Antarctica is a continental landmass much of which
is covered by an ice cap, consequently the ichthyofauna
is totally marine. Surrounding the continent is
the Southern Ocean, approximately 36 million km2,
continuous with the Atlantic, Indian, and Pacific
Ocean basins to the north and whose northern limit
is generally taken as the Antarctic Polar Frontal
Zone (APFZ). There is a clear separation between
the Antarctic and the Southern Hemisphere continents,
the nearest connection being with South
America via the Scotia Arc, a series of islands separated
from each other by deep water.
The Antarctic Circumpolar Current (ACC) and the
general oceanographic regime mean that marine
isotherms are more or less concentric around the
continent. Close to the continent the seasonal variation
in temperature is rarely more than 1 1C while
even at the northern limit, as for example at South
Georgia, the range is little more than 4 1C.
These two factors, geographical isolation and
constant low temperature, have a major effect on
Antarctic fish.

Fish Fauna
The Southern Ocean ichthyofauna is relatively sparse
and unusual in composition, consisting of 213
species belonging to only 18 families.
Nearly half the species belong to one
group, the perciform notothenioids, which make up
45% of the fish fauna. Restricting consideration to
the shelf, and particularly in the highest latitudes,
notothenioids make up 77% of the species and
90–95% of the biomass of fish. Notothenioids are
morphologically and ecologically diverse, and have
variegated into a wide variety of niches, mainly demersal,
and also in the water column and even within
sea ice. As a group, this makes them more diverse
than, for example, the finches of the Gala´pagos
archipelago. The concept of species flocks has been
developed for freshwater fish to identify groups that
have a close affinity; typically such flocks are to be
found in ancient lake systems and it is extremely
unusual for such a flock to be identified from a large
marine environment. Antarctic notothenioids with
their high species diversity and endemism form a
species flock comparable to that of Lake Baikal.
Early taxonomic studies were based on the traditional
methods of morphometric and meristic analyses.
Recent studies have used molecular biological
analyses not only of nuclear material but also of
antifreeze compounds to indicate phylogeny.

Cold Adaptation
Some of the earliest studies on the physiology of
Antarctic fish concerned the measurement of oxygen
uptake rates. Initially it had been assumed that,
since many biochemical processes are temperaturedependent,
the metabolic rates of Antarctic fish
might be very low. The initial experiments indicated
that rates were substantially higher than those of
temperate fish when studied at low temperature and
the degree of elevation of the metabolic rate in
Antarctic fish was attributed to a phenomenon
termed ‘cold adaptation’. Subsequent studies demonstrated
that the greater part of this elevation was
caused by handling stress and the extended recovery
time, of the order of 24 h or more, following introduction
into respirometers. In spite of this, it is now
accepted that some slight elevation of metabolic rate
remains that cannot be explained wholly by experimental
technique. Consideration of the phenomenon
has raised some controversy between different
workers. The existence of the phenomenon has been
demonstrated experimentally, although it does not
appear to confer any evolutionary advantage because
it implies a higher energy requirement on the part of
the fish. All these studies have been undertaken on
whole fish; the overall oxygen uptake rate being the
balance between all the component metabolic pathways
that are present. As such it has been argued that
the term ‘cold adaptation’ has little meaning and that
it is more sensible to consider each metabolic component
separately to provide an overall balance.

