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
they 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
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.
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.