VFS Latest revision: 1/4/12
Cold water and deep-diving adaptations of various animals
Relationship of temperature and gas solubility
The most dramatic cold-adapted phenotype among the Antarctic notothenioids is shown by the
Channichthyidae, also named "icefishes" or "white-blooded fishes". Uniquely, all species of this
family lack hemoglobin, and hence their blood
is colorless, while blood-containing organs
like the gills, spleen and heart are pale. In
addition, in Channichthyidae the myotomal
(fast-twitch muscles used for swimming)
muscles, and in some species also the heart, do
not contain myoglobin. (Myoglobin is the
protein that is normally found in high
quantities in muscle to provide a local supply
of oxygen when the partial pressure of oxygen
falls very low.) This highly specialized
condition is only compatible with the stably
cold and well oxygenated Antarctic waters.
Additional adaptations of the ice fish include a
large heart, wide blood vessels, and thin blood.
These cold-water adaptations cause the fish to suffocate at warmer temperatures because of the
reduced solubility of oxygen.
Freezing-point depression
The presence of high concentration of low molecular weight polyols or glucose in
physiological fluids serves to lower the freezing point of tissues in cold-adapted plants and animals by
colligative action.
Prevention of ice nucleation.
Antifreeze proteins are found in a wide variety of species, including certain plants, fish,
spiders, ticks and insects. Antifreeze proteins do not act colligatively, but bind ice crystals and stop the
addition of water molecules to lattice structures. There are currently five known classes of antifreeze
proteins (AFP); the mechanism of action of all of these classes is not well-understood, but one class of
AFP are glycoproteins that have carbohydrate groups covalently attached to amino acid sidechains.
The carbohydrate groups bind to water molecules through hydrogen bonding.
The Antarctic fish described have another interesting adaptation that allows them to avoid
formation of ice crystals: Their body fluids are matched in density to the surrounding seawater and
they do not have a swim bladder. This prevents them from floating up to the surface of the water,
where ice may be floating. Contact with ice would cause instant crystallization of the supercooled
fluids in the fish.
Membrane composition
Cell membranes must remain relatively fluid in order for cells to function properly. One way
that organisms maintain membrane fluidity at low temperatures is by increasing the degree of
unsaturation in the fatty-acid chain. The double-bonds introduce kinks in the long hydrocarbon chains
and prevent close-packing and stiffening of the membrane. Goldfish can actually alter their membrane
composition to adapt to the temperature of the water by controlling the production of the enzymes that
produce the various fatty acids. They can also increase the proportion of short chain fatty-acids, which
remain fluid at lower temperatures due to reduced van der Waals interactions. Finally, the presence of
VFS Latest revision: 1/4/12
cholesterol is an important factor in maintaining membrane fluidity at low temperatures: because of its
rigid, planar structure, insertion of cholesterol in a membrane prevents close packing of saturated fatty
acid chains, ensuring that the chains remain mobile.
High-pressure adaptations in the sperm whale
Sperm whales can dive to depths of 10,000 feet and stay submerged for an hour or more. At
these extreme depths and resulting high pressures, nitrogen gas readily dissolves in the blood. As the
animal returns to the surface, the nitrogen gas will become less soluble and there is the danger that it
will be released from the blood in gas bubbles. Although human divers must take special precautions,
such as timed decompression, to avoid the painful and possibly fatal condition known as “the bends,”
sperm whales and other deep-diving mammals have adaptations that allow them to prevent formation
of the large nitrogen bubble that cause joint and muscle pain and other symptoms of decompression
sickness.
Three adaptations are particularly useful: 1) the retia mirabilia, a network of tiny blood vessels
that filter out nitrogen bubbles before it the blood enters the brain; 2) a rigid trachea that is largely gas
impermeable, preventing nitrogen from entering the blood, and 3) a flexible ribcage that allows the
lungs to collapse easily at high pressure, forcing the air out through the rigid, non-absorbing trachea.
Other interesting adaptations that allow sperm whales to perform deep dives are shown below.