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A Brachiopod's Twelve-Decade Response to Ocean Acidification and Warming

Paper Reviewed
Cross, E.L., Harper, E.M. and Peck, L.S. 2018. A 120-year record of resilience to environmental change in brachiopods. Global Change Biology 24: 2262-2271.

Writing as background for their work, Cross et al. (2018) point out important shortcomings of present-day ocean acidification experiments, noting that they "can still only predict responses from exposures of relative short durations, of months or even a few years, to environmentally unrealistic conditions." What is more, such short-term studies fail to account for what Cross et al. refer to as "the fundamental role played by seasonal phenotypic plasticity and genetic change across generations," thus ignoring the "acclimation and/or adaptation potential in organisms with short generation times." And without adequate knowledge of these processes "identified as most important to confer resistance," predictions of marine organism responses to the perceived threats of ocean acidification and warming must be recognized for what they are -- half-baked estimations of a future that may well have little resemblance of reality.

So how can such estimates be improved? How can science incorporate the key influences of organism acclimation and/or adaptation into ocean acidification studies?

Cross et al. provide a novel approach. In their paper they "evaluated a unique specimen collection of a single species of brachiopod (Calloria inconspicua) collected every decade from 1900 to 2014 from one sampling site." In total, 15 specimens were hand-collected live from the sampling site (Paterson Inlet, Stewart Island, New Zealand) in 2014, while an additional 389 specimens were gathered from various research institutions and museums throughout New Zealand. The evaluation of the collection included the authors' analysis of eight key shell characteristics, including characteristics related to shell morphology (calcification), shell structure (shell density, punctal density and punctal width) and shell integrity (dissolution and total shell thickness). In addition, they also examined shell elemental composition by measuring the concentration of various minerals.

And what did their study reveal?

According to Cross et al., "the resilience of C. inconspicua to environmental change over the last century is clear from the data on various shell characteristics," where "six key aspects of the shells of this high calcium carbonate content species did not change since 1900 through to 2014 despite significant environmental shifts of a 0.6°C sea surface temperature increase over the last 60 years and 35.7 µatm increase in surface seawater pCO2 over the last 20 years." The two variables that did change were shell density and punctal width.

With respect to shell density, it increased by 3.43% over the period of study. Regarding this increase, the authors say it "cannot be explained by a change in shell morphology, elemental composition, shell thickness or the number of punctae, as none of these shell characteristics varied over this period." And that fact, in their words, "demonstrates the robust control of several aspects of shell production in C. inconspicua to changing environmental conditions."

With respect to punctal width, Cross et al. report that it declined by 8.26% over the period of study, which observation, they say, demonstrates "that this species appears to have laid down more shell by constructing narrower punctae," adding that producing narrower punctae is "likely a response to acidification by increasing calcification."

Other important findings included that (1) "the majority of shell surfaces (>55% in each decade) of specimens throughout the 120-year study were intact with the protective periostracum layer undamaged and with the outer pitted layer present," (2) "only minimal shell dissolution (0-13%) occurred in any specimen, and this did not vary throughout the time series," and (3) shell elemental composition for the main five components (Ca, Mg, Na, Sr and P) also did not change over the last 120 years."

In light of all their findings, Cross et al. conclude that "these rhynchonelliform brachiopods have therefore been unaffected in their abilities to construct and maintain their extensive skeletons by the change in ocean acidity and temperature over the last 120 years." And this is a noteworthy conclusion, given that C. inconspicua is one of the most calcium-carbonate-dependent species globally, and is therefore presumed to be highly susceptible to ocean acidification. It would thus appear that proper incorporation of species' adaptation and/or acclimation potentials is essential if scientists are to get predictions of the impacts of ocean acidification on marine life correct.

Posted 29 November 2018