Maintaining a Balance
Topic 20: Enantiostasis and Estuarine Organisms
Biology in Focus, HSC Course
Glenda Childrawi, Margaret Robson and Stephanie Hollis
DOT POINT
 Define enantiostasis as the maintenance of metabolic and
physiological functions in response to variations in the
environment and discuss its importance to estuarine
organisms in maintaining appropriate salt concentrations
Enantiostasis
Some organisms live in
environments where they
experience extreme
fluctuations in conditions.
To survive, these plants and
animals have evolved
adaptations that allow them
to cope physiologically with
these fluctuations, a survival
mechanism called
enantiostasis.
Enantiostasis
Enantiostasis is the
maintenance of metabolic and
physiological functions in
response to variations in the
environment. The survival of
species that live in an
environment such as an
estuary, where salt and water
concentrations fluctuate
broadly on a daily basis,
depends on their ability to
either avoid these changes or
tolerate them.
legacy.owensboro.kctcs.edu
Enantiostasis
Organisms that move freely between the sea and rivers
experience similar fluctuations in environmental conditions,
and they too have developed mechanisms of avoidance or
tolerance.
Enantiostasis
Enantiostasis is not limited to fluctuations in salt levels. For
example, extreme changes in environmental pressure are
experienced by diving birds and so these animals must also rely
on enantiostasis for their survival.
telegraph.co.uk
Estuarine Organisms
In estuaries, the daily change
in tides affects the salinity of
the environment. At high tide,
sea water flows into the river
mouth, creating an
environment with a high salt
concentration (a higher
osmotic pressure) than the
cytoplasm of cells and body
fluids in organisms. This salt
water has the tendency to
draw water out of cells by
osmosis.
flickr.com
Estuarine Organisms
At low tide, sea water flows out of the river mouth and fresh
water flows into the estuary. Plants and animals in the estuary
which are subjected to this predominantly fresh water
environment with a high water potential face the challenge of
water tending to move into living tissue.
tripwow.tripadvisor.com
Estuarine Organisms
Osmoregulation in organisms inhabiting an estuary is a
challenge – they need to maintain normal metabolic
functioning, despite these enormous fluctuations within the
environment. Living organisms employ one of two strategies in
enantiostasis.
treknature.com
Estuarine Organisms
Osmoconformers are organisms that tolerate the changes in
the environment by altering the concentration of their internal
solutes to match the external environment. Their body fluids
‘conform’ to that of the environment. Their metabolism is able
to tolerate changes in salinity in their own body fluids and
cells.
gatheringhisblessings.blogspot.com
Estuarine Organisms
Osmoregulators are organisms that avoid changes in their
internal environment and have the ability to keep the solutes at
an optimal level. They ‘regulate’ solute concentrations within
the body, regardless of the differing external environment.
absolutedivers.com
Estuarine Organisms
These organisms are unable to tolerate a range of salt
concentrations in their body fluids and cells and so they have
mechanisms to exclude salt to keep the internal fluid
concentration constant, despite fluctuations in the
environment.
guardian.co.uk
Activity
-Use ALARM Scaffold to answer DOT Point Question:
Define enantiostasis and discuss its importance to estuarine
organisms in maintaining appropriate salt concentrations