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Hello everyone, while in the previous video
I gave you some information on the biology
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of the upside-down jellyfish Cassiopea,
here I will briefly show you its ecology.
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Species of the genus Cassiopea are
characterised by a particular trophic
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strategy called mixotrophy. In fact the upside-
down jellyfish relies on both heterotrophy,
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capturing prey, and autotrophy, thanks
to the presence of the endosymbionts.
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Concerning heterotrophy, we can say
that Cassiopea feed on zooplankton,
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and in fact in aquariums they are
fed with Artemia salina nauplii.
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So zooplankton and also interstitial micro
-organisms are flushed across the oral
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appendages, entangled in mucus, and
consumed by the many secondary
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mouths present on the oral arms.
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With heterotrophy, both nitrogen and
phosphorus are provided to the host.
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On the other side, we have autotrophy.
As I told you before, Cassiopea lives in
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symbiosis with endosymbionts usually
of the clade A (Symbiodinium) and C
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(Cladocopium). Symbionts are able to
perform photosynthesis through the Calvin
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cycle. So during this process, inorganic
carbon and nitrogen are transformed into
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organic forms such as carbohydrates,
amino acids, and fatty acids that then
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are translocated to the host.
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Photosynthesis could theoretically provide
more than 100% of the carbon required.
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However the symbiotic jellyfish still ingest
zooplankton, which provide the essential
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nutrients that the zooxanthellae cannot.
One of the major gaps in the algal-cnidarian
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symbiosis has been the understanding of
the mechanisms of carbon translocation
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from symbionts to the host. Finally, we can
also say that photosynthesis seems to be
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supported by the uptake of regenerated
nutrients initially buried in the sediments
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and that are then resuspended thanks
to the pumping activity of Cassiopea.
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Thus, since symbionts perform photosyn
-thesis, these jellyfish depend on light
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availability. Experiments on photosynthesis
in the lab are carried out through incubation
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in these small chambers where
a probe can detect oxygen levels.
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Before incubation with light, it is common to
make a dark incubation to measure respiration.
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Then jellyfish are irradiated with increasing
light intensities, having in this way an idea
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of the oxygen production. It was observed,
for example, that photosynthesis is an
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essential part of the autoecology of
the species, and also at low light levels
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a basal photosynthesis is still carried out,
so there is no complete shift to heterotrophy.
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Cassiopea can easily withstand different
light conditions and this has been suggested
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as a key factor for adaptation in new habitats.
This is important especially for alien species
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such as Cassiopea andromeda, a
colonizer of the Mediterranean Sea.
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In fact, the ability to cope with fast changes
in abiotic conditions, such as light exposure
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and temperature, together with their dual
nutrition mode make the upside-down
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jellyfish a potential winner in a
changing Mediterranean Sea scenario.
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For mixatrophic species such as Cassiopea,
it is challenging to distinguish their food
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source and relate it with different
environmental conditions.
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The study of animal diets has been applied
to understand their basic ecology and trophic
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interactions in the food webs. Up to now,
many studies investigating jellyfish trophic
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position have relied on gut content.
However, direct tools are limiting since
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only ingested and not assimilated
food can be defined.
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That's why we can use indirect tools to solve
this problem. They are represented by
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some biomarkers such as stable isotopes,
fatty acids, lipids, and carbohydrates,
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and with these tools we can have the
integration of seasonal energy inputs.
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We can also use them to understand the
dominance of autotrophic or heterotrophic
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inputs in mixotrophic species such as
Cassiopea, and to make spatial or
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temporal comparisons. Stable isotopes
have been largely used to understand the
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trophic position of species, while the free
fatty acids are useful to understand the
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source of the energy used by the organisms
in different conditions or having different
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reproductive strategies. Biomolecules
such as carbohydrates or lipids help us
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to understand the nutritional condition of
species and their capability to overcome
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the reproductive period in different
environmental conditions.
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In the next video I will show you
how to feed and raise Cassiopea.