The Hidden Consequences of Ocean Acidification: How Low pH Could Shift Oyster Sex Ratios

Sex determination is controlled by a set of diverse regulatory mechanisms, but it can also be affected by the environment. For example, as some of us may remember from high school biology, the incubation temperature of a crocodile egg is the key environmental factor for determining the sex of the hatchlings. But with global climate change, environmental sex determination in many species risks disruption—and not just from changing temperatures. As carbon dioxide levels in the atmosphere rise, the oceans absorb much of this excess CO₂, leading to a drop in pH commonly referred to as ocean acidification. The National Oceanic and Atmospheric Administration estimates that human activity over the past 200 years has resulted in a 30% increase in acidity in our oceans.¹

Mollusks are the second largest animal group after arthropods. Interestingly, across the phylum they show a wide variety in their methods of sex determination and forms of sexual reproduction, indulging in simultaneous and sequential hermaphroditism, as well as unisex males or females.² Unlike mammals or birds, whose sex is determined at the point of fertilization, mollusks do not have characterized sex chromosomes. Oysters are especially interesting as all three sexual reproduction routes can coexist within the same species,³ making them ideal candidates for scientists investigating how the mechanisms of sex determination might be responding to environmental changes.

An international research team publishing in Environmental Science & Technology has done just that, looking specifically at how low ocean pH driven by high CO₂ levels is affecting sexual reproductive patterns in oysters.⁴ To do this, they subjected oyster larvae to controlled low-pH environments, testing them against a neutral pH control group. The researchers found that exposure to lower pH conditions during early development resulted in a shift toward female-biased populations. Even more striking, this feminization effect persisted across two generations, meaning offspring continued to exhibit skewed sex ratios even when raised in normal conditions. This pH-mediated sex determination (PSD) appeared to have no consequences for fecundity, gonadal development, or reproductive function in the offspring. Transcriptomic analysis indicated that the PSD could be linked to the inhibition of spermiogenesis functions in males, and the activation of the Wnt pathway—a signaling transduction pathway that is critical for embryonic and organ development—in females.

The authors note that this response to pH could be an adaptive strategy to cope with extreme events that occur in coastal zones. This is potentially because exposure to low pH negatively impacts larval fitness, since evidence suggests more acidic seawater corrodes young oyster shells before they can properly form.⁵ It may therefore be that increasing the number of fertile females is an evolutionary strategy to compensate for increased larval mortality and thereby maintain the population size.^6-7

Other groups have looked at how pollution affects oysters, finding that animals sampled at contaminated sites show delayed gametogenesis and shorter spawning periods.⁸ Here too, the proportion of females increased significantly among contaminated oysters, similar to the findings seen for oysters in low pH environments. There are temporal trends across the year, but as an example, the proportion of females in May was 90.5% at the contaminated site compared to 63.7% at reference site—an increase of 42%. Interestingly, previous reports have suggested increased expression of estrogen-regulated genes induced by cobalt, copper, nickel, lead, mercury, and chromium.⁹ It follows then that feminization of oysters might be due to an endocrine-disrupting effect of metal pollutants, although this may be species-dependent.

This research goes beyond biologic interest. Oysters are an important fishery and aquaculture species, with more than 5.5 million tons produced annually, providing high-quality protein and an industry valued at over $1.5 billion.¹¹ With the ocean's future at stake, continued research into phenomena like PSD and how human activities are affecting marine populations will be vital for guiding policy, conservation efforts, and sustainable aquaculture.

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