Episode 52: Father’s Day Special – Seahorses and Male Pregnancy

Photo of a female seahorse depositing eggs into a male seahorse's pouch.

Image: Transfer of eggs from female to male Hippocampus abdominalis. (Source: Wikimedia Commons, used and edited in compliance with CC BY 3.0).

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Transcript

Charles 0:22
Hello and welcome to Assigned Scientist at Bachelor’s. I’m Charles and I’m an entomologist.

Tessa 0:27
And I’m Tessa and I’m an astrobiologist.

Charles 0:29
And today it’s just the two of us. It’s almost Father’s Day, and I thought the perfect topic would be seahorses. Now Tessa… you know about seahorses.

Tessa 0:39
Right, they are well known for essentially… their reproductive strategy is such that, while the female seahorse produces the eggs…

Charles 0:50
Definitionally.

Tessa 0:51
The eggs are actually fertilized and cared for in the pouch in the male seahorse.

Charles 0:56
How equivalent do you think the male pregnancy of the seahorse is to the female pregnancy of other viviparous vertebrates?

Tessa 1:09
I mean, it’s not going to be exact, because technically speaking, it is still external fertilization because there is some seawater that is mixed in as they assume versus mate. But it’s pretty close in a lot of respects. Or at least it’s certainly close to other fish that bear live young. I mean, in the sense that there isn’t like, obviously a placenta and umbilicus. But you know, beyond that, you’re still producing a safe, nutrient rich environment for your young to gestate.

Charles 1:38
Yeah, I was… I was surprised – no spoilers – but I was surprised by how closely it approximates what we would think of an hour mammal centric view of the world as pregnancy. So let’s get into it.

Okay, so for a little bit of context, the seahorses as far as I can tell the like true, seahorses are only the members of the genus Hippocampus, which is named after an ancient mythological figure that has front half horse, back half fish, kind of like a horse mermaid.

Tessa 2:19
Yeah, it literally means like horse fish in Greek.

Charles 2:23
Yeah, the true seahorses have what we think of as seahorses of being fish that basically swim upright. They have prehensile tails, they’re not very good swimmers. And they’re called seahorses, because they and the other members of the family that they belong to have these elongated, fused snouts that look like a horse’s long face. Seahorses belong to a family called Syngnathidae. They belong to the ray-finned fishes and specifically the teleost fishes. And if you ask me to explain what that means, I would say… don’t ask me that because I’m not a vertebrate person.

So Syngnathidae includes the seahorses, of course pipe fishes, which are kind of… if seahorses, if Hippocampus the seahorses look like horse mermaids, pipe fishes are like snake mermaids.

Tessa 3:22
Yep. I mean, they’re literally you know, they look like pipes, which is why they’re called pipefishes.

Charles 3:27
Yes. And then also the sea dragons, which as far as I can tell, are three species in two genera that basically, and I’m sorry to say it this way, but it’s the only succinct explanation I can really think of – they look like yassified seahorses. So there are about 320 described species in this family, and there are like 46 to 48 species and hippocampus specifically. And so they’re characteristically like fused jaw, the long snout that makes them kind of look like a horse is used to suck up small invertebrates. So they’re, they’re just suctioning up tiny crustaceans, basically, and their bodies externally are covered in bony plates.

Members of this family are found in marine and freshwater and brackish environments, so they’ve kind of achieved the fish environmental hat trick – normally in relatively shallow environments because they are not strong swimmers, which is why the seahorses as we know them have a prehensile tail, where the end of the tail basically will wrap around something. And what’s interesting is that male pregnancy, as in a fully enclosed pouch which has no exposure to the external environment is, as far as I can tell, pretty much exclusive to the Hippocampus. However, there is a spectrum of traits of egg retention and brooding and paternal care in this family that sort of spans the gamut from relatively little to fully enclosed. And from phylogenetic research in this family, it appears it is not a straightforward evolutionary progression like…

In the mammals, as they… we’re starting off with the synapsids, which are our long distant, kind of reptilian ancestor within the tetrapods, and then you get to monotremes which are still egg laying, and then you have the marsupials, and then you get to the eutherians, which we all are, unfortunately, of full viviparity, full development inside of a totally internal organ, which, frankly, I think was an evolutionary mistake, and we should go back but…

Tessa 5:49
It does really inconvenience a lot of things.

Charles 5:51
Yeah, it’s not up to me, unfortunately. But mammals, the evolution of mammals indicates this like stepwise progression towards increasingly internal embryonic development within the body cavity, right, where we start off with our fully ancient ancestors that laid eggs and then you get into the monotremes where you’re developing the traits that are typical of mammals – mammary glands, hair, other synapomorphies, I dunno. And then another point of divergence and you get to the metatherians which are the marsupials which have pouches that they basically do a lot of their development in, and then finally, we get to you eutherians, where we have a whole organ internally, which is just for putting developing embryos in.

In seahorses and in the Syngnathidae overall, there is not this same kind of stepwise evolution towards increasingly internal… so it’s not like, you had external brooding and then a point of divergence, and then slightly more internal on that a point of divergence, and then you get to the Hippocampus with the fully enclosed specialized brood pouch, right? It turns out that, based on the evidence that we have now of the phylogeny of this group, all these different sort of characteristic ways of brooding eggs have evolved independently multiple times. And so there is the spectrum of different adaptations in the group, but it has not evolved in the same way that internal brooding did in mammals – it has kind of just like, relatively unrelated groups can have the same adaptation evolved.

Tessa 7:40
Convergent evolution.

Charles 7:41
Yeah. I wanted to talk specifically mostly about Hippocampus because I think seahorses are the most extreme, and also the most well known, of the examples within this family, but I did want to establish the diversity of reproductive strategies that are present.

