2 December, 1998
Okay, now that you've had time to think about adaptations of fish for living in Antarctic waters at below-freezing temperatures let me tell you some of their amazing adaptations. Most Antarctic fish belong to the suborder of bony fish called the Notothenioids which are found almost entirely in Antarctica. So these adaptations are the ones typical of this suborder.
A. WHY DON'T THEY FREEZE?
As I have mentioned earlier there are two major reasons why they don't freeze even though they live at a constant -1.86 degrees C. (Typically they won't even freeze until the temperature is lowered to -2.2 C which never happens in their natural environment.)
1. The salt concentration in their blood is twice that of a normal temperate zone marine fish. The amount of salt and other solutes in these fish lowers their freezing point to about -.7 C. That is still not enough to keep them from freezing in these Antarctic waters, but it helps.
2. The rest of the protection comes from 8 different types of glycoprotein antifreeze molecules. A glycoprotein is a combination of a protein and a sugar. This antifreeze is 300 times more effective than regular car antifreeze but no one knows exactly how it prevents ice crystals from forming in the fish. Possibly the glycoproteins coat the surface of a growing crystal preventing new water molecules from joining the crystal.
Pagothenia borchgrevinki, (also known as borchs), live among the shards of platelet ice that hang from the frozen surface of the sea. If an ice crystal penetrates the skin of a fish it can easily serve as a seed crystal for more ice to grow and quickly freeze the fish. To prevent this, borchs have more antifreeze than any of the other fish that have been studied. These other fish live in the relatively ice free zones at the bottom of the sea or in the water column so they can get by with less antifreeze.
The antifreeze molecules are produced by the liver. They are not poisonous like car antifreeze.
3. Dr. Art DeVries and his students are trying to find out how any ice crystals that do start to form are removed by Notothenioid fishes. Right now they believe that the spleen is removing the ice crystals and somehow destroying them, perhaps by endocytosis, but this has not been proven yet.
B. CONSERVATION OF ANTIFREEZE AND ENERGY
In the typical vertebrate kidney the glomerulus of the nephron acts like a sieve as it separates blood cells and large blood proteins from the wastes and other solutes dissolved in the blood plasma. Then the tubules of the nephron use ATP energy to rebasorb the useful nutrients and water back into the blood. What is left behind becomes urine and is excreted. It costs a lot of energy to pump all these valuable nutrients back into the blood. The glycoproteins produced by the Antarctic fishes are too small to be filtered out of the waste by the glomerulus, so even more energy would have to be spent to absorb these crucial molecules back into the blood. So Antarctic fishes have solved the problem by having a kidney that doesn't have glomeruli. Instead of using ATP energy to rebasorb all those antifreeze molecules and other useful blood solutes like sugar and ions, energy is only used to excrete the wastes out of the blood into the tubules. That's thrifty; that's nifty; that's neat. I wonder why we all don't have this kind of kidney.
I have previously mentioned that the Notothenioid fishes lack the swim-bladder that most fishes have to control their buoyancy. For most of them, like the Trematomus bernacchii which I studied, the swim bladder is unnecessary; since they are bottom dwellers they don't need to float. However some of them, such as the Antarctic Cod (Dissostichus mawsoni), and the silverfish (Pleuragramma antarcticum)are pelagic, swimming freely at middle depths or near the surface. They need to be able to float with minimal effort to conserve energy. In order to be nearly "neutrally buoyant", in other words to weigh almost nothing in the water so they don't sink, they have a number of nifty adaptations to reduce their weight and their density and to increase their buoyancy:
1. a reduced amount of calcium in the skeleton, or having a cartilaginous skeleton instead of a bony one
2. fewer skin scales with less minerals in them 3. an abundance of lipids distributed throughout their body such as in their muscles, liver, belly flaps, and mesentery (connective tissues). Lipids are molecules such as oils, waxes, and fats that are less dense than water (which is why salad oil floats on vinegar in salad dressing). The lipids in the mawsoni (Giant Antarctic Cod) muscle are why the mawsoni I ate at the barbecue a couple weeks ago was so moist and oily.
As a result of these adaptations, a 200 pound D. mawsoni weighs less than a one pound T. bernacchii in the water. In fact, the average mawsoni weighs absolutely nothing in water. That's pretty amazing!
D. HOW CAN THEIR METABOLISM WORK AT SUCH A LOW TEMP?
It is generally true that since there is less molecular motion at low temperatures, chemical reactions--and therefore metabolism--are slowed way down. In fact, for every 10 degree (C) drop in temperature, metabolic rate generally slows down by a factor of about 2.5. You have probably felt the effects of this on a cold windy day outside. Your fingers get numb and clumsy. Your nerves can't send sensory messages to your brain or motor messages to your muscles because their chemical reactions can't happen fast enough.
Well then, how can these fish swim, catch food, digest their food, sense their environment, mate, and do everything else they need to do at temperatures that would bring our metabolism to a standstill? Their metabolism seems to work 2 to 10 times faster than we would expect based on the above formula.
