Hypoxia at Work

Sad but interesting story. There's a tie-in here for a possible bioevolutionary hypothesis, which I'll approach, as always from a layman's perspective, near the end.
Lack of Oxygen Can Mean More Male Fish

WASHINGTON - Dead zones — oxygen-starved patches of ocean — may be turning normal breeding grounds into the equivalent of male-dominated locker rooms for fish.

In lab experiments, newly born male zebrafish outnumber females 3-to-1 when oxygen is reduced. And the precious few females have testosterone levels about twice as high as normal, according to a scientific study released Wednesday.

Scientists are concerned that might reflect life in the dead zones, too.
And of course this is caused by human byproducts. According to the article, and other stories I've seen on topic, Nitrogen from fertilizers is one of the primary culprits, so it's not even as if we were talking about super high tech or paper mill bleaches or power plants, though all of those things have their own catastrophic side-effects* **when contributed in such large quantities.

The world's dead zones add up to about 100,000 square miles and most of those zones are man-made because of fertilizer and other farm run-off, said Robert Diaz, a professor of marine sciences at the College of William and Mary. More than 30 dead zones are in U.S. waters and are part of key fisheries.

The stress of hypoxia — the lack of oxygen in water — tinkers with the genes that help make male and female sex hormones, said study lead author Rudolf Wu, director of the Centre for Coastal Pollution and Conservation at the City University of Hong Kong. Wu's peer-reviewed study will appear in the May issue of the journal Environmental Science and Technology.

Wu restricted the oxygen of zebrafish, which are freshwater aquarium fish, but said similar changes are possible in other species of fresh and saltwater fish. Fish often change genders during their lives, but this is different, he said.

"Since development of sex organs is modulated by sex hormones, hypoxia may therefore affect sex determination and development," Wu wrote in an e-mail interview. "Hypoxia covers a very large area worldwide, many areas and species may be affected in a similar way."

But I got another wonder aloud.

If hypoxia promotes testosterone, did hyperoxygenation first promote sexual reproduction by stimulating the synthesis and mutation of female hormones? Maybe stupid but brutal male hormones are how the biological practicum of sex began on Earth.

What's the geological evidence for peak O2 concentrations in the atmostphere or the oceans? What are the earliest known fossils suggestive of sexual behavior? You could probably study it in a lab if you figure out what bacterium and/or other life were around at the time sexual characteristics start occurring, and if O2 levels were extraordinally high at the time.
In the Gulf of Mexico, sexual development problems have been found with shrimp and croakers, said Nancy Rabalais, executive director of the Louisiana Universities Marine Consortium.

The trend is worrisome, said Peter Thomas, professor of marine sciences at the University of Texas.

"Hypoxia is emerging as a really important stressor, possibly of even greater significance than chemicals," Thomas said. "When it does act, it shuts things down completely."

I really wanna know which came first; the chicken or the egg. It's sounding like 'twas the cock.

* Sorry, but a sub to SciAm Digital is needed. Either that or the rag itself, which is how I know that the story actually covers such effluents. The **expletive**ty thing is that you don't get one with the other. Capitalist scientists.. .

** Hhhmmm... Actually that is NOT the correct story. I'll have to check the mag when I get home today. Updated it shall be. (1:49PM 03-30-06)


  1. Hi, Michael,

    That's interesting, especially in light of the various reports of human-caused chemicals feminizing other species.

    Your analysis of it seems a little off though: If females dominated the period before oxygen became common (and I think it's generally accepted that female is the template sex, but do algae have gender?), why would low-ox circumstances breed males now?


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