Rising ocean acid levels skew predator/prey relationship

Normally, black turban snails (top) escape predation by sea stars by crawling out of tide pools(Credit: Gabriel Ng, UC Davis)

The rising tide (no pun intended) of acid in the Earth's oceans could cause a major disruption in the delicate balance of its ecosystem. A new study suggests that changes in the ocean's acidic levels due to atmospheric carbon dioxide could also change the predator-prey relationship of ocean life by tipping the scales in favor of the predator.

The study was conducted by the University of California, Davis at the UC Davis Bodega Marine Laboratory. It examined the behaviour of ocre sea stars (predators) and black turban snails (the sea stars' prey), two common species taken from tide pools at the Marine Reserve, in seawater with varied pH levels. Researchers simulated "16 discrete levels of pH" starting from present acidity levels to the levels they are expected to reach by 2100, according to the study's abstract.

The snails started to show signs of a weakened defense when the pH level in the seawater fell to 7.1 and lower. Black turban snails ordinarily crawl out of the tide pool to avoid being eaten, since ochre sea stars rarely exit the water to capture their prey. The study says that the snails not only spent less time in such "refuge locations," but they also failed to recover their ability to escape from predators in waters with fluctuating acidity levels.

The study does not show why the rise of acid in seawater causes these snails to move slower when responding to a predator, or if the snails could learn to adapt to their surroundings.

An observation based study of the Atlantic Ocean's basin published last year in the journal Proceedings of the National Academy of Sciences of the United States of America (PNAS) found large decreases in pH levels in the South Atlantic Central and North Atlantic Central waters that "trended towards zero in deep and bottoms waters."

Brian Gaylord, a professor of evolution and ecology at the Bodega Marine Laboratory, says that these tide pool experiments could lead to the discovery of future disruptions in other ocean organisms' predator-prey relationships due to rising acidity levels.

"Dozens of West Coast species display escape responses to sea stars," Gaylord says. "We don't yet know the extent to which ocean acidification could alter these additional predator-prey interactions, but there is clear potential for broader disruption of links within shoreline food webs."

Ocean acidification has lead to mass extinctions of oceanic life in the past. A study conducted by the University of Edinburgh found that more than 90 percent of marine life and two-thirds of land animals died off more than 252 million years ago due to a rise in ocean acidity levels over a 60,000 year period. Data taken from rock samples collected in the United Arab Emirates region of the Middle East showed that eruptions from the large volcanic rock region known as the Siberian Traps poured carbon dioxide into the atmosphere and led to ocean acidification that lasted around 10,000 years.

The study was published in the journal Proceedings of the Royal Society B.

Source: UC Davis

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