Their molecules are designed to work at these low temperatures. The enzymes of most organisms work best at temperatures of 20-40 C, but the enzymes of Antarctic fishes work best at around 0 C. In fact the enzymes are so well-designed for these temperatures that they start to fall apart when the temperature increases to more than 4 degrees C! (Goldfish--from the temperate zone--can survive a range of at least 40 degrees celsius, but these fish can't survive outside a narrow range of about 6 degrees C.) Apparently the enzymes of Antarctic fishes are more open, allowing their active sites to be more accessible for the chemical reactions they are designed to catalyze. Very little energy is needed to activate these enzymatic reactions. Most enzymes in warmer temperature environments are held together more tightly by internal bonds such as disulfide bonds. But the enzymes of Antarctic fish seem to have less internal binding. This would explain why they fall apart (become denatured) when they jiggle around more with increasing temperature.
Researchers studying the physiology of Notothenioid fishes have found biochemical adaptations for the cold in the nerves, muscles, hemoglobin, and liver. They have found that the nerves can still conduct nerve impulses even if they are cooled to -5 C!
F. SPECIALIZED LIPIDS--LIKE BUTTER OR SQUEEZE PARKAY?
I am sure that you have noticed that when you take the butter out of the refrigerator it gets more and more soft and spreadable as it gets warmer and warmer. Butter is a lipid and it demonstrates what happens to most lipids as they change temperatures. Every cell is surrounded by a double-layer of lipids called phospholipids. It is VERY important that this layer be soft an malleable. Here's why.
Scattered through this bilayer of phospholipids are all kinds of important proteins that control many of the special characteristics of a cell that enable it to do its function. Specialized carrier proteins carry nutrients in; others carry wastes out. Receptor molecules receive messages from other cells to tell the cell what to do. Marker proteins label what kind of cell or membrane it is. In order for most of these proteins to do their jobs they must be able to change their shape. If the lipid surrounding the proteins is hard like refrigerated butter rather than soft like "Squeeze Parkay" the proteins can't change shape and they can't do their jobs. If they can't do their jobs, neither can the cells. If the cells can't do their jobs, neither can the fish!
Sodium-potassium ATPase is one of these carrier proteins. In addition to regulating the fish's salt balance as I described in my journal earlier, it is crucial in the way a nerve functions. If the nerves can't work, the critter becomes "numb". The brain can't sense the environment or control the body's response to the environment. As I mentioned earlier, the nervous system of Antarctic fishes works fine, even as cold as -5 C, thanks to their specialized lipids.
Researchers have found that the types of lipids (unsaturated) found in the membranes of Antarctic fishes cells are still soft and fluid at below-freezing temperatures.
G. FINDING FOOD BELOW THE ICE
It's dark below the ice, especially in the long winter night when the sun doesn't shine for several months. So how do these fish find food in the dimly lit or unlit conditions below the ice? Here are some of the adaptations that have been noticed in various types of Antarctic fishes:
1. They all have large bulging eyes designed for capturing as much light as possible.
2. Mawsoni have lots of rods rather than cones. Rods are the cells in the retina of the eye that are more sensitive to light. They allow for black-and-white night vision. Cones allow for high-resolution color vision, but they require more light. Color vision is not important in the dingy depths of the ice-covered sea so it is better to have more of the cells that can detect low levels of light--the rods.
3. Also in mawsoni, the nerve impulses from several rod cells in the retina are converged together and funnelled onto one brain cell. Although this makes for blurrier vision, it acts as an amplification system, magnifying the ability to detect low light levels. Since mawsoni eat large kinds of prey the blurrier vision probably doesn't affect their ability to catch prey as much as not being able to see the prey at all would.
4. Borchs have extensions of their lateral lines onto their heads and lower jaws, making this essential part of their body sensitive to the water-born vibrations produced by their swimming prey, presumably allowing them to find their prey in the dark.
Hot maple syrup pours much more quickly than maple syrup straight out of the refrigerator. Likewise, blood becomes more viscous (thicker and harder to pump) at lower temperatures. To solve this problem, Antarctic fishes have fewer red blood cells. This reduces the viscosity of the blood making it easier to pump it through the small capillaries.
But red blood cells contain hemoglobin which carries the oxygen that all cells need. How can they survive with fewer red blood cells? First, at colder temperatures cells need less oxygen since they perform less metabolism. Second, there is more oxygen dissolved in freezing cold water than there is in warm water since gases such as oxygen are more soluble in cold water than they are in warm water. (You have probably noticed this when you have seen the bubbles that form in your glass of water when you leave it on your bedside table overnight.When you poured the glass of cold water at night the gas was dissolved in the water. As the water warmed up to room temperature overnight the gas came out of solution and formed bubbles.) Third, fish that are more active (like mawsoni) have more red blood cells than less active fish, enabling the more active fish to get more oxygen to their tissues.
One of the most amazing adaptations of Antarctic fishes is that of the Channichthyids, the icefish. The icefish has saved on the expense of creating hemoglobin and red blood cells by having very few red blood cells and by totally lacking hemoglobin! Instead of having red blood like all other vertebrates, its blood is clear! The icefish can get away with this mostly because of reasons 1 and 2 above. But also it has a larger blood volume (which can carry more oxygen dissolved in its plasma), its heart is larger than other fish so it can pump its less viscous blood faster through its wider blood vessels with less work, its large, well-developed gills can easily extract oxygen from the oxygen-rich water, and it can absorb some oxygen directly into its body through its scaleless skin. AMAZING!
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