NEWS113
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The freshwatermedusa in thephotowasdiscoveredbyour assistantMichelleGerner inapondnear
Darmstadt.
Thisneat caricatureby theartistHartmuth
Pfeil (1893-1962) shows a sectionof an
exhibitionput onby theHottonia, the
Darmstadt aquarium society.The text
(translated) reads: ”Andherewehave the
medusa -which is a freshwater jellyfish.Quite
rare too.”Towhich thevisitor is replying:
”Just lookat that,andwehave themhere?
Andnow I knowwhat I just trodon in the
swimmingpool..”
but of these only 3-5 species of
Craspedacusta
and2-6speciesof
Limnocnida
are regarded as valid. The latter genus is
commonplace inLakeTanganyika.
Butback to
Craspedacustasowerbii.
In1905 it
turnedup inGermany (inMunich),in1969 in
Sweden,and in2002 inLithuania.Butonlythe
medusaeareconspicuous,thetinypolypsare
probablyalmostalwaysoverlooked.
Medusaeonlywhenhot
Bynowthebiologyof the freshwater jellyfish
hasbeenso intensivelystudiedthatweknow
why themedusa stage occurs so rarely and
then usually comes as a surprise. Medusae
developonlyatwater temperaturesof 25 °C
upwards, which at our latitudes aren't
guaranteed every year by a long chalk. The
polyp stage may thus have lived for many
years in a body of water before it suddenly
springs intomedusa production in a warm
summer. This also explains why freshwater
jellyfishes sometimes "appear out of
nowhere" inwarm-wateraquaria.
Only females
Themedusa stage actually serves for sexual
reproduction. Male and female jellyfishes
release their sperm and eggs into the open
water. Fertilized eggs settle on a substrate
and develop into polyps. The polyps
reproducebydivision.
For some strange reason there are only
femalemedusae,at least inEurope,sosexual
reproduction isprecluded.Thismeansthatall
the freshwater jellyfishes outside China are
apparentlyclonesof the individuals that first
turnedup inRegent
´
sPark in1880!
Intheaquarium
Freshwater jellyfishes can easily be
maintained in the aquarium. They will eat
Artemianaupliiand liveforaround2-3weeks,
thoughhere inEuropetheycanonlybebred
via the polyps. In China, on the other hand,
there is a patented process for breeding
Literatur
:
Didz
̌
iulis,V.andZ
̇
urek,R. (2013):NOBANIS–
InvasiveAlienSpecies Fact Sheet –
Craspedacusta sowerbii.–From:Online
Databaseof theEuropeanNetworkon
InvasiveAlienSpecies –NOBANIS
access 7/10/2014
Jankowski T.,CollinsA.G.&R.Campbell
(2008): Global diversityof inlandwater
cnidarians.Hydrobiologia598:35–40
freshwater jellyfishes specially for the
aquarium! But they aren't exported, as the
medusastage is toodelicate for that.
