Australia: The Land Where Time Began |
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Jellyfish - The Ocean
About 72 % of the surface of the Earth is covered by water, and
jellyfish are present throughout the global ocean from pole to pole, and
from the surface to the abyssal plain. There are many habitats that
characterise different ecosystems, and these habitats define the flora
and fauna occupying those ecosystems.
The ocean has been divided into zones based on the vertical position in
the water column or horizontal position, according to their relationship
to landforms. There are many habitats which characterise and define the
biota living there. These zones are analogous to formations such as
mountains, deserts, rivers and lakes that divide up the terrestrial 3D
habitats, defining the ecospace in which certain organisms can thrive.
Ocean zones – horizontal
The coast, where land and water meet, is the starting point for these
oceanic horizontal zones. The area of the shoreline that is inundated by
the tides, the intertidal zone, is a harsh environment where only the
hardiest organisms survive, as a result of extreme changes in
temperature, salinity, and wet and dry conditions and motion pressure
applied by the incoming and outgoing tide. When jellyfish are stranded
on the intertidal zone by the outgoing tide they don’t often survive
long enough to recover when the tide returns water to the shore, as they
are generally already too overheated and dehydrated to survive.
The subtidal zone is the next seaward zone after the intertidal zone,
beginning at the point on the shore where the beach is never uncovered
by the outgoing tide. The subtidal zone has been divided into the
subzones:
1)
the shallow subtidal zone, within diving depths, the neritic,
2)
the neritic zone, the water covering the continental shelves, and
3)
the oceanic, or open ocean zone, which is off the shelf.
Different species of jellyfish characteristically occupy the different
zones, though all these biomes are occupied by jellyfish.
Ocean zones – vertical
The pelagic realm is the water column extending from the surface of the
ocean to the bottom. The animals living in the ocean realm, that
includes many jellyfish, have a lifestyle that is termed pelagic. The
benthic zone is the bottom, or sea bed. Examples of the animal
inhabitants of the benthic zone are starfish, clams and burrowing worms.
The larvae of many benthic species are pelagic, and many jellyfish, as
well as other animals, have life stages that are benthic. There are also
jellyfish, such as stauromedusae and platyctene ctenophores, that are
primarily benthic, not pelagic.
Animals living in the pelagic realm may specialise in a particular
lifestyle. Those drifting in the currents are planktonic forms, and
those strong enough to swim against the current are nektonic species.
Good examples of nektons are tuna and swordfish
The animals inhabiting the pelagic zone vary, and the pelagic realm is
not uniform.
1)
The epipelagic zone is that uppermost layer of the ocean extending from
the surface to the depth at which there is enough light penetration to
support photosynthesis, which is a depth of about 200 m (650 ft).
2)
The mesopelagic zone extends from 200 m to a depth of about 1,000 m,
(about 3,280 ft), also known as the twilight zone, where there is only
faint light penetration. According to Gershwin the life forms in the
mesopelagic zone commonly have adaptations that appear peculiar in order
to cope with the small amount of light penetration, such as large eyes,
bioluminescence, the behaviour of migrating vertically, i.e. moving to
the surface at night and then returning to the depths as sunlight
returns the next morning.
3)
The bathypelagic zone extends from a depth of 1,000 m to 4,000 m (13,000
ft).
4)
The abyssopelagic zone is from 4,000 m to 6,000 m (13,000 ft to 20,000
ft).
5)
The hadopelagic zone drops to the greatest depths. In deeper zones;
where there is no light penetration there is permanent darkness. Many of
the organisms living at these depths are blind. There are fewer species
of jellyfish in these zones than those above.
6)
The neuston zone is the top few inches of the water column, the surface
acts as a barrier that can’t be crossed, and planktonic organisms are
concentrated just below it.
7)
The pleuston zone is at the air-water interface. In this zone some
organisms live on top of the water, while other cling to the underside
of the air, the majority having body parts in both air and water. The
Portuguese Man-O’-war (bluebottle) has its float in the air and its
tentacles in the water, is an example of a pleustonic organism.
Habitats
There are many habitats in the oceanic horizontal and vertical zones,
with some commonly being inhabited by jellyfish. Examples are the box
jellies and Irukandji’s, both highly venomous, are often encountered
along sandy beaches where they search for food.
Not all jellyfish species live in marine environments; there are some
that live in freshwater reservoirs and lakes, where there are no tides
and where the water is generally too shallow to have complex horizontal
and vertical zones.
Gershwin, Lisa-Ann, 2016,
Jellyfish: A natural history, Ivy Press
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |