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Home » Fish Sex – It’s Weirder Than You Think: Marah J. Hardt (Transcript)

Fish Sex – It’s Weirder Than You Think: Marah J. Hardt (Transcript)

Full text of Marah J. Hardt’s talk: Fish sex – it’s weirder than you think at TEDx MileHigh conference. In this hilarious talk, marine biologist Marah J. Hardt explains why (fish) sex in the sea matters, how we humans impact it, and what we can do instead to protect our oceans, and future generations of fish.


Marah J. Hardt – Marine Biologist

Right now, beneath a shimmering blue sea, millions of fish are having sex. And the way they’re doing it and strategies they’re using looks nothing like what we see on land.

Take parrotfish. In this species, all fish are born female. And they look like this. Then later in life, she can transition into a male. And she’ll look like this.

But it’s not just a spectacular wardrobe change. Her body can reabsorb her ovaries and grow testes in their place. In just a few weeks, she’ll go from making eggs to producing sperm. It’s pretty impressive, and in the ocean, it’s also pretty common.

In fact, I bet nearly all of you have at some point had a seafood dish made up of an individual that started life as one sex and transitioned to another: oysters, grouper, shrimp… seeing some heads nodding, yeah?

But not all fish that change sex start as females. Those clownfish we know from Finding Nemo … they’re all born male. So, in the real world, when Nemo’s mother died, Nemo’s dad, Marlin, would have transitioned into Marlene, and Nemo would have likely made it with his father-turned-mother.

You can see why Pixar took a little creative license with the plotline, right?

So sex change in the ocean can happen in either direction and sometimes, even back and forth. And that’s just one of the many amazing strategies animals use to reproduce in the ocean.

And trust me when I say, it’s one of the least surprising. Sex in the sea is fascinating. And it’s also really important and not just to nerdy marine biologists like me who are obsessed with understanding these salty affairs. It matters for all of us.

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Today, we depend on wild caught fish to help feed over 2 billion people on the planet. We need millions of oysters and corals to build the giant reefs that protect our shorelines from rising seas and storms. We depend on medicines that are found in marine animals to fight cancer and other diseases.

And for many of us, the diversity and beauty of the oceans is where we turn for recreation and relaxation and our cultural heritage.

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In order for us to continue to benefit from the abundance that ocean life provides, the fish and coral and shrimp of today have to be able to make fish and shrimp and coral for tomorrow. To do that, they have to have lots and lots of sex.

And until recently, we really didn’t know how sex happened in the sea. It’s pretty hard to study. But thanks to new science and technology, we now know so much more than even just a few years ago. And these new discoveries are showing two things:

First, sex in the sea is really funky.

Second, our actions are wreaking havoc on the sex lives of everything from shrimp to salmon.

So today, I’m going to share a few details about how animals do it in the deep, how we may be interrupting these intimate affairs, and what we can do to change that.

So, remember those sex-changing fish? In many places in the world, we have fishing rules that set a minimum catch size. Fishers are not allowed to target tiny fish. This allows baby fish to grow and reproduce before they’re caught.

That’s a good thing.

So fishers go after the biggest fish, but in parrotfish, for example, or any sex changer, targeting the biggest fish means that they’re taking out all the males. That makes it hard for a female fish to find a mate, or it forces her to change sex sooner, at a smaller size. Both of these things can result in fewer fish babies in the future.


In order for us to properly care for these species, we have to know if they change sex, how, and when? Only then can we create rules that can support these sexual strategies, such as setting a maximum size limit in addition to a minimum one.

The challenge isn’t that we can’t think of these sex-friendly solutions. The challenge is knowing which solutions to apply to which species because even animals we know really well surprise us when it comes to their sex lives.

Take Maine lobster. They don’t look that romantic …or that kinky. They are both. During mating season, female lobsters want to mate with the biggest, baddest males, but these guys are really aggressive, and they’ll attack any lobster that approaches – male or female.

Meanwhile the best time for her to mate with the male is right after she’s molted, when she’s lost her hard shell. So she has to approach this aggressive guy in her most vulnerable state.

What’s a girl to do? Her answer?

Spray him in the face, repeatedly, with her urine. Under the sea, pee is a very powerful love potion. Conveniently, lobsters’ bladders sit just above their brains, and they have two nozzles under their eye stalk with which they can shoot their urine forward.

So the female approaches the male’s den, and as he charges out, she lets loose a stream of urine, and then gets the hell out of there.

Only a few days of this daily dosing is all it takes for her scent to have a transformative effect. The male turns from an aggressive to a gentle lover. By the week’s end, he invites her into his den.

After that, the sex is easy.

So how are we interrupting this kind of kinky courtship?

Well, the female’s urine carries a critical chemical signal that works, because it can pass through seawater, and lobsters have a smell receptor that can detect and receive the message. Climate change is making our oceans more acidic. It’s the result of too much carbon dioxide entering seawater.

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