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SEB Bulletin July 2006 - Sink or Swim: coping mechanisms of fish to extreme environments

26-1.

Some fish have evolved remarkable abilities to survive extreme changes in conditions in their environment. Researchers presenting their work at the 2006 SEB Annual Main Meeting in Canterbury described some amazing ways in which certain fish have adapted to extreme conditions such as living in freezing temperatures, coping with high acidity or even managing for months without oxygen.

Don’t hold your breath

How long can you hold your breath? Whilst maverick magician David Blaine recently managed only seven minutes, the Crucian Carp, a close relative of the humble goldfish, is able to live for months without oxygen. During winter, the pool in which the carp lives can freeze over, reducing the amount of oxygen that dissolves in the water, sometimes shutting off the supply completely.

Professor Göran Nilsson’s group at the University of Oslo have found that this extraordinary fish can change the structure of its gills to avoid becoming anoxic. In addition its blood has a much higher affinity for oxygen than any other vertebrate, and it makes tranquilizers and produces alcohol when oxygen supplies are limited. These mechanisms allow the fish to survive for days or even months without oxygen depending on the temperature, whilst still maintaining physical activity.

“Anoxia related diseases are the major causes of death in the industrialized world. We have here a situation where evolution has solved the problem of anoxic survival millions of years ago, something that medical science has struggled with for decades with limited success”, said Professor Nilsson. The researchers hope that understanding how some animals cope with anoxia might give clues as to how to solve this problem in humans.

Fish on acid

The Pacific Hagfish feeds by gnawing its way into a carcass and staying inside to feed for up to three days. Researchers at the University of British Columbia believe the Hagfish’s gruesome method of feeding may cause the stagnant water inside the carcass to become acidic from the build up of CO2 produced by the fish, which could explain why the fish is able to cope with environmental conditions of up to 7% CO2 (350 × that found in normal air).

Just as cold-blooded animals have an equal body temperature to their surrounding environment, the Hagfish has the same concentration of salt in its blood as the surrounding seawater. This trait previously led scientists to believe that these fish (known as osmoconformers) could only poorly regulate their pH.

“It turns out that Hagfish can not only regulate their acid-base balance, but that they have a greater capacity for rapid pH compensation than any marine or fresh water fish studied to date”, explained Dan Baker who is studying Hagfish as part of his PhD.

The researchers next want to find the mechanisms by which they do this, and if prolonged exposure to high levels of CO2 causes any long term effects.

Cod makes sense out of junk DNA

26-2.Fish such as cod that live in subzero polar waters have evolved to avoid freezing to death by using antifreeze proteins that work by binding to ice crystals to prevent the crystals growing larger and causing problems. Most of these antifreeze proteins evolve by natural selection from existing proteins when the DNA coding for them duplicates itself and changes over time to give new functions. However, Professor Christina Cheng’s group from the University of Illinois has found the gene for the cod antifreeze protein has come from a non-coding region of their DNA sometimes referred to as “junk DNA”.

26-3.“This appears to be a new mechanism for the evolution of a gene from non-coding DNA”, said Professor Cheng, “3.5 billion years of evolution of life has produced many coding genes and conventional thinking assumes that new genes must come from pre-existing ones because the probability of a random stretch of DNA somehow becoming a functional gene is very low if not nil. This cod antifreeze gene might be an exception to this because it consists of a short repetitive sequence that only needs to be duplicated four times to give a fully functioning protein”.

Since the creation of these antifreeze proteins is directly driven by polar glaciation, by studying their evolutionary history the scientists hope to pinpoint the time of onset of freezing conditions in the polar and subpolar seas.

Vicky Just
SEB press officer

  • References
    DeVries, A.L. and Cheng, C.-H.C. (2005) Antifreeze proteins in polar fishes. In Fish Physiology, Vol.22. (Eds. A.P. Farrell and J.F. Steffensen), Academic Press, San Diego (in press).
  • Cheng’s website: http://www.life.illinois.edu/ccheng/
  • Brauner’s website: http://www.zoology.ubc.ca/%7Ebrauner
  • Sollid and Nilsson (2006) Plasticity of respiratory structures – Adaptive remodelling of fish gills induced by ambient oxygen and temperature. Respiratory Physiology and Neurobiology (in Press – available online at ScienceDirect.)

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