NEWS113
46
Most people know jellyfishes only from the sea,where they aregenerally
unpopularas theycandeliver serious,even fatal,stingswith thestingcells
situated on their tentacles. But hardly anyone knows that there are also
jellyfishes that live in freshwater, andwhich are completely harmless to
humans.
Invertebrates
Freshwatermedusa,
Craspedacustasowerbii.
The"umbrella"ofthe jellyfish isaround2cmacross.Photos:F.Schäfer
A small jellyfishconquers
theworld
byTobiasKörbel
he freshwater jellyfish was first seen in
Europe in 1880, at Regent
´
s Park in
London,where this delicate creature,which
attainsadiameterofonlysome2cm,turned
up in a tank housing tropical water lilies. At
that time there was far more interest in
animals than there is today, and the
freshwater jellyfish hit the headlines
worldwide.This, the first andonly freshwater
jellyfish then known, was scientifically
described in the same year as it was
discovered - twice! - as
Craspedacusta
sowerbii
L
ANKESTER
, 1880 and
Limnocodium
victoria
A
LLMANN
, 1880. The first name was
published somewhat earlier and hence is
valid.So thescientificnameof the freshwater
jellyfish is
Craspedacustasowerbii.
Detectivework
But where did this creature appear from so
suddenly? It was already known that
jellyfishes are no more than the sexually
reproductive stage of a polyp. Jellyfishes
belong to the phylum Cnidaria and their
closest relatives are the sea anemones and
corals.Hence thenormalhabitusofa jellyfish
looks like a small sea anemone. This can
perhaps best be comparedwith toadstools
andmushrooms. The normal state of these
fungi is a network of filaments that live
underground. The parts that appear above
groundandcanbemade intotastydishes(as
longas theyaren'tpoisonous fungi) areonly
the fruitingbodies,not the fungus itself.
Itwasthussupposedright fromthestartthat
the jellyfish arrived in the greenhouse in
polyp form, attached towaterlilies. Because
the greenhouse in Regent
´
s Park was
dedicated to thequeenof all thewaterlilies,
the
Victoriaregia
fromSouthAmerica (hence
the name
Limnocodium
victoria
), it was
supposed that the original home of the
freshwater jellyfishwas inSouthAmerica.
Thepolyp isdiscovered
Accordinglyasuccessfulsearchwasmadefor
an unknown polyp in the waterlily tank in
Regent
´
s Park. A tiny polyp, only 2 mm
across,was discovered. It looked a bit like a
Cluedo playing piece with a bristly hair-do.
Sometimes 2 to 4 polyps unite into a small
colony 5-8 mm across. The jellyfish comes
into being when the head of the polyp
transforms into a little ball, which, when it
detaches from the polyp, is open on the
underside, inotherwordsbell-shaped. In the
courseof itsgrowth the "bell"becomesan
ever-flatter disc. The fully-formed, adult
jellyfish - another word for this stage is
medusa - is eventually around 2 cm
across.
In 1885 the polyp was also found in
Philadelphia, but it wasn't recognized as
the polyp of
Craspedacusta sowerbii
and
hence was described as a new species,
Microhydra ryderi.
The first jellyfishes
weren't found in Philadelphia until 12
years later, inAugust 1897.
T
Asmall colonyof
Craspedacusta
polyps.
After Jankowskietal.,2008,modified
Worldwidedistribution
Nowadays this freshwater medusa is found
on all the continents of the world except
Antarctica, though it remainsamysteryhow
it has managed to spread so extensively.
However, the puzzle of where it came from
originally appears to have been solved.This
jellyfishalmost certainlyoriginated from the
Yangtze drainage in China, which is also
home to 2-3 further species of the genus
Craspedacusta
. The actual number of
freshwater medusae in existence remains
unclear.More than 20 species of freshwater
jellyfishes havebeen scientificallydescribed,
