How does rising atmospheric CO2 affect marine organisms?

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Ocean Acidification: Separating the Winners from the Losers
Schlegel, P., Havenhand, J.N, Gillings, M.R. and Williamson, J.E. 2012. Individual variability in reproductive success determines winners and losers under ocean acidification: A case study with sea urchins. PLOS ONE 7: e53118.

The authors write that "environmental factors directly affect populations by selecting resilient individuals," noting that "selection at the gametic level, or during early life, has strong and immediate effects at the population level, carrying over into subsequent life stages," such that "heritability of this resilience leads to cascading adaptive effects in subsequent generations." And as an example of this process, they report that "in free-spawning marine organisms, sperm selection during fertilization plays a key role by determining the nature and diversity of genotypes in the subsequent generation (Levitan, 1996, 2008) and thus their resilience to environmental change."

What was done
Schlegel et al. investigated "the effects of CO2-induced ocean acidification on the early life history stages in the Australasian sea urchin Heliocidaris erythrogramma, focusing on intra-specific variation in responses, which can be highly variable for this species (Evans and Marshall, 2005)." More specifically, and "following the A1FI-scenario from the IPCC's 4th assessment report," they "compared the effects of present day conditions for southeast Australia with the end-of-century scenario (pCO2=970 ppm; pH=0.3 unit reduction) and a high-CO2 scenario (pCO2=1600 ppm; pH=0.5 unit reduction)," after which the "observed effects on sperm swimming behavior were applied within an established fertilization kinetics modeling framework (Vogel et al., 1982; Styan et al., 2008) to predict fertilization outcomes of single urchin pairs at each pCO2 level." Last of all, these results "were then compared to observed results from fertilization experiments conducted in the laboratory."

What was learned
The four researchers report that "acidification significantly decreased the proportion of motile sperm but had no effect on sperm swimming speed," and they go on to say that the subsequent fertilization experiments "showed strong inter-individual variation in responses to ocean acidification, ranging from a 44% decrease to a 14% increase in fertilization success."

What it means
Schlegel et al. opine that their results suggest that (1) "some individuals will exhibit enhanced fertilization success in acidified oceans, supporting the concept of 'winners' and 'losers' of climate change at an individual level." And they say that if these differences are heritable, it is likely that (2) "ocean acidification will lead to selection against susceptible phenotypes as well as to rapid fixation of alleles that allow reproduction under more acidic conditions," which phenomena "may ameliorate the biotic effects of climate change if taxa have sufficient extant genetic variation upon which selection can act."

Evans, J.P. and Marshall, D.J. 2005. Male-by-female interactions influence fertilization success and mediate the benefits of polyandry in the sea urchin Heliocidaris erythrogramma. Evolution 59: 106-112.

Levitan, D.R. 1996. Effects of gamete traits on fertilization in the sea and the evolution of sexual dimorphism. Nature 382: 153-155.

Levitan, D.R. 2008. Gamete traits influence the variance in reproductive success, the intensity of sexual selection, and the outcome of sexual conflict among congeneric sea urchins. Evolution 62: 1305-1316.

Styan, C.A., Kupriyanova, E. and Havenhand, J.N. 2008. Barriers to cross-fertilization between populations of a widely dispersed polychaete species are unlikely to have arisen through gametic compatibility arms-races. Evolution 62: 3041-3055.

Vogel, H., Czihak, G., Chang, P. and Wolf, W. 1982. Fertilization kinetics of sea urchin eggs. Mathematical Biosciences 58: 189-216.

Reviewed 24 July 2013