New research suggests larger marine invertebrates are more vulnerable to environmental changes than smaller invertebrates and fish.

As global warming heats the planet's oceans, oxygen levels are declining across a variety of marine ecosystems. Since the middle of the 20th century, scientists estimate marine oxygen levels have declined between 2 and 5 percent.

The changes are already affecting the abundance and distribution of marine species, shifting the makeup of marine food chains. Through a series of lab tests, scientists showed the ill effects of diminished oxygen levels are experienced more intensely by larger invertebrate species.

Researchers exposed amphipod species of different sizes to varying oxygen levels. The performance of larger species decreased more dramatically when oxygen levels declined.

Scientists estimate larger body size requires greater levels of oxygen intake. With less oxygen available, large bodies generally can't function as well. But researchers suggest some larger species likely benefit from oxygen-binding pigments in their body fluids and other adaptive advantages.

While larger species are more vulnerable to change, they may also be better suited to adaptation. Climate change is already testing the adaptability of many marine species.

"Unless they adapt, many larger marine invertebrates will either shrink in size or face extinction, which would have a profoundly negative impact on the ecosystems of which they are a part," John Spicer, professor of marine zoology at the University of Plymouth, said in a news release. "This is obviously a major cause for concern."

Spicer and his colleagues detailed their work this week in the journal Philosophical Transactions of the Royal Society B.

While some species have already adopted adaptations to cope with limited oxygen supplies, authors of the new study argue it would be foolish to expect vital marine species to safely adapt to warmer, acidic and oxygen-deprived waters.

"Marine animals thrive in the Southern Ocean but life in these freezing waters has led to the evolution of many distinct characteristics," said Simon Morley, an ecophysiologist with the British Antarctic Survey. "These 'strategies,' which allow animals to survive in the cold, are expected to make many Antarctic marine invertebrates and fish vulnerable to the impact of climate change. Understanding these impacts will not only help us to predict the fate of marine biodiversity at the poles but will also teach us much about the mechanisms that will determine the survival of species across the world's oceans."

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