The gender mutability of many fish in response to changing population density and other environmental factors is well known. It is one in the bag of tricks evolved to succeed in an environment that can change rapidly.
A new study documents that low levels of dissolved oxygen can change embryonic fish from female to male, radically altering sex ratios of the population. If widespread enough this could easily hasten species' decline. These conditions exist, sometimes intermittently, in the oceans' "dead zones," which human activity is enlarging and multiplying.
Dive.
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