Pure water freezes at 0 1C, but the presence of salts
causes the freezing point to be depressed such that
normal seawater freezes at around 1.85 1C. At
McMurdo Sound the annual mean water temperature
is 1.87 1C and varies within the range 1.40 to
2.15 1C. Body fluids, such as the blood plasma, of
most teleost fish have a freezing point of
c. 0.7 1C. Even though this difference is small it is
important, because living in waters close to the
freezing point of seawater, Antarctic fish require some
mechanism to prevent their body fluids from freezing.
In the absence of ice, fish could live in a supercooled
state. Unfortunately this is not a stable state
because very few ice crystals are required to cause a
supercooled liquid to freeze. An alternative
adaptation is required. The ionic concentration
of the blood of most marine teleosts is
320–380mOsmkg1, only about one-third of that of
Antarctic seawater (1050mOsm kg1). The freezing
point depression of some notothenioids at McMurdo
Sound is 2.2 1C, although their blood osmolality is
550–625mOsmkg1, equivalent to a freezing point
depression of 1.02 to 1.16 1C. Thus although
there appears to be some compensation as measured
by the osmolality, it is insufficient to explain all of
the depression in freezing point. Compensation for
this difference comes in the form of antifreeze glycopeptides
(AFGPs) which exert their effect by a
mechanism known as adsorption-inhibition.
Even though ice crystals can form, their
further growth is prevented when AFGPs are adsorbed
onto them because the AFGP molecule prevents
growth of the ice crystal along its main axis.
Thus the AFGPs have an antifreeze function, lowering
the freezing point beyond that which would be
expected from the osmolality. The AFGPs however
do not lower the melting point.
The AFGP molecules are of such a size that they
would be lost through the glomeruli of normal teleost
kidneys. In glomerular nephrons of normal teleosts,
molecules with a molecular weight of
o68 000 Da pass through the filtration barrier. As
the urine passes through the different parts of the
nephron, it is modified by reabsorption of nonwaste
products and secretion of waste products. The AFGP
molecules are of such a size that they would pass
through the glomeruli but would need to be reabsorbed
later on in the nephron. The kidneys of all
Antarctic fish which possess AFGPs are aglomerular,
obviating this requirement. Thus the evolution of the
aglomerular trait in Antarctic fish complements that
of the presence of antifreeze.

A continuous low water temperature means that the
oxygen-carrying potential of seawater is high. Thus,
as long as the partial pressure of oxygen in the
seawater remains high, so will the available oxygen.
It is against this background that further cardiovascular
adaptations have evolved.
Early taxonomic studies relied on specimens preserved
in alcohol or formalin, both of which affect
the color of the fish. Because fish typically possess red
blood, until the 1950s no mention was made of the
anemic appearance of the gills of some species of
Antarctic fish. At that time, it was noticed that
members of the Channichthyidae (at that time called
Chaenichthyidae) were white, as a result of which
they were called ‘white-blooded fish’ or ‘icefish’. The
blood of channichthyids is devoid of hemoglobin,
although small numbers of nonfunctional erythrocytes
have been described in a few species.
Initial consideration was given to determine whether,
because channichthyids do not possess scales,
cutaneous respiration might be a major factor in
oxygen uptake. However, the absorptive area and
vascularization relative to the gills mitigated against
that mechanism. Alternatively it was thought possible
that channichthyids possessed either a more
efficient oxygen utilization mechanism or else lowered
oxygen requirement. This second consideration
was being examined at a time when the concept of
metabolic cold adaptation was under discussion.
The viscosities of the plasma of red-blooded
notothenioids and channichthyid fish are very close,
although the blood of the former is approximately
25% higher than the latter. Studies on oxygen
uptake rates indicated that channichthyids utilized
oxygen at a slightly lower rate as compared with
equivalent red-blooded notothenioids. In the absence
of hemoglobin, the oxygen-carrying capacity of
channichthyid blood is only about one-tenth that of
red-blooded fish. Two mechanisms are possible to
compensate for this effect: either channichthyid
blood is circulated at a much faster rate or there is
much more of it in the system. The latter has proven
to be the case and channichthyid blood takes up
8–9% of the total volume of the fish (2–4 times that
of other teleosts); the heart rate and blood pressure
are low but the stroke volume and resultant cardiac
output are large. To reduce the resistance to flow, the
capillaries are larger than in other teleosts and the
blood is less viscous.
Even though the hemoglobin-less condition is
clearly effective, it is a feature that confines the fish
to areas of high oxygen tension such as those present
in Antarctic waters. Only one channichthyid species,
Champsocephalus esox, is found outside of the
Antarctic zone. Experimental studies have demonstrated
that channichthyids are particularly sensitive
to hypoxia, indicating that in their natural habitat
the oxygen saturation is always consistently high.
source: Encyclopedia of Ocean Sciences, Second Edition, Volume 1-6.