Over the last decade, the effects of global climate change have become the subject of hot debate. And while the politicians have difficulty agreeing how to deal with the threat, ecologists regularly record species shifting territory as their environment alters. The effect of temperature fluctuations on animal physiology also intrigues Hans-Otto Pörtner. In recent years,Pörtner has investigated the effects of extreme temperature and found that water-breathing vertebrates experience difficulties, resorting to anaerobic metabolism, as their oxygen supply becomes limited at both high and low temperatures. But how widespread is this problem? Pörtner and his colleagues Frank Melzner and physicist Christian Bock decided to investigate the effects of high and low temperatures on oxygen supply in a sophisticated invertebrate: the cuttlefish, Sepia officinalis(p. 891).
According to Melzner, cuttlefish are the `couch potatoes' of the cephalopod world so were perfectly content to sit in Bock's state-of-the-art MRI system for days at a time, having their metabolism monitored with a finely tuned phosphate probe. Melzner explains that animals that derive energy from anaerobic metabolism regenerate spent ATP supplies viaphospho-L-arginine, liberating inorganic phosphate as ATP is synthesised. Monitoring the cuttelfish's inorganic phosphate levels allowed the team to keep track of the animal's metabolism allowing them to spot the moment when it switched to anaerobic metabolism if oxygen became limited at extreme temperatures.
But cuttlefish mantle muscles also respire anaerobically whenever they set off for a quick jet. How could the team distinguish a bout of activity from an oxygen-limited switch to anaerobic metabolism? By monitoring the pressure surges in the cuttlefish's mantle cavity. Melzner explains that cuttlefish breathe by gently pulsing their mantles, generating pressures around 30 Pa;however, propulsive jets result in massive pressure surges, as great as 10 000 Pa. By measuring the pressure fluctuations in the cuttlefish's mantle cavity,Melzner and Bock were confident they could distinguish between a temperature-induced switch to anaerobic metabolism and a burst of cuttlefish exuberance.
Settling individual cuttlefish in an aquarium surrounded by a powerful MRI magnet, Melzner and Bock began the painstaking task of recording the cephalopod's phosphate levels as the team gently changed the temperature. Melzner remembers that it was a nail-biting experience, keeping the cuttlefish supplied with specially shipped-in clean seawater, while Bock monitored the magnets and water temperature. Measuring phosphate levels as the temperature rose, the team were amazed to see a sudden increase in muscle inorganic phosphate levels; the cuttlefish had switched to anaerobic metabolism at 26.8°C. And as the team dropped the water temperature, they saw the cuttlefish recruit anaerobic metabolism again, at temperatures ranging between 5-8°C. Somehow the cuttlefish's oxygen supply had become limited.
Melzner admits that he was surprised by how clear the switch was when the cuttlefish shifted to anaerobic metabolism. He adds that he is keen to discover the mechanisms that limit the cephalopod's oxygen supply and suspects that they will be different for high and low temperatures.
