Australia: The Land Where Time Began

A biography of the Australian continent 

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.


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.

Sources & Further reading

Gershwin, Lisa-Ann, 2016, Jellyfish: A natural history, Ivy Press  


Author: M.H.Monroe
Last updated: 30/01/2017
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