater. There were tens of thousands of them. I've been looking for them ever since, but I've never seen them again." Doug Smith, who, like Boyd Kynard, spends a large part of his life around fresh water, says he has seen the medusae only twice.

So why are the medusae so rarely seen, and what triggers them in certain places at certain times? Those are questions that Dr. Terry Peard and his team of graduate students at Indiana University of Pennsylvania are trying to answer by creating a national database of Craspedacusta medusae sightings. Temperature seems to play some role in triggering the medusa stage, which is why there is a freshwater jellyfish season, typically from August to September, extending a little later into the fall further south. But temperature alone doesn't seem to be the answer. In some lakes and ponds there will be jellyfish one summer and not the next, even though the water temperature reaches the same levels. In other lakes, the jellyfish appear nearly every summer.

The other great mystery of Craspedacusta is why the jellyfish produced in any one body of water are, with only a couple of documented exceptions, all one sex or the other, most often female. The only place where male and female medusae are regularly found together is in the Yantzge River in China. That's one of the reasons that the Yantzge River basin is considered to be the original home of Craspedacusta. We do know for sure that Craspedacusta isn't native to North America. One theory is that Craspedacusta polyps arrived attached to water lilies imported from China by way of Great Britain in the 1890s. However it got here, Craspedacusta spread rapidly so that it is now found all over the United States. In fact, Craspedacusta polyps are now found on every continent except Antarctica. And everywhere except China, the appearance of the medusa is sporadic and unpredictable, and almost always of one sex or the other.

So the idea of going out looking for Craspedacusta jellyfish is not an altogether practical one. But now that I know that they are out there somewhere -- a freshwater jellyfish -- I'm going to be watching for them. Maybe, if I'm lucky, some day I'll be in the right place, and it'll be jellyfish season.

freshwater jellyfishFor pictures of freshwater jellyfish, or to report a sighting of freshwater jellyfish, click here to go to the website of Dr. Terry Peard, the expert on Craspedacusta.




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