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Seahorses have two sexes. One has mobile gametes, the other gives live birth. I imagine that a biologist who discovered the seahorse would initially call the first "male" and the second "female".

That is not the case: for some reason (why?), the ones with the mobile gametes are called "female", and the ones that give live birth from a pouch are called "male". I would like to know why.

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    $\begingroup$ I would imagine they compared the male and female ones, perhaps morphologically but more likely by chromosome spreads and determined that way. Male has the heterogenous chromosome (XY in our case, other species have other genetics), but I don't know what that might be for seahorses. $\endgroup$
    – bob1
    Sep 2, 2023 at 23:27
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    $\begingroup$ Of course its arguable that our sperm need some transfer too, but they are mobile once ejaculated. $\endgroup$
    – bob1
    Sep 2, 2023 at 23:31
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    $\begingroup$ One has mobile gametes, the other gives live birth - this is not true. In reality it is one has mobile gametes and gives live birth while the other does not have mobile gametes but does not give birth (they lay eggs instead - inside the males - inside the ones with mobile gametes) $\endgroup$
    – slebetman
    Sep 4, 2023 at 9:30
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    $\begingroup$ @Ivo - Yes. That's the case here. The confusion comes from thinking that the eggs are mobile - they're not. Like any other species it is the sperm that's mobile. The eggs happen to be ejaculated/laid and the sperm is not ejaculated. $\endgroup$
    – slebetman
    Sep 4, 2023 at 9:57
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    $\begingroup$ 1. Please provide links to papers describing these different gametes, otherwise it is not possible for me (and others) to analyse the situation. 2. The general assumption in the question and comments is that the designation of gametes as male or female is made on the basis of “mobility”. However a recent(ish) question about giant sperm in Drosophila professed that the “generally accepted view” is that “size” is the determinant (the bigger is the female). I got severely squashed when I expressed surprise at this, but nobody could provide an original reference to this idea. And mobility? $\endgroup$
    – David
    Sep 5, 2023 at 22:22

1 Answer 1

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Seahorses are fish. Most fishes use external fertilization to reproduce (there are certainly exceptions, though), so there's not really any aspect of "giving birth" to sex definitions (that's a mammalian bias perhaps): both sexes release gametes externally. One or both or neither fish may guard the fertilized eggs; if one is on guard, it's usually the male that does so, which isn't too different than the male seahorse's behavior, which is just more extreme.

Usually it's gamete size that is used to identify male versus female sex; female seahorses have larger gametes they include the yolk that will feed the developing embryo. Male gametes are also typically mobile. Both of these fit with seahorses: even though the female deposits her eggs in a male pouch, the eggs don't themselves move, any more than eggs move when a female fish simply releases gametes for external fertilization.

Wilson, A. B., Ahnesjo, I., Vincent, A. C., & Meyer, A. (2003). The dynamics of male brooding, mating patterns, and sex roles in pipefishes and seahorses (family Syngnathidae). Evolution, 57(6), 1374-1386.

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