PHYSIOLOGICAL MECHANISMS
Islay D. Marsden
School of Biological Sciences, University of Canterbury,
Christchurch, New Zealand
Definition
Physiological mechanisms refer to the physiological adaptations needed for life in estuaries and coastal lagoons.
Introduction
Perhaps the most important specialization needed by an
estuarine organism is the ability to live in brackish water
of varying salinities. Living sometimes in rapidly changing external media, animals are also exposed to wave
activity, high silt levels, freshwater inputs, and temperature extremes. The ability to avoid adverse conditions or
regulate their ionic and water balance allows species to
occupy habitats where they are able to grow rapidly and
exploit readily available food sources (Lockwood,
1976). Being close to human settlements, estuaries and
their brackish-water organisms are subject to anthropogenic inputs from industry, fishing, and waste discharges,
including untreated and treated sewage. These can result
in highly elevated nutrient levels, eutrophication, and hypoxic sediments. Many estuaries are contaminated with
trace metals, PCBs, and other substances, and the combinations of stressors can affect all aspects of the biology
of estuarine organisms. Adaptations include morphological, behavioral, and physiological mechanisms that allow
animals to evade the stresses, those which regulate
osmotic and ionic balance, and specialist mechanisms to
survive in hypoxic conditions. Estuaries are under pressure from climate change, and one can ask whether the
organisms that live in this habitat have physiological
mechanisms that allow them to adapt to changes in water
levels and ocean acidification.
Adaptation to salinity changes: osmoconformers
and osmoregulators
Brackish-water organisms are euryhaline, tolerating
a wide salinity range (0.5 to 35 psu). Some are conformers,
species where the body fluids and cells are usually equal in
osmotic pressure (isosmotic) to the external environment,
and others are osmoregulators where the body fluids are
regulated homeostatically, maintaining a constant body
fluid concentration despite changes in the external environment. As shown in Figure 1 (from Willmer et al.,
2005), there are varying levels of osmoconforming and
osmoregulating among different animals. Normal seawater is 34 psu, yielding about 1,000 mOsm, and the figure
shows the variation of internal body fluid with external
salinity for a range of brackish-water species. Although
there is no single pattern of osmoregulatory ability of estuarine organisms, three main mechanisms are involved in
the adaptation to osmotic changes. These are (1) altering
external permeability, (2) varying salt uptake, and
(3) conducting cellular osmoregulation (see Willmer
et al., 2005).
Polychaetes and mussels, which are generally thought
of as osmoconformers, have limited or no osmoregulatory
control; they are successful in estuaries because they
tolerate a wide range of internal concentrations and tissue
volume changes, although they may possess some
intracellular isosmotic regulation using organic solutes
compatible and counteracting organic osmolytes.
Osmoconforming is a good mechanism because it avoids
energy use in active transport, and this lowers the metabolic costs and reduces food requirements.
Fish and marine vertebrates are osmoregulators and are
able to maintain a steady internal osmotic pressure regardless of osmotic changes in the external environment. In
fish, osmoregulatory organs are the digestive tract, gill,
kidney, urinary bladder, and liver in ureotelic regulators.
Some crustaceans, such as the brine shrimp Artemia, are
also osmoregulators, living in pools of varying salinities
and maintaining a low internal osmotic pressure. They
drink large amounts of water (8 % of body weight per
day), retain water with a highly impermeable exoskeleton,
and excrete excess ions with special salt glands on their
gills.
Bony fish (teleosts) and higher vertebrates are
hypoosmotic, maintaining a low cellular osmolarity
between 250 and 400 mOsm regardless of the external
salinity. This adaptation allows them to migrate between
the sea- and freshwater for spawning and feeding. They
tend to lose water and gain excess salts through their external surfaces. One mechanism to cope with this problem is
for marine fish to drink seawater. The digestive tract
restricts some ions, but some are imported allowing water
to follow. Excess salts are carried in the blood to the gills
where specialized epithelial chloride cells actively transport
NaCl outward. Salts are lost in the feces, and there is very
little urine production, again reducing the water loss.
A more detailed description of the mechanisms used in
other marine vertebrates is given by Sherwood et al. (2005).
Cartilaginous (elasmobranch) fish, sharks, and rays are
different. They maintain high osmotic pressures often
above their external medium by producing urea which is
stored throughout the body. Urea is toxic and such concentrations would be fatal to many organisms. These fish,
however, are peculiar in that they have evolved specialized structures that are urea resistant and they also produce
methylamine osmolyte (TMAO), with concentrations
about half of that of the urea, that counteracts the
destabilizing effects of urea on protein structure and
function.
