You probably didn't expect to learn about flounder sex changes today, but here we are.
There aren’t any southern flounder farms yet, but the fish has some traits valuable for aquaculture, or fish farming. “It's one of the few food-sized flounders that can grow in freshwater,” said Harry Daniels from North Carolina State University. And that's great, because freshwater farms – which are typically built on land – avoid some of ocean aquaculture’s environmental problems, like pollution and the disruption of native fish populations.
There's just one problem – while female southern flounders are plump and juicy, their male counterparts are puny in comparison. But Daniels and his colleagues have a solution: raise all-female populations, change some of the female fish into males (more on that in a second), and freeze the resulting “neomale” sperm so farmers can one day run all-female southern flounder farms.
So, want to farm flounder? From an economic standpoint, you probably only want the biggest flat female doormats (yes, people really call them that) which grow to around four pounds, quadruple the weight of a male. Farmers of other flounder species have historically killed hundreds of thousands of small, mostly male flounder in favor of the big females. You, on the other hand, probably don’t want to kill any fish. Probably.
About a decade ago, scientists figured out how to apply the process of gynogenesis – literally “woman making” – to southern flounder. It’s a tedious process, but you don’t want your all-female southern flounder farm to flounder, so bear with me for a sec.
If you think back to high school biology, you'll recall that every animal cell has two sets of chromosomes containing all of the information on how the animal should look. It's like having two print edition sets of the World Book encyclopedia, one from mom and one from dad. The encyclopedia from mom always has an X-chromosome volume, while dad’s has either the same X chromosome volume or the Y chromosome volume. If cells have two X volumes, the animal usually winds up female. If they have an X and a Y, the animal is generally male.
The sperm and egg cells stand out. They have only one chromosome encyclopedia set, because they split in half and lose their second set through a process called meiosis. The female’s eggs keep mom’s X chromosome volume, and sperm end up with dad's X or a Y volume. The baby gets one sex chromosome volume from each when the sperm fertilizes the egg.
To make female-only babies, scientists blast sea bass sperm with ultraviolet light to wreck its chromosomal encyclopedias, then use that sperm to fertilize flounder eggs. At this point in flounder meiosis, the egg would usually spit out its second chromosome set and take the sperm’s — but it can’t read the sperm’s ruined encyclopedia, which (since it's from the wrong species) is basically written in a foreign language to boot. Instead, scientists put the now-fertilized egg under high pressure, about 580 times the pressure of our own atmosphere, forcing the egg to hold onto both sets of its own chromosome encyclopedia. The eggs have two sets of X chromosome volumes, and the babies are female.
“It’s like science fiction,” said Daniels.
This gynogenesis process is slow, so you’ll need a quicker way to breed your farm’s fish. Well, some of those female flounders still develop into smaller males with male genitals if they’re raised in warm enough water. Genetically, they’re females and their sperm cells will only have X chromosomes — the “neomale” flounders don’t have any Y chromosomes in their body to give to the sperm cells. So you can let this new generation of flounders have babies the good-old-fashioned way, and there won't be a single Y chromosome amongst their offspring.
But wait, you say. This sounds hard and expensive and science-y. If I wanted to be a scientist, I wouldn't be starting a southern flounder farm! (Apologies to all of our fish-farmer-slash-scientist readers, we can't cater to every demographic every day).
That's where the cryogenically preserved sperm comes in. If you want to start a female-only flounder farm on the fly, you can take a small female population and buy this X chromosome-only-sperm wholesale, guaranteeing you mostly female offspring. Daniels’ group found an ideal freezing process that allows them to preserve and ship fish sperm to folks who want it. They published their most recent results on this method in the Journal of the World Aquaculture Society last month.
Scientists use gynogenesis and frozen sperm for other aquaculture species where females are more desirable stock. “[Daniels’ group] is applying a well known method that’s already been done for Japanese flounder and cobia,” said Daniel Benetti, director of the University of Miami’s aquaculture program, who wasn't involved in the new study. But “the southern flounder has extraordinary properties for aquaculture. I’m strongly behind the research and I’m keeping a close eye.”
The last major hurdle is the flounder-killing Edwardsiella tarda bacteria, said Daniels. NCSU and other researchers are developing a vaccine, after which Daniels thinks we’ll start seeing southern flounder farms in the United States.