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Cephalopods and other molluscs likewise have a wider zoogeographic distribution
and a lower degree of endemism.
Wide bathymetric ranges (eurybathy)
During the Cenozoic, and particularly during the Miocene and Pleistocene
(23
5million years ago), considerable extensions of the Antarctic ice sheet across
the continental shelf have clearly had drastic impacts on the shelf biota. There is
geomorphological evidence of ice scouring to depths of at least 1000m in places,
and it is generally accepted that the ice shelves extended uniformly to the edge of
the continental shelf around the Antarctic Peninsula and also much of East
Antarctica. Although it has been suggested that this has led to periodic complete
eradication of the shelf fauna it is very likely that this has not happened. However,
the wider bathymetric distribution of Antarctic invertebrates when compared to
other seas does suggest that many continental slope species found shelter in
deeper waters off the shelf during these glaciation events. The historical changes
in ice-shelf extent probably led to adaptive radiations and speciation processes on
the Antarctic continental shelf and slope, again in effect generating a
-
'
biodiversity
pump
. While we cannot quantify directly the extent to which species have migrated
up and down the Antarctic continental shelf and slope, these processes are likely
to have selected for the wide bathymetric ranges (eurybathy) that are characteristic
of many Southern Ocean species today. The boundary between the shelf /slope and
true deep-sea marine faunas lies between 1500 and 2500m depth in the Weddell
Sea. At these depths, groups such as Foraminifera show close links with deep-
water faunas of other oceanic regions, perhaps through the in
'
uence of the
thermohaline circulation. However, cryptic speciation has also been documented
for Foraminifera, suggesting a relationship between populations from widely
separated geographic regions of the Southern Ocean (e.g. Weddell and Ross
Seas), while other species are adapted to a wide bathymetric range.
Macroalgae
The Antarctic macroalgal
flora and its relationship with those of surrounding
continents is well characterised, and provides an important structural component
of nearshore marine ecosystems particularly of the sub-Antarctic islands. The
Desmarestiaceae, which originated in the southern hemisphere, occur worldwide.
Bipolar species developed in either of the two hemispheres and crossed the equator
during the Last Glacial Maximum. Polar macroalgae are adapted to weak light
conditions but, as with plants on land, in order to obtain the light required for
photosynthesis, exposure to UV radiation is inevitable. Brown, green and red algae
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