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The Impact of Ocean Acidification on Blue King Crab

Paper Reviewed
Long, W.C.,Van Sant, S.B., Swiney, K.M. and Foy, R.J. 2017. Survival, growth, and morphology of blue king crabs: effect of ocean acidification decreases with exposure time. ICES Journal of Marine Science 74: 1033-1041.

Writing as background for their work, Long et al. (2017) note that the blue king crab (Paralithodes platypus) is an important and commercially valuable species in the Bering Sea, yet they say that "nothing is known about blue king crab sensitivity to ocean acidification." Therefore, they made it their goal to learn how projected changes in future seawater pH might affect blue king crab populations; and to do so, they performed a year-long laboratory study to investigate the effects of ocean acidification on the growth, morphology and survival of this particular species.

The experiment was conducted at the Alaska Fisheries Science Center's Kodiak Laboratory in Kodiak, AK. There, juvenile blue king crabs were exposed to one of three seawater pH treatments corresponding to present (pH of 8.1) and future projections of ocean acidification (pH of 7.8 and 7.5). Each crab was checked daily for moulting or mortality and morphometric measurements of various growth-related parameters that were made over the course of the one-year study.

Results of the experiment revealed that overall mortality was higher for crabs reared in the lowest pH treatment (pH of 7.5), whereas the middle pH treatment (7.8) was not significantly different from ambient seawater conditions (pH of 8.1). However, as stated by Long et al., "the overall mortality is not the whole story."

According to the four researchers, nearly all of the observed mortality differences can be explained by a high initial rate of mortality in the pH 7.5 treatment. Subsequent to this initial die-off, the mortality rate in the pH 7.5 treatment "declined quickly over the first couple of months such that, by the end of the experiment, it was similar to or even lower than the rate in the other treatments." Indeed, they add that "there was not a single mortality in the pH 7.5 treatment after day 135 through the end of the experiment on day 356," which was a feat unmatched in the other two treatments.

Commenting on this mortality rate decline in the lowest pH treatment, Long et al. suggest it to likely be indicative of "phenotypic variability or plasticity," which enabled the juvenile organisms to acclimate to ocean acidification. And if this is indeed the case, they opine that "phenotypic plasticity (altering physiology to adapt to different conditions) or phenolic variability could substantially reduce or eliminate any potential large-scale population effects" of ocean acidification on blue king crab. Furthermore, they add that "because the pHs at which blue king crab is affected will not be reached for many decades, there is ample time for evolutionary adaptation to work on phenotypic variability in the population." Thus, the researchers ultimately conclude that "blue king crab may prove resilient to changes in ocean carbonate chemistry over the next two centuries."

Posted 3 August 2017