Effects of Ocean Water pH on Sperm Motility and the Fertilization Kinetics of Oysters

Many people are worried that projected increases in the air's CO₂ content could lead to what is generally referred to as ocean acidification, characterized by pH declines of 0.3 to 0.4 by the year 2100. Most of this concern derives from theoretical considerations that suggest that a pH decline of this magnitude may greatly impede the calcification process in various shellfish and corals; but the concern is widening to also include, in the words of Havenhand and Schlegel (2009), "the very earliest, and critical, process of fertilization."

Working with specimens of the oyster Crassostrea gigas -- which they collected from a mixed mussel/oyster bed on the coast of western Sweden and kept within flow-through tanks of filtered sea water that they maintained at either (1) the normal ambient pH level or (2) a level reduced by about 0.35 units that was created by bubbling CO₂ through the water -- Havenhand and Schlegel observed and measured the species' sperm swimming behavior and fertilization kinetics.

Results indicated that in water of pH 8.15, mean sperm swimming speeds were 92.1±4.8µm/s, while in water of pH 7.8 they were actually slightly higher at 94.3±5.5µm/s, although the difference was not statistically significant. Likewise, mean fertilization success in water of pH 8.15 was 63.4%, while in water of pH 7.8 it was also slightly higher at 64.1%; although this difference, too, was not statistically significant.

Based on these findings, the Swedish scientists state that "the absence of significant overall effects of pH on sperm swimming behavior and fertilization success is remarkable," emphasizing that power analyses they conducted "showed clearly that these results were not due to inadequate statistical power," and adding that "the absence of significant effect is likely a true reflection of the responses of Crassostrea gigas gametes and zygotes from the Swedish west coast to levels of CO₂-induced acidification expected by the end of this century," which finding is very encouraging.