Chapter 8
THE POLAR DEEP SEAS
Andrew CLARKE
INTRODUCTION
At a first glance, the Arctic and Antarctic regions
might seem to be rather similar. Both are sited
over the geographical poles and are hence subject to
extreme seasonal variations in solar radiation; both
are cold, receive relatively little precipitation and
are dominated by ice. From a biological viewpoint,
however, the differences between the two polar regions
outweigh their similarities. The Arctic is essentially a
landlocked basin allowing only a limited exchange of
water with adjacent oceans, and receives an enormous
input of freshwater and sediment. The Antarctic is a
single landmass isolated on all sides by deep oceans.
The two areas thus differ greatly in topography and
oceanography; they are also very different in their
tectonic and evolutionary history. In this review I shall
therefore deal with the Arctic and Antarctic deep seas
separately, before describing aspects common to their
biology.
First, however, it is necessary to define what is
meant by the term ‘deep sea’ in a polar context.
Gage and Tyler (1991) considered the deep sea to
start at the continental slope. This slope marks the
boundary between continental (granitic) crust and
oceanic (basaltic) crust. It typically starts at a depth
of about 200 metres, with the gradient easing into the
continental rise or abyssal plain at around 2000 metres
depth. This topographic definition of the deep sea,
however, runs into problems in the Southern Ocean
where a combination of isostatic adjustment to the
mass of the icecap and scouring by ice-sheets during
previous glacial maxima means that the continental
shelf around Antarctica may be as deep as 500 or even
1000 m in places. As a result many biological studies
of the deep sea in Antarctica are actually concerned
with the biota of the deep continental shelf rather than
truly abyssal or continental-slope organisms. Animals
living on the deep continental shelves of the Antarctic
will share some physiological features with true deepsea organisms (since pressure effects on metabolism
become apparent at depths as shallow as 500 m:
Hochachka and Somero, 1984). In faunistic terms,
however, they belong to a continental-shelf fauna rather
than a true deep-sea fauna.
In this review I shall therefore use the functional
definition of Gage and Tyler (1991), confining the
use of the term “deep-sea” to those organisms of the
abyssal plain or continental slope. Some comparison
with studies of the deep continental shelf around
Antarctica will be inevitable, however, for organisms
living there share many ecological features with true
deep-sea organisms.
Exploration of the polar deep seas
Despite their inhospitable nature, the polar regions
have played an honorable role in the history of
deep-sea biology. The British explorer John Ross,
whilst searching for the North-West Passage in 1818,
collected a specimen of the basket-star Astrophyton
when this was snagged on a sounding line at about
1600 m depth. Thirty years later, James Clark Ross
(John Ross’s nephew) and the eminent botanist Joseph
Hooker, working from HMS Erebus and Terror in the
Southern Ocean, collected abundant organisms whilst
sounding as deep as 1800 m.
These results led directly to the pioneering work
of HMS Lightning and Porcupine in the deep waters
of the Atlantic, and then to the seminal work of
HMS Challenger (1872–76). The circumnavigation
by HMS Challenger was probably the single most
important expedition in the history of oceanography,
and laid the foundations for current knowledge of the
239
THE POLAR DEEP SEAS
Andrew CLARKE
INTRODUCTION
At a first glance, the Arctic and Antarctic regions
might seem to be rather similar. Both are sited
over the geographical poles and are hence subject to
extreme seasonal variations in solar radiation; both
are cold, receive relatively little precipitation and
are dominated by ice. From a biological viewpoint,
however, the differences between the two polar regions
outweigh their similarities. The Arctic is essentially a
landlocked basin allowing only a limited exchange of
water with adjacent oceans, and receives an enormous
input of freshwater and sediment. The Antarctic is a
single landmass isolated on all sides by deep oceans.
The two areas thus differ greatly in topography and
oceanography; they are also very different in their
tectonic and evolutionary history. In this review I shall
therefore deal with the Arctic and Antarctic deep seas
separately, before describing aspects common to their
biology.
First, however, it is necessary to define what is
meant by the term ‘deep sea’ in a polar context.
Gage and Tyler (1991) considered the deep sea to
start at the continental slope. This slope marks the
boundary between continental (granitic) crust and
oceanic (basaltic) crust. It typically starts at a depth
of about 200 metres, with the gradient easing into the
continental rise or abyssal plain at around 2000 metres
depth. This topographic definition of the deep sea,
however, runs into problems in the Southern Ocean
where a combination of isostatic adjustment to the
mass of the icecap and scouring by ice-sheets during
previous glacial maxima means that the continental
shelf around Antarctica may be as deep as 500 or even
1000 m in places. As a result many biological studies
of the deep sea in Antarctica are actually concerned
with the biota of the deep continental shelf rather than
truly abyssal or continental-slope organisms. Animals
living on the deep continental shelves of the Antarctic
will share some physiological features with true deepsea organisms (since pressure effects on metabolism
become apparent at depths as shallow as 500 m:
Hochachka and Somero, 1984). In faunistic terms,
however, they belong to a continental-shelf fauna rather
than a true deep-sea fauna.
In this review I shall therefore use the functional
definition of Gage and Tyler (1991), confining the
use of the term “deep-sea” to those organisms of the
abyssal plain or continental slope. Some comparison
with studies of the deep continental shelf around
Antarctica will be inevitable, however, for organisms
living there share many ecological features with true
deep-sea organisms.
Exploration of the polar deep seas
Despite their inhospitable nature, the polar regions
have played an honorable role in the history of
deep-sea biology. The British explorer John Ross,
whilst searching for the North-West Passage in 1818,
collected a specimen of the basket-star Astrophyton
when this was snagged on a sounding line at about
1600 m depth. Thirty years later, James Clark Ross
(John Ross’s nephew) and the eminent botanist Joseph
Hooker, working from HMS Erebus and Terror in the
Southern Ocean, collected abundant organisms whilst
sounding as deep as 1800 m.
These results led directly to the pioneering work
of HMS Lightning and Porcupine in the deep waters
of the Atlantic, and then to the seminal work of
HMS Challenger (1872–76). The circumnavigation
by HMS Challenger was probably the single most
important expedition in the history of oceanography,
and laid the foundations for current knowledge of the
239