Life in a cold environment - Antarctica: Global Science from a Frozen Continent - page 166

Figure 6.2 The blood of ice sh (Channichthyidae) is transparent as it lacks haemoglobin and so

the fish must absorb oxygen directly through their scale-less skin. (Credit: Peter Bucktrout, BAS)

The Southern Ocean hosts several pelagic ecosystems whose borders are de ned

by a series of physical features. Close to the coast is a community that is most
affected by ice and which has the greatest connection between the surface waters and
the sea oor. The Antarctic continental shelf is unusually deep (typically ~500+m)

because of the weight of the ice sheet pressing down on the continent itself. None the
less, there are a number of species, such as Antarctic silver sh and the coastal ice

krill Euphausia crystallorophias , which can be found throughout the water column,
in association with the benthos and even deep beneath the great ice shelves. There
is a fairly diverse fish fauna on the shelves and on the continental slope, which

includes numerous members of the notothenoids, an endemic order of Antarctic sh

which are characterised by their lack of swim bladder. They include, in the ice sh,

the only vertebrates that completely lack haemoglobin in their circulatory system
and myoglobin in their muscle tissue. The Antarctic silver sh ( Pleuragramma

antarcticum ) is a schooling species that resembles the more northern herrings or
anchovies and which can, at times, form an important element in the diet of
emperor or Adélie penguins. The ice krill is another species that can be important
food for vertebrate predators including minke whales, seals and penguins. The
distribution of both the silver sh and the ice krill cuts out sharply at the edge of

the shelf break where a strong westward flowing current jet forms a demarcation

between the coastal zone and the more offshore communities. The phytoplankton
communities of the coastal zone are less distinct than their animal counterparts but
the productivity of this zone is the highest of the region, in the short period when the
continental shelf is ice free. Vast blooms of algae such as Phaeocystis can turn the
waters intensely green and the detritus from these blooms sinks to the sea oor

feeding the benthic communities there.