The Long Voyage of the Antarctic Bullhead Notothen Fish

2017.08.14 11:40:25 | 2017.08.14 11:41:35

Professor Arthur L Devries [Photo from University of Illinois]이미지 확대

Professor Arthur L Devries [Photo from University of Illinois]

A unique group of fishes inhabits the Antarctic Ocean whose waters are at their freezing point (-1.9O C) for much of the year. These fishes are known by the unwieldly handle, notothenioids (classification sub order) or the shortened form notothens. Within this group there are approximately 120 species and they make up approximately 90 percent of the fish biomass in the Antarctic Ocean. These fishes are interesting from an evolutionary point of view because they evolved from a bottom dwelling fish lacking a swim bladder. This hypothetical ancestor gave rise to a variety of closely related species which differ in size, shape, color and occupy distinct ecological niches in the ocean.

Studies of this group of fishes is interesting and important because when the Antarctic Ocean cooled to the freezing point of sea water about 20-30 million years ago the existing fish fauna became extinct except for a sculpin like bottom fish which gave rise to a closely related diverse fauna in terms of appearances. There are five Antarctic fish families in this suborder and these closely related species are what is referred to as a marine species flock, much like the African fresh water cichlids. From this single progenitor the notothens arose and evolved into diverse body morphs and most importantly they evolved to exploit the empty ecological niches vacated by the extinction of the prior temperate fish fauna. Some look like the common near shore bottom dwelling sculpin of the northern oceans while others live in the water column and look similar to a smelt or herring. Most are bottom fishes, and a few even have a "fishing barbell" attached to their lower lip like some unrelated deep sea fishes, while others have adapted to inhabit the water column despite lacking a swim bladder.

In comparative studies of the notothens biology (physiology, biochemistry, morphology and life histories) one does not have to contend with traits that are largely due to phylogenetic differences because the notothens all arose from a common ancestor. Such a model system makes it easier to identify adaptive traits that arose from selection in response to environmental factors in their particular habitat. One example is the evolution of traits that resulted in neutral buoyancy, without a swim bladder, for efficient swimming in the water column. These mid-water species have become neutrally buoyant by accumulation of lipid in sacs beneath their skin or their muscle mass; lipid is less dense than sea water. They have also reduced the mineral content of their boney skeletons so that their skeletons are essentially cartilage which is lighter than bone. Another example within the notothens is that some have lost their red blood cells and thus hemoglobin needed for oxygen transport from the water that baths their gills to the tissues where it is required. They can survive because the freezing water is saturated with oxygen and the fish`s metabolism is very low compared to temperate fishes. Because of the absence of red blood they are translucent and have often been called "ice fish". Studies of the origin of neutral buoyancy and mechanism employed for survival in the hemoglobin less ice fishes are important because they provide insights into how selective forces in nature lead to adaptive traits.

One of the key adaptations that allowed survival in the freezing Antarctic waters was the evolution of an antifreeze protein. All notothens at one time or another during their life cycle are exposed to freezing seawater laden with ice crystals. Because they only have about the one third the amount of salt in their blood as present in sea water, and thus they would face freezing and death when exposed to freezing sea water. Therefore they require a protective antifreeze protein to lower the freezing point of their body fluids below that of seawater. All the Antarctic notothens have antifreeze proteins circulating in their blood and those living in the year round freezing habitats such as McMurdo Sound have higher levels than those of the Antarctic Peninsula where temperatures are a few degrees (0 to+1 OC) above the freezing point of seawater (-1.9 O C) during the summer. Studies of the origin of the antifreeze protein is important because it provides insights into the adaptive processes of evolution. The selective force for the evolution of the antifreeze proteins was very strong because if the fish lacked it, freezing and death ensued and thus it would be eliminated from the gene pool. Understanding of the mechanism of how the fish utilizes the antifreeze proteins to avoid freezing is also important because it may be possible to insert antifreeze genes into cold water fishes and allow them to be raised in aquaculture pens in northern waters where occasional freezing occurs in the surface waters which would cause the entire populations to freeze to death with the loss of thousands of dollars for the aquaculture industry. Also the adsorption inhibition mechanism of antifreeze activity involves inhibition of ice crystal growth at temperatures where it should grow is not only of academic interest but has also found use in the food industry. It is used to inhibit growth of large ice crystals at the expense of small ice crystals, a process called inhibition of recrystallization. It is used to prevent small ice crystals in ice cream from being transformed into undesirable large crystals which gives ice cream a grainy texture. Ice cream containing fish antifreeze proteins is marketed by Unileiver Corp in Great Britain an apparently a successful product. Because of these interesting adaptations and potential uses the notothens haves become one of the foci of biological research for several nations` polar programs.

