Unlike most animals, many fish produce two forms of a gene responsible for the enzyme aromatase, which in turn makes estrogen. One form of aromatase is in the ovaries; the other is in the brain.
The study results suggest that direct chemical disruption of the brain aromatase gene, which directly affects production of brain estrogen, may be a key mechanism behind the endocrine-related disturbance of normal reproduction and development in fish.
The researchers first found that the differential expression of the brain aromatase gene was associated with sex differentiation. "It became clear that compounds that affect this gene will thereby affect sex and sexual behavior in fish," said Trant.
In other words, the researchers found that the endocrine-disrupting compounds can regulate the aromatase gene in the brain, affecting more than a fish's sex.
"What is dangerous is that in-between stuff," said Trant. "You might get males who do not display the correct behavior. In order to mate with a female, he may have to court her, build a nest, chase, or show some dominance. So, even if the concentration of these disrupting compounds in the water are not sufficient to completely reverse their sexual physiology, small adjustments in their behaviors would be equally fruitless."
A growing number of scientists theorize that endocrine-disrupting compounds in the water behave chemically like hormones in fish tissues and cells.
When compounds such as polychlorinated biphenyls, dioxins, certain plasticizers, and some detergent additives are in streams or rivers, groups of fish, birds, frogs, and other animals are sometimes found to be all male, or all female, or are partially both sexes in their genitalia.
Historically, scientists have suspected actual estrogens or chemicals that mimic estrogens in pollution as the causes of the "gender-bending" effects on fish.
Estrogen (or estrogen-like) molecules dock onto a structure called an estrogen receptor in the cells of the liver, ovaries, fat, breast, brain, bone, and many other target tissues in man. The activated receptor initiates a series of changes into action related to sexual physiology. Many of the pollutants, such as PCB's, some pesticides, and petroleum products in the Chesapeake waters, are recognized as estrogen molecules by fish and human cells.
"That's why scientists have focused there," said Trant. "But, this is worse than we thought before. This is not simply toxicology. It is interfering with the reproduction of the adults, and potentially skewing sex ratios of the populations."
The research team reported that the aromatase gene expression in zebrafish was changed by multiple classes of pollutants such as estrogen mimics (or xenoestrogens, such as surfactants in detergents and pharmaceuticals), arylhydrocarbons (PAH's and benzo(a)pyrine), peroxisome proliferators (pharmaceuticals and plasticizers), and herbicides (atrazine).
"For people looking for a magic bullet of why productivity of the Chesapeake Bay is down, this is not it," said Trant. "There are probably many causes. But this is certainly affecting the reproductive health of animals that spawn here and the developmental health of animals that are raised in the Chesapeake Bay."
Scientists have just been looking too narrowly at estrogen mimics, said the researchers. They added that it's almost certain that the multiple compounds are affecting all the fish in the Bay and beyond, not just zebrafish in the laboratory.
Liquid Planet Resources
A. Animals and Nature
Online Jellyfish Forecast Warns Chesapeake Swimmers
South Africa Sardine Migration Draws Crowds
Was this Earth's First Predator?
Was the Humble Sponge Earth's First Animal?
Tiny Mandarin Fish Reveal Surprisingly Complex Mating Ritual
Scientists Mount Assault to Save Endangered Right Whales
Scientists Track Giant Sunfish by Satellite
Right Whales Get Boost from U.S. Navy
National Geographic Animals and Nature Guide: Go >>
Interactive Feature: National Geographic's World of the Crocodilians
Follow the progress of this National Geographic expedition to the Florida Keys:
Expedition Report One: Scientists Study Nurse Shark Mating Habits
Expedition Report Two: Researchers Tag Sharks to Study Breeding Habits
Expedition Report Three: Crittercams Provide Insights into Nurse Shark Behavior
Creature Feature: Great White Sharks
Ten Cool Things That You Didn't Know About Great White Sharks
Print 'N' Go Coloring Book: Great White Sharks
Shark Surfari: Online Quiz
Related Lesson Plans:
Lesson Plan: A Trip to the Beach
Lesson Plan: Are Sharks As Dangerous As We Think They Are?
Lesson Plan: Does the Hammer Help?
Lesson Plan: SharksSetting the Record Straight
Lesson Plan: SharksShould They Be Afraid of Us?
Lesson Plan: What's the Hammer For?
C. Archaeology and Paleontology
JFK's PT-109 Found, U.S. Navy Confirms
New Fossil: Link Between Fish and Land Animals?
Ancient Walking Whales Shed Light on Ancestry of Ocean Giants
New Underwater Finds Raise Questions About Flood Myths
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Scientists Prepare to Open Civil War Sub
Human Remains Found in Civil War Submarine Hunley
Curious Find on Confederate Sub Links North and South
Captain's Remains Found in U.S. Civil War Submarine
Captain's "Lucky Coin" Found in Civil War Submarine
U.S. Civil War Sub "Photo" Disproved as Image of Captain
Forensic Team Studying Skeletons of Hunley Crew
Journals of Captain Cook Go Online
Interactive Feature: Black Sea @ NationalGeographic.com
Maryland Suffers Setback in War on Invasive Walking Fish
Study Calls Into Question Global Quotas on Bluefin Tuna
Expedition Reveals Black Coral's Bleak State
Artificial Reefs: Trash to Treasure
Hermaphrodite Frogs Caused By Popular Weed Killer?
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Caviar Crisis Spurs Caspian Sea Summit
Despite Predictions, Viagra Hasn't Stemmed Trade in Threatened Wildlife
Raw Human Waste Killing Off Coral Reefs?
Is Bleaching Coral's Way of Making the Best of a Bad Situation?
Artificial Reefs: Trash to Treasure
World Has Enough Water for All, Experts SayBut Only if People Pay
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Coral Reef Paradise Found in Remote Indonesian Islands
Iceland Lake Disappearing Into New Crack in Earth
Inner Earth May Hold More Water Than the Seas
National Geographic Magazine: Deep Sea Vents: Science at the Extreme
F. Lesson Plans
David Doubilet (Oceans and Photography):
Into the Ocean
Using Photography to Help Save the Ocean
Ocean Conservation: Getting the Word Out With Photographs
What's Wrong With the Oceans? Can Photography Help?
Aquarium vs. Natural Environments:
Grades 3-5: Aquarium Habitats
Grades 9-12: The Pros and Cons of Artificial Reefs
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