Oxygen-starved Atlantic croakers change sexes
By Ker Than, for National Geographic News
A low-oxygen “dead zone” in the Gulf of Mexico is causing sexual deformities in fish, a new study says.
The Gulf dead zone occurs when agricultural and waste runoff from the Mississippi River spark blooms of algae and microbes. These organisms gobble up oxygen, starving other marine life and creating huge swaths of “dead” ocean.
Between 2006 and 2007, nearly a quarter of female Atlantic croaker fish caught in the northern Gulf’s dead zone had developed deformed, testes-like organs instead of ovaries.
It’s unclear how long the fish were living in hypoxic—or low oxygen—waters before they began developing such sexual defects. But lab experiments showed that ten weeks of exposure is all that’s needed.
The Gulf dead zone, which occurs annually, generally persists between May and September, and has more than doubled since the 1980s.
This zone, which often fluctuates in size, currently occupies a patch of ocean larger than the state of Connecticut.
Low Oxygen Screws Up Fish Hormones
Lab analysis of the fish revealed that the masculinized female croakers had decreased levels of a key chemical found in the brain and ovaries called aromatase.
This enzyme regulates the production of the female sex hormone estrogen, which is critical for proper development of the ovaries.
The brain uses about 20 percent of the oxygen that the croakers breathe, said study co-author M.S. Rahman, a marine biologist at the University of Texas in Austin’s Marine Science Institute.
"If the oxygen levels go down, it affects the brain and the neurohormones and neuropeptides that it produces."
In croakers and many other fish species, the sex organs are male by default—estrogen exposure is required to transform the testes into ovaries.
Rahman and colleague Peter Thomas, also at the University of Texas, think that when the croaker’s estrogen levels were reduced as a result of hypoxia, some of the cells in the animals’ ovaries reverted back to testicular tissue.
The sex organs of the masculinized female fish were smaller and less developed than normal male testes. While some of malformed organs even contained sperm, they were incapable of fertilizing normal female eggs, Rahman said.
The study also found that male croakers were affected by hypoxia, although to a lesser degree. Males caught in the Gulf dead zone, as well as those bred in hypoxic lab conditions, had smaller than average testes and lower sperm counts, according to the study, published online recently in the journal Proceedings of the Royal Society B.
Sexual impairments of both male and female croakers help explain the low hatching rates among fish exposed to dead zones, the scientists added.
While normal croaker hatching rates vary between about 40 to 80 percent, the hatching rate of the oxygen-starved fish was as low as 10 percent. What’s more, affected females produced 1.5 times more male offspring than females.
(Source: National Geographic)