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Warmer and More Acidic Seawater Threatens Shellfish



Warming ocean temperatures and rising levels of acidification are harming mussels' ability to cling to hard surfaces, said Emily Carrington of the University of Washington. | Lynchaos/licensed and modified under CC BY-NC-SA 2.0

Rising ocean water temperatures and increasing levels of acidity – two symptoms of climate change – are imperiling sea creatures in unexpected ways: mussels are having trouble clinging to rocks, and the red rock shrimp’s camouflage is being thwarted, according to presenters at the AAAS Pacific Division annual meeting at the University of San Diego in June.

Mussels of the genus Mytilus tether to hard surfaces with collagen-like fibers – called byssal threads – that they create one at a time and continually replenish, said Emily Carrington at the University of Washington. Byssal threads that are too weak to withstand currents and other forces cause mussels to dislodge and fall to the sea floor. Carrington estimates that 20% to 35% of the Mytilus population is lost each year, both in farms and in the wild.

The strength of the byssal threads varies seasonally, Carrington said, with mussels creating significantly weaker threads in late summer when the oceans reach higher temperatures and high levels of acidity – both of which are also on the rise due to climate change.

“Mussels are losing their grip under conditions that we think we’re going to be seeing in the future,” Carrington said.

To understand this, Carrington and her team tested byssal thread strength at different temperature and pH levels.

Carrington found the attachment strength for the species M. trossulus is strong until water acidity levels fall below a pH of about 7.6. (Normal seawater conditions in the locations where Carrington conducted her research are less acidic, with pH levels of 7.8, she said.)

“High pH seawater is necessary for the animals to form their glue, and if we don’t allow them to experience that, they’re going to have a weak attachment,” she said.

Carrington also found that M. trossulus, which is native to the Pacific Northwest, weakens dramatically in water temperatures that exceed 18 degrees Celsius, with its attachment strength “crashing” in waters hotter than 20 degrees Celsius.

A different species, M. galloprovincialis, also known as the Mediterranean mussel, displayed a very different response to higher temperatures, Carrington said.

“It really loves warm water. It makes more byssal threads and makes better byssal threads,” she said.

One species may come to dominate the other off the West Coast if water temperatures shifts take hold. Eighteen degrees Celsius – 64 degrees Fahrenheit – is the “magic number” that separates conditions favoring the native M. trossulus and those favoring the Mediterranean mussel, Carrington said. If the water temperature off the West Coast rises 2 degrees Celsius over the next 100 years as expected, the Mediterranean mussel “takes over most of the West Coast, simply based on its ability to attach well under these warming conditions.”

Mussels play several important roles in their ecosystem, Carrington said. Not only do they provide predators food, mussels regulate water quality and clarity through the filter system that is engaged when they feed, and their colonies provide a habitat for other species. Additionally, the harvesting of mussels for restaurants and grocery stores affects local economies, she said. Shellfish aquaculture contributed $184 million to the economy of Washington state in 2010, according to the National Oceanic and Atmospheric Administration.

Mussel growers “need to know which eggs, literally, to put in their baskets,” Carrington said.

Jennifer Taylor of the Scripps Institution of Oceanography also shared research on how ocean warming and acidification impairs the structure and function of crustaceans.

Some crustaceans have been observed with increased amounts of calcification in their exoskeletons in response to ocean acidification, Taylor said.

“Once you start messing with the calcification of a structure, that can have severe consequences for animals,” Taylor said.

Increased calcification tends to make a structure more brittle and less tough, Taylor said. Crustaceans are particularly vulnerable, she said, because a single structure – their exoskeleton – performs a host of critical functions, including body support, movement, defense, producing sound for communication, capturing and eating food, and camouflage.

Taylor and her colleagues also tested water temperature and pH levels in the laboratory to study the impact of ocean warming and acidification on the exoskeletons of several species of crustacean.

Studying the mineralization of the mantis shrimp’s specialized forelimb for striking prey – its raptorial appendage – revealed a significant increase in magnesium under more acidic conditions. The change, though, did not impact the material properties of the forelimb enough to reduce its hardness or flexibility, Taylor said.

Red rock shrimp, which rely on camouflage as they remove parasites from moray eels, doubled the amount of calcium in the cuticle that makes up its exoskeleton when the pH level was reduced, meaning the seawater was more acidic.

The influx of calcium diminishes the refractive index of the cuticle, making transparency more difficult for the red rock shrimp, she said. Under more acidic conditions, red rock shrimp displayed more than a fivefold decrease in its transparency.

Taylor likened the decrease in transparency to a window turning opaque, “making it harder for all that underlying color to show through.”

[Associated image: Heal the Bay/licensed and modified under CC BY-NC-SA 2.0]


Andrea Korte

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