Notothenioids [Photo from Korea Polar Research Institute(KOPRI)]이미지 확대

Notothenioids [Photo from Korea Polar Research Institute(KOPRI)]

One notothen fish which is one of the hardiest of all is the fish Notothenia coriiceps and has been a research model for several short term physiological experiments in Antarctica. This fish has been given the common name, the Bullhead notothen because of shape of its head. Many of the notothens are rather delicate and do not survive in captivity, but the bullhead is an exception in that it covered with robust scales that do not easily come off. Thus KOPRI scientists have captured 50 specimens of this hardy fish and have transported them from the South Shetland Islands in the Antarctic Peninsula to their laboratory aquarium in KOPRI. This fish although hardy does require cold water to survive and will begin to die off at +6 OC if kept at that temperature for a week. Therefore when they transported them via their ice breaker the ARAON to Korea they were kept in a refrigerated room where the temperature was maintained at 0 OC. Almost all survived the 6 week trip and are now housed in aquaria in a cold room at KOPRI maintained at 0 OC. Researchers from a few other countries have transported Antarctic fishes to their home institutions, but they were a different species and very small (~100 grams) and easily handled. This is the first transport of Antarctic fishes as large as a kilogram that have been transported to the northern hemisphere and in part was possible because the ice breaker was able to dock in Incheon near KOPRI. Such an endeavor in America would not be possible because their ships home port far away from the locations where researchers study Antarctic fishes and the fishes this large would not survive the heat and time involved in air transport.

So what research do KOPRI researchers plan for these fishes? Obviously they would not want to sacrifice them in the short term with all the effort spent to transport them to Korea. One possibility would be to investigate their response to non-lethal elevated temperatures to determine whether they have a robust heat stress responses similar to that observed in temperate fishes. It is possible that they lack a robust heat stress response because they have evolved for millions of year in a constant cold environment and therefore it would appear that they do not need one. If they do have a robust response, it would be most interesting to analyze its genomic bases-that is to see whether protective heat stress proteins become elevated as their genes are turned on and other non-stress genes are momentarily turned off. How could this be done without sacrificing the fish? The fishes could be exposed to temperatures of 10 OC for an hour, a short enough time which is not lethal, and then returned to their environmental temperature of 0 OC. Then RNA could be isolated from a small blood sample taken from their tail with a small needle and syringe. Such a procedure is tolerated without detrimental effects. Unlike other vertebrates including humans fish red blood cells are packed with DNA and RNA. Analysis of the RNA isolated from the red blood cells that are translated into the heat stress proteins would allow one to determine whether they have a heat stress response even though they evolved in a constant cold environment and one might predict that they would not need one. If a heat stress response is found, it would be important to know the magnitude of it after exposure to temperatures well above their normal habitat temperature in nature. In the big picture such information may tell us something about how this common Antarctic species would respond to climate change with the associated increase in water temperature that is taking place around the Antarctic Peninsula and whether it would survive warming of the ocean above +6 OC. All indications are that water temperatures will continue to increase in the future as the earth`s climate continues to change. Thus studies of this fish`s heat stress response could be a model system to investigate whether the Antarctic notothens as a group could survive ocean warming above their present low lethal temperatures of +6o C, the lowest of all fishes in the world. Loss of the Antarctic notothens would have a large effect on the Antarctic marine ecosystem because they are prey for many birds and marine mammals that inhabit the Antarctic Ocean.

Another possible research project that could be carried out without sacrificing the fishes is to analyze their antifreeze proteins after long term exposure to temperatures warmer than their habitat temperature. As pointed out the bullhead notothen experiences winter temperatures of -1.9 OC and inhabits the shallow sub-tidal where slush ice is abundant during the winter but is immune to freezing because of the high level of antifreeze glycoproteins circulating in their blood. If they are maintained at temperatures well above freezing (+2OC) will they begin to lose their antifreeze protein because it is no longer required for survival? Again this experiment is possible because collecting a small blood sample from the tail has little effect on the health of the fish. Antifreeze protein concentrations in the blood could be analyzed with a nanoliter freezing point osmometer which accurately determines the temperature at which their blood will freeze in the presence of a small seed crystal. This instrument requires less than a microliter to accurately determine the freezing point. If no change in the freezing point occurs with warm acclimation then this experiment would indicate that the blood concentration of antifreeze protein is genetically fixed-that is it is maintained at a constant high level despite living at temperatures where it is no longer required for survival. A genetically fixed expression of antifreeze protein would be in contrast to what is observed in some northern hemisphere fishes where antifreeze protein is expressed at high levels during the winter but is shut down during the summer when it is not needed. Investigations of the genomes may shed light on the control points that are involved in "turning on" antifreeze protein synthesis as well as turning it off. Presently there are over 1000 references concerning antifreeze proteins attesting to the interest and importance as to their role in survival in polar and cold water fishes. As well there are about 30 patents that have issued indicating the potential commercial exploitation of antifreeze proteins.

Finally little is known about reproduction in the notothens. It would be most interesting if this species could be induced to spawn and obtain fertilized eggs. The time from fertilization to hatching could be documented which is not known and most interestingly whether they hatch with an adult level of circulating antifreeze protein or if it is synthesized only after hatching as has been shown in another Antarctic notothen from McMurdo Sound.

These are some of the interesting experiments that can be done with these unique fishes without causing their death. Since some of these experiments require long periods of acclimation they cannot be done in Antarctica where often the experimental field season is too brief to carry them out or the facilities are limited. With an aquarium where temperature can be precisely controlled such experiments can be carried out at KOPRI. They would provide results that would be of wide spread interest not only to Antarctic fish biologists but to all fish biologists.

By Arthur L Devries, Professor, The School of Molecular and Cellular Biology, University of Illinois

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