NEAR FREEZING, NOT FROZEN – The primary purpose of the McMurdo Oceanographic Observatory (MOO) is to support studies on the freezing-avoidance abilities of Antarctic notothenioid fishes. The 130 species of this group live almost exclusively in the coldest and iciest waters of Antarctica's Southern Ocean. While your everyday fish would freeze solid almost instantly under these conditions, notothenioids possess special antifreeze proteins in their blood and other body fluids that protect them from freezing to death in icy waters.
Cold-blooded marine fishes risk freezing in polar oceans because they, like most animals with backbones, have blood that is much less salty than seawater. While the concentrated salts in seawater depress its freezing point to around -1.9°C (28.5°F), the fresher fish body fluids will freeze at much warmer temperatures, around -1.0°C (30°F) or higher.
So, how do these fishes manage to survive in freezing seawater at temperatures as much as 1.0°C (1.8°F) below their expected freezing point?
A major piece of the puzzle was solved by one of our team members. In the 1970's, while researching fishes in McMurdo Sound, Art DeVries discovered that the resident species possessed proteins that lower the freezing temperature of their body fluids. He called these special molecules antifreeze proteins. In being long macromolecules of biological origin, fish antifreeze proteins are completely different than the antifreeze fluid you would put in your car.
By binding strongly to the surface of ice crystals antifreeze proteins stop their growth. Since the ice can't grow any larger, the effective freezing temperature of antifreeze-fortified fish body fluids is depressed. In this way, the antifreeze proteins prevent the fish from freezing to death even at lowest temperature they may experience in the wild–the freezing point of seawater (see chart).
Almost fifty years after Art's discovery, the evolutionary origins of the antifreeze glycoprotein gene has been described, the mechanisms by which the proteins recognize and arrest ice growth have been extensively probed, and the variation in antifreeze proteins across fish species and habitats has been studied in depth. Nevertheless, it's still unclear exactly how the antifreeze proteins work inside fishes in the wild to prevent freezing and death over a lifetime of exposure to ice-filled, near-freezing seawater.
By establishing a long-term record of seawater temperature, salinity and tide data from one of our prime fish research sites in McMurdo Sound, Antarctica, the MOO is providing–for the first time–an in-depth view into the nature of the environment that these fishes call home. In having access to oceanographic condition data in real time, we can now conduct experiments on the fishes under known, real-world conditions–such as seawater supercooling–that are impossible to recreate in the lab.
MOO team members fishing through a hole in the sea ice along shore in McMurdo Sound, Antarctica.
Most fishes are less salty than seawater, so they should freeze to death in near-freezing seawater. Antarctic notothenioid fishes have antifreeze proteins that lower the freezing point of their body fluids to below that of seawater.
By virtue of their antifreeze proteins, the juvenile notothenioid fishes shown here on anchor ice in McMurdo Sound, do not freeze under conditions where most fishes would perish. Seastars, sea spiders, and other invertebrates are the same saltiness as seawater–they don't risk freezing to death.
A DOUBLE-EDGED SWORD - Thir evolution of antifreeze proteins, some 10 to 15-million years ago as Antarctica was cooling drastically, is unquestionably the major reason that notothenioid fishes can survive and thrive in the coldest seawater on the planet. Nevertheless, despite being immune to the rapid freezing death that their more temperate fishy counterparts would experience, this special adaptation didn't solve all of the notothenioids' icy problems.
We've recently discovered that, in eating, drinking and living among ice crystals in the frigid Southern Ocean, ice rapidly invades the fishes' bodies. Although antifreeze proteins protect the fishes from quickly freezing solid, over the course of their ten- to fifty-year-long lives in near-freezing seawater, we expect that ice crystals could accumulate internally to the point of harming the fish. Antifreeze protein-bound ice crystals can't grow inside the fish, but they could become so numerous as to turn the body fluids to slush or interfere with vital processes.
Despite the benefits that antifreeze proteins confer to the fishes, the evolution of this adaptive trait thus appears to have come along with a secondary problem: To survive the harsh Antarctic environment these fishes may have also had to evolve methods to deal with the potentially lethal accumulation of internal ice crystals.
In McMurdo Sound, nearly all of the shallow-living fishes possess ice inside their bodies. Yet, under what ocean conditions and how the ice crystals get inside the fishes' bodies in the first place is still unknown. Moreover, no one knows what happens to the ice crystals once they enter the body. In their frigid environment, could ice persist inside the fishes for the duration of their lives? Or, is it broken down by enzymes, for example, or melted when the Sound's waters warm slightly during the brief Antarctic summer?
The capabilities of the MOO allow us to explore this chilling question in greater detail. With its environmental data available in real time during our field research seasons at McMurdo Station, we can now use specialized techniques to count the number of microscopic ice crystals in individual fishes as it changes as a function of the ocean conditions the animals have experienced. As seawater warms in the brief Antarctic summer, we can determine whether the slightly elevated temperatures melt the fishes internal ice crystal load.
Answering these seemingly simple questions is a first step to gaining a better understanding of the extraordinary story of the evolution of antifreeze proteins in Antarctic fishes.
These miniscule ice needles, grown in a sample of antifreeze protein-containing Antarctic fish blood under a microscope, may resemble those that exist naturally inside the fishes' bodies.
Antarctic notothenioid fishes thrive in icy, near-freezing seawater in which normal fishes would freeze solid almost instantly. These fishes, however, risk accumulating environmental ice crystals inside their bodies, with potentially detrimental consequences.
AN UNDER-EXPLORED REALM – Besides contributing directly to our primary research projects, the MOO is providing new insight into the basic ecology and biology of McMurdo Sound. Although our research site, in close proximity to McMurdo Station, is very likely one of the most explored marine habitats in Antarctica, discoveries await the intrepid explorer. Indeed, during a routine under-ice SCUBA dive in 2004, our project leader, Paul Cziko, discovered a never-before-seen species of fish (Cryothenia amphitreta) just a few feet from where the MOO now stands.
In taking daily 360-degree video surveys and hourly still images from more than thirty predefined camera orientation waypoints, the MOO is recording a unique multi-year, high-resolution dataset. This visual record of a remote Antarctic marine environment may prove useful for understanding how the local marine animals' interactions and abundances change over time, who eats who, or how and when various species mate and spawn.
While MOO may not identify another new species at its research site, in this under-explored realm at the ends of the earth, even seemingly mundane occurrences may be new to science.
The MOO watched as a pack of Trematomus fishes devoured a patch of unguarded dragonfish eggs ahered to a flat rock.