Laboratory experiments indicate that many marine creatures could theoretically tolerate temperatures far higher than what they encounter today. But these studies don’t mean that marine animals can maintain their current ranges in warmer oceans, says Curtis Deutsch, a professor of oceanography at the University of Washington.

“Temperature alone does not explain where in the ocean an animal can live,” says Deutsch. “You must consider oxygen: how much is present in the water, how well an organism can take up and utilize it, and how temperature affects these processes.”

Species-specific characteristics, overall oxygen levels, and water temperature combine to determine which parts of the ocean are “breathable” for different ocean-dwelling creatures.
The findings in Nature also provide a warning about climate change: since warmer waters will harbor less oxygen, some stretches of ocean that are breathable today for a given species may not be in the future. “Organisms today are basically living right up to the warmest temperatures possible that will supply them with adequate oxygen for their activity level—so higher temperatures are going to immediately affect their ability to get enough oxygen,” says Deutsch.

Temperature affects both how much oxygen that seawater can hold, and how much oxygen an animal needs to maintain the same level of activity. The already-tight overlap the researchers saw between breathability and current ranges indicate that long-term rises in temperature, as expected under climate change, will likely restrict the ranges of many marine animals.