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Sassy Scallops: How will shellfish fare as ocean conditions shift?

Woods Hole Oceanographic Institution’s Meredith White researches how ocean acidification affects the larval stage of commercial scallops

By Elizabeth Halliday

Months of experiments at the Woods Hole Oceanographic Institution gave Meredith White ample time to observe her laboratory subjects: scallops. And perhaps for them to observe her, too. Resting in their trays, drawing water across their gills for food and oxygen, the scallops reveal rows of shiny eyes ringing the edge of their shells.

“You just feel like they’re watching you,” White said. “And they do react to shadows and movement.”

In her research as a graduate student in the MIT-WHOI Joint Program in Oceanography, she got well acquainted with scallops’ idiosyncrasies.

“Scallops have the most personality of any bivalve,” she said.

But personality won’t carry the day as scallops confront a precarious future. The oceans are becoming more acidic, threatening shellfish’s ability to build shells.

Through funding in part from Woods Hole Sea Grant, White took a close look at Argopecten irradians, the commercially valuable bay scallop. In particular, she focused on how changing ocean conditions might affect scallops during their sensitive, crucial larval stages.

Since the Industrial Revolution, humans have pumped huge amounts of greenhouse gases into the atmosphere, and the ocean has soaked up 30 percent of the excess.

Carbon dioxide reacts with water to form carbonic acid and bicarbonate. That not only makes seawater more acidic, it makes carbonate less readily available for corals, shellfish, and some plankton to build their calcium-carbonate shells or skeletons. Plants and animals that have evolved in an ocean where the pH hadn’t changed for millennia may not readily adapt to rapid shifts.

Average world ocean pH has dropped from 8.2 to 8.1 over recent decades and is expected to drop to 7.8 by the end of the century. Some coastal waters, however, already surpass the acidity prognosis for 2100 because of a double-whammy of human impacts: excessive atmospheric carbon dioxide and excessive nutrients entering the ocean from fertilizer, sewage, and septic tank runoff.

The nutrients fertilize blooms of algae, which die and provide a banquet for bacteria. These decomposing organisms crank up their metabolism the same way we do, gobbling up oxygen and releasing carbon dioxide, which acidifies waters to a pH as low as 7.4 in summer months. These summer pH dips coincide with scallop spawning season.

Twelve to 24 hours after fertilization, the larvae begin the critical task of using calcium and carbonate to calcify a shell. Importantly, this task is fueled by the energy reserves put into the egg by the parent scallop; the larvae can’t eat, because they have yet to develop mouths.

In her research, White isolated spawning scallops in deli-style plastic containers to capture the eggs and sperm and carefully raised water temperatures. She initiated their fertilization in an aquarium environment in which she could monitor and control carbon dioxide levels and acidity.

For seven days, White grew a subset of larvae in only ambient seawater; a subset spent their first 12 hours in ambient seawater and then were subjected to more acidic seawater with a pH of 7.4; another subset of larvae grew only in acidified water; and a final subset spent their first 12 hours in acidified seawater and then were switched to ambient water. 

White measured the survival rate and shell size in larvae to test whether larvae spending their first days of life in normal seawater fared differently than larvae spending their first days in seawater acidified with high carbon dioxide. She also wanted to learn if any impacts are reversible, since larvae in coastal waters can be swept between polluted, acidic coastal areas and open ocean areas within spans of days or even hours.

Across the board, “any exposure to the acidified seawater consistently decreased survival,” White said. Among the larvae that survived in all experiments, she found striking differences in their shells. She targeted her investigations on the critical 12-hour window in which the larvae initiate shell calcification.

Larvae raised in ambient seawater, as well as larvae that had been switched from acidic to ambient seawater before calcification began, had bigger shells than larvae raised in the acidified seawater or switched into acidified water before calcification began. Even though all the larvae subsequently grew at similar rates, the ones with initially stunted shells never fully compensated for those undersized shells later on, she found.

White, who received her doctorate last year, published her findings with Woods Hole Sea Grant-funded researchers Dan McCorkle, Lauren Mullineaux and Anne Cohen, in PLOS ONE.

A version of this article previously appeared in Oceanus magazine, a publication of the Woods Hole Oceanographic Institution.

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