Essay D, Student 7, Expert Marker
Define the terms osmoconformer and osmoregulator and used named examples to
explain these different osmotic strategies in water-breathing animals.
The osmolality (mOsmol/kg) or osmolarity (mOsmol/l) of an organism’s internal fluids is crucial
to maintain intracellular conditions at optimum concentrations. To much deviation either way
(ions or water) can decrease the efficiency of processes occurring in the cells and lead to the
cell death. This is due to the diffusion across permeable membranes like in gills and lungs,
essential for diffusion of oxygen, but this also means it allows the diffusion of water and ions in
or out of the organism; along their concentration gradients through the membrane. Organisms
that actively alter their internal osmolality, independently from the external are called
osmoregulators and organisms follow the external osmolality as it changes are called
osmoconformers (Randall, et al., 2001).
Most osmoconformers are marine invertebrates, such as the Lugworm (Arenicola), because it
is simple and efficient to allow diffusion of salts and water across their surface membranes with
large surface area to volume ratios. It requires no special systems or energy expenditure on
behalf of the Lugworm as diffusion is a passive process; making it very efficient. There are no
freshwater osmoconformers as the osmolarity of fresh water is <50mOsmol/l and general blood
osmolarity for freshwater vertebrates is 200-300mOsmol/l (Randall, et al., 2001). This would
cause swelling as water diffused into the cells, down the concentration gradient into the
vertebrate, causing lysis of the cells since they physically cannot contain enough water to
dilute the internal osmolarity to equal the external. The Hagfish (Myxine) is only one vertebrate
known that is iso-osmotic and iso-ionotic to the seawater, it is suggested that this is because
Hagfish have an entirely marine ancestry (Hill, et al., 2008; Randall, et al., 2001).
Elasmobranchs are also roughly iso-osmotic with the external seawater, but they do
ionoregulate as, their blood has high concentrations of urea and TMAO (plus other organic
osmolytes) instead of high levels of inorganic ions found in the environment. The excess salts
are removed by the kidneys and via the rectal gland.
Some marine invertebrates can slightly hyper-osmoregulate at low salinities such as marine
springtails; when they would otherwise behave as osmoconformers at higher salinities. For 2
species of Collembola (Isotoma viridis and Hypogastrura viatica) hyper-osmoregulate at salt
concentrations lower than 50% of seawater. Another species, Anurida maritima, is a strict,
Essay D, Student 7, Expert Marker
stenohaline, osmoconformer seeing that it has lost the ability to hyper-osmoregulate, unlike the
other species it cannot survive at low salinities (Witteveen, et al., 1987).
Freshwater organisms are all hyper-osmotic to their external environment as they actively
regulate their ionic and osmotic concentrations because the external osmolality is to low to
support internal metabolic processes. In teleosts, like the Goldfish (Carassius auratus), this is
done by active uptake of salts from food and from the mitochondria rich cells in the gills that
counteract the passive loss of salt to the environment from the gills. This is achieved by the
use of the electrochemical gradients set up by the Na+ - K+ ATPases and H+ ATPases as the
protons and Na+ are pumped out of the cell (and 2 K+ in to every 3 Na+ out into the blood
stream). This makes the interior of the cell negatively charged and exterior positive, which in
turn attracts Na+ from the freshwater down the electrochemical gradient into the cell. Gaining
Cl- ions is done by the diffusion CO2 into the gill cells, then reacting with metabolic water
creating bicarbonate ions (and protons). The bicarbonate ions follow the electrochemical
gradient and diffuse out the cell into the external environment via a counter-transporter that
brings in a Cl- ion at the same time increasing the cells salt concentration (Hill, et al., 2008;
Evans, et al.,, 2005). As they are so significantly hyper-osmotic to the fresh water around them
they have to produce the large amounts of dilute urine to cope with the amount of water that
diffuses in through their gills and to avoid losing too much salt. They don’t drink the water as
they gain all they need from metabolic processes in the body. A Goldfish cannot tolerate a
wide range of salinities, so it is therefore known as a stenohaline osmoregulator.
For Marine teleosts, like the Atlantic wolfish (Anarhichas lupus) and other animals in the
opposite situation where they are hypo-osmotic to the surrounding environment they have to
produce very little, concentrated urine to remove Mg2+ and SO2- ions. They also do drink sea
water to replace the fluids lost from the urine and the gills by diffusion, but this means that they
are gaining more salt than they require, so they remove the excess through the mitochondria
rich cells, as in freshwater fish but the mechanisms work in the opposite direction to remove
salts from the cells and blood plasma. The Atlantic wolfish is called a euryhaline osmoregulator
as they can tolerate a broad range of salinities, but for long periods of time this can impair
growth rates if they have to increase osmoregulation rates to obtain a constant internal
osmolarity (Le Fran et al., 2004).
Essay D, Student 7, Expert Marker
For water-breathing animals the osmoregulation strategy used is dependent on their
environments osmolality (seawater, freshwater or estuarine mix) and how constant the
conditions are to whether they are euryhaline or stenohaline. This is because it is
advantageous to be an osmoconformer, if there is not a large difference in the osmolalities
between the blood and the external environment, as it relies on the passive movement of water
and ions through permeable membranes; so no energy is expended. But being an
osmoregulator means that they don’t have to rely on being in an environment with suitable
osmolality as their internal osmolality is controlled this allows them to live in more extreme
environments, where osmoconformers couldn’t exist. It can also be influenced by heritage,
since the Hagfish is an osmoconformer due to having a completely marine ancestry. When
compared to marine teleosts (like the Atlantic wolffish), that are believed to have evolved from
freshwater teleosts they have inherited the mitochondria rich cells to aid in the regulation of salt
and their osmotic water potential (Hill, et al., 2008; Randall, et al., 2001).
References:
Evans, David H.; Piermarini, Peter M. & Choe, Keith P. (January 2005). The multifunctional
fish gill: dominant site of gas exchange, osmoregulation, acid-base regulation, and excretion of
nitrogenous waste. Physiological Reviews, Volume 85, Number 1, pp. 97-177.
Hill, Richard W.; Wyse, Gordon A. & Anderson, Margaret (2008). Animal physiology, 2nd
edition. Sinauer Associates, Inc., Massachusetts, USA.
Le Fran, N.R.; Lamarre, S.G. & Blier, P.U. (2004). Tolerance, growth and haloplasticity of the
Atlantic wolffish ( Anarhichas lupus ) exposed to various salinities. Aquaculture, Volume 236,
Numbers 1-4, pp. 659-675.
Randall, David; Burggren, Warren & French, Kathleen (2001). Eckert Animal physiology:
mechanisms and adaptations, 5thedition. W.H. Freeman and Company, New York, USA.
Witteveen, J.; Verhoef, H.A. & Letschert, J.P.W. (1987). Osmotic and ionic regulation in
marine littorial collembola. Insect Physiology, Volume 33, Number 1, pp. 59-66.