Crustaceans have a hard relatively impermeable external surface, and there are interspecific differences in
osmoregulatory ability between species. Some are
osmotic and ionic conformers in full strength seawater
PHYSIOLOGICAL MECHANISMS
485
Islay D. Marsden
School of Biological Sciences, University of Canterbury,
Christchurch, New Zealand
Definition
Physiological mechanisms refer to the physiological adaptations needed for life in estuaries and coastal lagoons.
Introduction
Perhaps the most important specialization needed by an
estuarine organism is the ability to live in brackish water
of varying salinities. Living sometimes in rapidly changing external media, animals are also exposed to wave
activity, high silt levels, freshwater inputs, and temperature extremes. The ability to avoid adverse conditions or
regulate their ionic and water balance allows species to
occupy habitats where they are able to grow rapidly and
exploit readily available food sources (Lockwood,
1976). Being close to human settlements, estuaries and
their brackish-water organisms are subject to anthropogenic inputs from industry, fishing, and waste discharges,
including untreated and treated sewage. These can result
in highly elevated nutrient levels, eutrophication, and hypoxic sediments. Many estuaries are contaminated with
trace metals, PCBs, and other substances, and the combinations of stressors can affect all aspects of the biology
of estuarine organisms. Adaptations include morphological, behavioral, and physiological mechanisms that allow
animals to evade the stresses, those which regulate
osmotic and ionic balance, and specialist mechanisms to
survive in hypoxic conditions. Estuaries are under pressure from climate change, and one can ask whether the
organisms that live in this habitat have physiological
mechanisms that allow them to adapt to changes in water
levels and ocean acidification.
Adaptation to salinity changes: osmoconformers
and osmoregulators
Brackish-water organisms are euryhaline, tolerating
a wide salinity range (0.5 to 35 psu). Some are conformers,
species where the body fluids and cells are usually equal in
osmotic pressure (isosmotic) to the external environment,
and others are osmoregulators where the body fluids are
regulated homeostatically, maintaining a constant body
fluid concentration despite changes in the external environment. As shown in Figure 1 (from Willmer et al.,
2005), there are varying levels of osmoconforming and
osmoregulating among different animals. Normal seawater is 34 psu, yielding about 1,000 mOsm, and the figure
shows the variation of internal body fluid with external
salinity for a range of brackish-water species. Although
there is no single pattern of osmoregulatory ability of estuarine organisms, three main mechanisms are involved in
the adaptation to osmotic changes. These are (1) altering
external permeability, (2) varying salt uptake, and
(3) conducting cellular osmoregulation (see Willmer
et al., 2005).
Polychaetes and mussels, which are generally thought
of as osmoconformers, have limited or no osmoregulatory
control; they are successful in estuaries because they
tolerate a wide range of internal concentrations and tissue
volume changes, although they may possess some
intracellular isosmotic regulation using organic solutes
compatible and counteracting organic osmolytes.
Osmoconforming is a good mechanism because it avoids
energy use in active transport, and this lowers the metabolic costs and reduces food requirements.
Fish and marine vertebrates are osmoregulators and are
able to maintain a steady internal osmotic pressure regardless of osmotic changes in the external environment. In
fish, osmoregulatory organs are the digestive tract, gill,
kidney, urinary bladder, and liver in ureotelic regulators.
Some crustaceans, such as the brine shrimp Artemia, are
also osmoregulators, living in pools of varying salinities
and maintaining a low internal osmotic pressure. They
drink large amounts of water (8 % of body weight per
day), retain water with a highly impermeable exoskeleton,
and excrete excess ions with special salt glands on their
gills.
Bony fish (teleosts) and higher vertebrates are
hypoosmotic, maintaining a low cellular osmolarity
between 250 and 400 mOsm regardless of the external
salinity. This adaptation allows them to migrate between
the sea- and freshwater for spawning and feeding. They
tend to lose water and gain excess salts through their external surfaces. One mechanism to cope with this problem is
for marine fish to drink seawater. The digestive tract
restricts some ions, but some are imported allowing water
to follow. Excess salts are carried in the blood to the gills
where specialized epithelial chloride cells actively transport
NaCl outward. Salts are lost in the feces, and there is very
little urine production, again reducing the water loss.
A more detailed description of the mechanisms used in
other marine vertebrates is given by Sherwood et al. (2005).
Cartilaginous (elasmobranch) fish, sharks, and rays are
different. They maintain high osmotic pressures often
above their external medium by producing urea which is
stored throughout the body. Urea is toxic and such concentrations would be fatal to many organisms. These fish,
however, are peculiar in that they have evolved specialized structures that are urea resistant and they also produce
methylamine osmolyte (TMAO), with concentrations
about half of that of the urea, that counteracts the
destabilizing effects of urea on protein structure and
function.
Crustaceans have a hard relatively impermeable external surface, and there are interspecific differences in
osmoregulatory ability between species. Some are
osmotic and ionic conformers in full strength seawater
PHYSIOLOGICAL MECHANISMS
485