WEBVTT
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Hello everyone, I'm Marta Mammone, a
PhD student from the University of Salento
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in the South of Italy. The topic of my
PhD is the so-called "upside down jellyfish"
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Cassiopea. So we are in the phylum of
Cnidaria, classe schyphozoa, order
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Rhizostomaea. The family is Cassiopeaidae,
however the species-level relationships of
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the genus Cassiopea are still unresolved,
due to the presence of plastic and
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polymorphic features. To date, only 10 species
are considered to be valid on molecular data.
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The upside-down jellyfish Cassiopea is a
benthic scyphozoan commonly found in
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tropical and subtropical shallow coastal
ecosystems, such as mangroves
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and seagrass meadows. Cassiopea
has a unique behaviour: in fact, it is
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able to lay down with the exumbrella on
the bottom of the sea or of the aquaria,
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and the oral arms face upwards. One
of the main features of Cassiopea
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is the presence of endosymbionts, which are
dinoflagellates of the family symbiodinaceae.
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Cassiopea follows the life cycle typical of
schyphomedusae. So we have one sexual
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part of the life cycle with the production of
larvae, and an asexual part with budding
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and strobilation. So starting from the sexual
part of the life cycle, we can say that
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Cassiopea is gonochoric so we have the
presence of both males and females.
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With histology we can see, for example
the male follicles where spermatogenesis
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takes place, and on the other side we
have female gonads with the presence
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of oocytes at different stages of maturation.
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During spawning, sperm are released in
the water, and fertilisation occurs in the
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gastrovascular cavity of the female. Then,
embryos will be released on the oral disc.
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In fact, Cassiopea are brooders.
Cleavage begins 1-2 hours after
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fertilisation and then at 48 hours post-
fertilization ciliated embryos are visible,
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and by 96 hours post-fertilisation
the planuae is able to swim with
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clear directionality and then once it finds
a suitable substrate it will then settle down
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and become a polyp
through metamorphosis.
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At the polyp stage, we have the
important acquisition of endosymbionts.
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In fact, for example, eggs and
planulae are free of symbionts.
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Asexual reproduction includes budding
and strobilation. In Cassiopea, we have
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two different kinds of budding: the first
one is the production of a planuloid bud
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forms through evagination of the calix.
The most distal part of the bud is the
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future aboral pole, so after detachment
it will lead the swimming and then settle
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down and become the stalk of the future
polyp. Within 24 hours, more or less,
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the settled bud will develop a hypostome
and tentacles, providing the capacity of
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capturing prey. In Cassiopea we have
another kind of budding, the so-called
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lateral budding. In this case, the new
polyp calix develops just below the
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original polyp calix. Contrary to swimming
buds, it's the arboral pole to be attached,
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and during the growth the new polyp
with the stalk will migrate down the
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original stalk of the mother polyp.
Strobilation is another kind of asexual
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reproduction, so in this case the polyp will
grow into a strobila that then will go through
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a transversal division of the body, forming
ephyrae. The strobilation in Cassiopea is a
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monodisc so we have the formation of one
ephyra at the time. So the ephyrae will form
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from the oral part of the polyp, which
during the process will decrease the
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length of its tentacles. At this point we
have also the formation of the sensory
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organs of the jellyfish, the so-called
rophalia. Once the ephyra has developed,
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also the umbrella and the oral arms,
start to pulsate and then after 1-2
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days it will detach from the strobila.
Before the detachment, the polyp starts
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to develop 10-12 tentacles that then
will increase up to 40, more or less,
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once the ephyra leaves. While the lifespan
of a medusa is limited to 16-17 months,
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budding can occur continuously
during the lifecycle.
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Asexual reproduction like budding and
strobilation are phenomena regulated
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by factors such as food and temperature.
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Under good conditions in fact we have, for
example, the formation of a chain of buds,
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while strobilation on the other side cannot
occur with a temperature below 18-20°C.
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Another important thing to take into account
is the presence of symbionts, which is
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required for strobilation. Then the ephyrae will
grow into juveniles and finally mature adults.