Thursday, May 13, 1999 Published at 16:59 GMT 17:59 UK
Oxygen boosts polar giants
Pallasea cancellus is an opportunistic predator
Some animals living near the poles become super-sized monsters compared to their lower-latitude cousins and now scientists know why - they have more oxygen to breathe.
Antarctic sea spiders, for example, can be over 30 centimetres (12 inches) long and are up to 1,000 times heavier than those living off Europe.
But Dr Gauthier Chappelle, at the Royal Belgian Institute of Natural Sciences, and Professor Lloyd Peck, at the British Antarctic Survey, studied 1,853 species of amphipod crustaceans to prove this wrong.
They had 12 study sites around the globe, from Lake Baikal, Russia to Madagascar to Antarctica. The only factor they could find to explain the variation in the sizes of the crustaceans was the amount of oxygen in the water.
"The physiological limit is the amount of oxygen they can get into their blood," said Professor Peck.
Better blood, bigger body
If more oxygen is available to be absorbed into an animal's blood, then that blood can travel further around a body delivering the oxygen. Therefore, an animal can support a larger body.
A clinching piece of evidence was that the biggest creatures of all were found in Lake Baikal. Its temperature is six Celsius, much warmer than the zero Celsius of the Antarctic oceans.
The explanation is that Lake Baikal contains freshwater, meaning it can dissolve more oxygen than salty sea water.
The amount of oxygen sea water can dissolve does depend on temperature, explaining why creatures living nearer the poles tend to be bigger.
Oxygen availability may also explain the giant insect fossils that have been found in Carboniferous era rocks. At that time, 350 million years ago, the Earth's atmosphere contained 30 to 35% oxygen, compared to only 20% today.
"We know that they disappeared from the Earth at about the same time that oxygen levels came down to 20 percent," Professor Peck said.
He believes that global warming could have a similar effect in the near future. Warmer oceans would mean they contained less oxygen.
"Under many of the expected scenarios of global changes over the next 50 to 100 years, the places where you will see extinction of species is likely to be in polar seas," he said.
The research is published in Nature magazine.
Pictures: copyright G. Chapelle, IRSNB