YSM Issue 86.1

Thousands of feet underneath the
shifting ice sheets of Antarctica lurks
one of the world’s greatest evolutionary
success stories. A variety of distinct fish
species, collectively called the notothenioids,
have developed the ability to avoid freezing
under extreme conditions through the evolution
of antifreeze glycoproteins. Today, these
fish make up approximately 75% of the biodiversity
and 95% of the biomass in Antarctica.
Their story not only illustrates how relatively
small changes in temperature have led to
major differences in species survival but also
the consequences that global warming may
have on this intrepid family of fish.
Antarctica’s Changing Landscape Shaped the Evolution
of the Notothenioids
The story of the notothenioids begins
roughly 40 million years ago with the separation
of Antarctica from the supercontinent
Gondwana. The split led to the creation of
an isolated continent and the Antarctic Circumpolar
Current. Driven by strong westerly
winds found in the latitudes of the Southern
Ocean, the Antarctic Circumpolar Current
blocked warm water from reaching Antarctica’s
shores. Antarctica’s formerly tropic
climate shifted dramatically and the temperature
plummeted. These changes exerted great
evolutionary pressure on the endemic species
of the region and, for many species, resulted
in mass extinction.
The notothenioids would have likely suffered
the same fate as their relatives had it not
been for the evolution of antifreeze glycoproteins
approximately 35 million years ago. Antarctic
marine fish drink water that contains
small ice crystals. Antifreeze proteins bind to
the crystals and prevent their growth, which
otherwise would lead to complete freezing of
the organism. However, the exact mechanism
of action for these antifreeze proteins is still
poorly understood.
The Diversification of Notothenioids Occurred Long
After Antifreeze Glycoprotein Evolution
For many years, researchers argued that
the evolution of antifreeze glycoproteins was
the driver of the diversification of Antarctica
notothenioids. In a recent paper, Yale University
researcher Dr. Thomas Near instead suggests
that the spread and diversification of the
notothenioids is due to climate change events
occurring at least 10 million years following
the evolution of these proteins. Near states
that while antifreeze glycoproteins are critical
for notothenioid survival, the morphological
and ecological diversity in Antarctic notothenioids
is correlated with events of the Late
Miocene, a time period approximately 11.6 to
5.3 million years ago.
During the Middle Miocene, approximately
20 to 15 million years ago, a warming
occurred in Antarctica that resulted in
temperatures significantly higher than those
today, causing the melting and shifting of ice
sheets in Antarctica. “This ice destruction,”
reports Near, “may have led to the extirpation
of [many Antarctic species] and created
all these open niches for notothenioids to
occupy and subsequently diversify.” The subsequent
Middle Miocene Climatic Transition
led to the polar conditions that exist today
in Antarctica.
This period of climatic turmoil resulted in
the extinction of many of the notothenioids’
competitors and a changed geographic environment.
The notothenioids expanded into
open niches and became physically and thermally
isolated by the cooling temperatures of
the Middle Miocene Climatic Transition. Near
states, “It is thought that dynamic history of
Vomeridens infuscipinnis, a semi-pelagic “dragonfish” species. This specimen was
captured at 410 m near the South Orkney Islands. The specimen is approximately
19 cm in length. Courtesy of Dr. Thomas Near, Yale Department of Ecology and
Evolutionary Biology.
12 Yale Scientific Magazine | January 2013 www.yalescientific.org