Many fish change sex, which in ichthyology is called sequential hermaphroditism. When I worked as a marine biologist, I worked with a few species of fish that did this and it was fascinating.
  1. Hermaphroditism in fish is pretty common.
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    It has evolved independently in many species of fish. This is a picture of the gonads (reproductive organs) of gilt-head bream in different stages of transition (photo credit T. Yamamoto)
  2. Sequential hermaphrodites are the most common.
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    Meaning they start as one sex and switch to another later in life as needed (usually to keep the correct ratio for mating). This picture is of rainbow trout, the top one has mostly sperm, the middle has half and half, and the bottom has mostly eggs (Guzel et. al 2006).
  3. Some fish are born male and have the possibility of switching to female.
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    This is called protandry. Clownfish are a good example (which is why in real life, Nemo's dad would have transitioned into his mom after Coral was killed at the beginning of the movie).
  4. Some fish are born female and have the possibility of switching to male.
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    This is called protogyny. Groupers and snappers do this (this is a Goliath grouper).
  5. I studied protogynous fish.
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    My undergraduate thesis was about red porgies and my masters thesis was about black groupers.
  6. Seeing the gonadal tissue of a transitioning female under a microscope was always fascinating.
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    o= oocyte (produced eggs), sp= spermatocyte (produced sperm). (Guzal et. al 2006)
  7. Transsexualism can really affect a fish's life. The groupers I studied were especially vulnerable to fishing because of their sexual behavior.
  8. Since the bigger fish are males, the fishermen targeting the larger fish mostly catch males.
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    This upsets the mating ratios.
  9. In addition to this, groupers often spawn in large aggregations.
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    We are talking tens of thousands of fish meeting at one site to spawn together.
  10. They return to the same sites every year, and fishermen know this.
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    Some sites get protected from fishing, even if just during the spawning season.
  11. And to make matters worse, they experience "spawning stupor", meaning that they act kind of goofy and less afraid when they're mating (don't we all?😏)
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    This makes them especially easy to catch. Fishermen know about this (and it's actually not well studied).
  12. All of this means that these fish are easily over exploited.
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    I never eat groupers or snappers because of this.
  13. So have a little thought about what you eat at the sushi counter. 😊
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    Check out Seafood Watch to see which species of fish are the most sustainable to eat.