Page 82 - AlertDiver_Winter2013

68 | WINTER 2012
be problematic for cells. The seals minimize the challenge of
lactic acid buildup by using oxygen with maximum efficiency.
As a seal starts its dive, it reduces its heart rate to half of
normal. With the immense amount of oxygen in its blood,
this is more than sufficient to supply normal amounts of
oxygen to the brain and nervous system, but blood flow to
most other organs slows to a trickle. Finally, it slows, but
does not stop, blood flow to its muscles. As the muscles burn
through their oxygen and become hypoxic, muscle cells start
to absorb the oxygen bound to myoglobin around them. By
balancing its oxygen use, the seals ensure that stores in the
blood and muscles are depleted at the same rate. With these
adaptations, Weddell seals can substantially limit reliance on
anaerobic metabolism for up to 20 minutes on a hunting trip.
Thus, the seals can make long series of 20-minute dives with
little buildup of lactic acid, diving again and again for up to 12
hours with very little time between dives.
Twenty-minute dives are routine for a
Weddell. But the seal can also push its limits by
somehow informing its body that a long dive is
ahead and cutting off blood flow entirely to its
muscles and nonessential organs at the start of
the dive. Its kidneys cease all function, and its
muscles quickly deplete their own oxygen stores
and switch to anaerobic processes. With no blood
flow, the anaerobic muscle cells are sequestered
from the rest of the body, keeping the building
lactic acid out of the seal’s circulatory system and
away from its organs until it resurfaces. The seal
also carries its own internal scuba tank: It keeps
more than half of its oxygen-rich red blood cells
in its spleen. Over the course of the dive, the seal
slowly releases more and more of the oxygenated
cells into its bloodstream, keeping oxygen levels high for its vital
organs. With this system of oxygen management, the seal can
stay submerged for up to 80 minutes, but after such a dive, as you
might expect, the seal needs to rest.
A s the seal disappeared below us, we
turned our attention back to the crack
and followed it toward the island.
Every few minutes a seal would either
ascend from the depths or drop out
of the sky. Some just dropped in and
swam the length of the tunnel, making
brief forays under the ice to hunt for hidden prey.
The bottom of the sea ice is not smooth like the surface,
but a complex of loosely packed crystals, or platelet ice,
that provides a labyrinth of diminutive hiding places. Tiny
fish like Pagothenia borchgrevinki, the “bork,” live hidden
in the platelets. The seals dive down and look up at the
ice, identifying these little fish as they move from place to
place, black silhouettes against the glowing blue. Once a seal
hones in on a bork, it rises and blows bubbles of air into the
platelet ice to force the small fish out of hiding. Deep-diving
Weddells may use a similar technique — diving down and
looking up to see dark shapes moving against the distant ice,
high above. It is even suggested that the seals continue the
strategy through the winter, hunting by moonlight during
the four-month-long night.
I followed the seal’s lead, dropping down 50 feet, then
stabilized and looked back up. Silhouetted against the ice
were a mother and pup, just entering the water. The pup
swam cautiously under the ice after its mother. A few feet
from their hole, the pup reared back, apparently scared, and
made a quick escape back to the surface. The mother seal
swam lazily back, and a few minutes later she managed to
lure the pup back into the water. This trip was a little longer
A N T A R T I C A
R O S S I S L A N D
R O S S
S E A
PACIFIC
OCEAN
Turtle Rock
Erebus Glacier
McMurdo Station
McMurdo
Sound
ATLANTIC
OCEAN
ROSS
SEA
Ross
Ice
Shelf
Ross
Ice
Shelf
INDIAN
OCEAN
South
Pole
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