The textbooks say there isn't anything like it in Wisconsin. But there it
was, undulating through the water at Redgranite Quarry right before my eyes.
A freshwater jellyfish, mini-version of the saltwater kind! The first I'd
ever seen in 16 years of diving.
Taken to the marine lab at Lawrence University, it was identified as
Craspedacusta sowerbyi, a true jellyfish and the only known freshwater
species in America. Hardly anyone, including professional limnologists even
knows they exist. And no wonder! An on-again, off-again critter with a
weird life cycle, Craspedacusta may show up in a pond once and never be seen
there again. Or it may reappear only after many years. Asking around
revealed records of freshwater jellyfish in seven Wisconsin spots since
1973. They include Mendota, Half Moon, Leesome, Devil's, White Sand and
Pine Lakes.
Upon searching, we found thousands in Redgranite Quarry in depths up to 10
meters where water temperature was 68 degrees F. Deeper than that, where
reading dropped to 59 in the thermocline, jellyfish were absent. Feeding on
zooplankton, they rose to the surface, then dropped through the water column
swimming actively up, down, and sideways in a dancing sort of movement.
Size ranged from about a fifth of an inch to nearly an inch.
This jellyfish form is the mature medusa stage of Craspedacusta's life
cycle. It is shaped like an umbrella or an upside-down soup dish and can
release sperm and eggs into the water where fertilization occurs. There are
also asexual ways of reproduction and many freshwater jellyfish populations
are either all male or all female. At Redgranite Quarry they're both.
When the fertilized egg falls to the bottom, it develops into a colony of
two to 10 individuals. These often break off and start new colonies.
Hydroids also sprout buds which drop off and grow into new individuals.
Budding also produces the jellyfish or medusa form which is almost
microscopic in size when it drops from the parent.
For many years it was thought to be a separate species called Microhydra
ryderi. Because they're tiny, covered with debris, and live on the bottom,
hardly any of these have been found. It was not until 1924 that scientists
learned the hydroid was a stage in development of the freshwater jellyfish.
Craspedacusta itself was discovered in England in 1880 and in the US in
1908. It has been reported in nearly all states east of the Mississippi
River but not in New England. There are related species in China, Africa,
India and Trinidad.
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