Particularly what is striking about seahorses is that, I think, because they are relatively upright in the way that we perceive humans as being upright, their pregnancy looks more like human pregnancy than I think…

Tessa 8:17
You know, I think that’s a good point.

Charles 8:19
Yeah. Even like, can you see like a pregnant cat, that cat could just be fat. You see a pregnant seahorse, you’re like, Oh, I recognize that as pregnancy. So getting into seahorses, let’s start at the beginning, with courtship. I don’t want to talk a great deal about courtship, because, listen, this isn’t a Valentine’s Day episode, this is a Father’s Day episode. But all that to say that seahorses are, from a certain perspective, romantics – in two major ways. One is that they pretty much typically form long term monogamous partnerships, and not monogamous partnerships in the way that a lot of birds form monogamous partnerships, where socially it’s monogamous, but they have an open marriage. Seahorses are much more typically… they partner for a long time and the genetic parentage of their offspring is pretty much limited to that partnership. And then they also, their courtship rituals, they will basically do kind of, in water courtship dances for like, hours, like hours before the actual oviposition into the brood pouch.

Tessa 9:33
Yeah. Which is not something you normally associate with fish.

Charles 9:36
I don’t think I’ll be honest with you. I do not know enough about fish overall, to say whether it is or isn’t. But I would say again, the sort of uprightness of seahorses is very emotionally compelling to humans, because it really does look like they’re kind of doing a fun little dance together.

And then eventually they get to a point where the female transfers eggs to the male, and I really did my best to try to find in for very specific information on the actual mechanism by which this happens and frankly, it seems like this is one of those situations where we’ve all kind of thrown up our hands and gone, I’m, I’m gonna be real candid with you. We don’t really know how this happens.

Tessa 10:23
Yeah, I mean, I know the females have an ovipositor, but…

Charles 10:27
Kind of, but I couldn’t even find a description of like a specific, identifiable organ. I found a video, even, because seahorses are pretty big in like the aquarium trade, it kind of, honestly, it seems a little bit haphazard for my tastes, but you know, I’m not a seahorse man trying to get pregnant. It’s, it basically seems like they just line up to their bodies very closely. And through an opening, the female pushes out eggs and they get collected into that anterior pore of the brood pouch that is opened up. It’s amazing to see, especially because you’d think, well, this is gonna be a disaster.

Continuing on the road of things that we haven’t been able to really specifically described, the way that these eggs actually become fertilized, to this day, as far as I can tell, remains a mystery. Because it used to be believed that there was maybe, you know, the seminal duct opened into the brood pouch, specifically, the male would accept the eggs and then provide sperm into the pouch directly. And it would be a kind of a kind of internal fertilization. But that doesn’t happen, because through like dissections of various seahorses, it would it would be physically impossible for that to be the way that the sperm gets to the eggs. However, they don’t have an intermittent organ, AKA a penis, because most fish don’t.

Tessa 12:13
Right.

Charles 12:13
So there’s no like specific organ whose job it is to orient directly over the eggs to provide sperm as they get accepted to fertilize them that way. So it seems like the best working hypothesis right now is that the sperm is sort of deposited into the marine environment with the eggs and then potentially seminal fluid or even ovarian fluid kind of provides an environment around the eggs into which the sperm will be accepted so that it doesn’t kind of just get scattered hither and yon. So it is external fertilization, but in a kind of a, it’s, it’s weird in the water, it’s wild out there. But that’s, that appears to be a remaining mystery that we just kind of, we know that it happens because we keep getting more seahorses that are not just genetically identical to the female parent – but how it happens precisely is… your guess is maybe as good as mine. Probably not as good as an ichthyologist’s. But who knows?

So now we’ve gone through the courtship dance. We have deposited the eggs, we have also deposited sperm, somehow. And here’s the thing. I used to think, before reading up about seahorses, I didn’t have like really solid ideas on probably what was happening because I always kind of imagined that the benefit of the pouch was just kind of like having a nest that’s connected to your body. Do you know what I mean?

Like a specific environment where you, the seahorse dad, can watch over and protect and prevent predation, prevent environmental harm, but that it was basically like just erecting a little house that was the same environment as the surrounding water. You know, I haven’t talked to enough people about their preconceived notions on seahorse male pregnancy to know if my idea of how it happened is how most people think it happened, but I was surprised by how wrong, how wrong I was, as you said there is not the same, a full placenta that is then attached to the eggs by an umbilical cord, because fish have managed to evade all the weird mammal stuff that we do.

Tessa 14:55
Right.

Charles 14:56
As one article put it, “very simply the brood pouch, like the mammalian placenta, performs the functions of gas exchange, waste removal, and osmoregulation.” So in Hippocampus, they have – not fully spherical eggs, they have kind of pear shaped eggs, and the narrow end actually implants into the like specialized epithelial cells lining the brood pouch. So they’re not all just kind of jumbled up to… it’s not like if you pressed on the brood pouch of a male seahorse, I always thought that it would be kind of like those, you know, fidget toys where there’s kind of a goo and a bunch of little plastic balls. Do you know what I mean?

Tessa 15:42
Yeah, yeah.

Charles 15:43
It wouldn’t be like that. And I will provide, I will provide a link to an article I read that had a great sort of cross section diagram, where there was a folded inner epithelium, where the epithelium is like a layer of skin cells, right. And it involves specialized cell types, which are probably used in providing nutrition to the developing eggs. So there are different kinds of specialized cells that make up the layers within the pouch, and there was a quote “vascularized elastic dermis,” as this article puts it, “a layer of stratum spongiosum, made up of loosely associated connective tissue and scattered smooth muscle and a layer of stratum compactum made up of tightly packed collagen.” So like it can expand in size, right, obviously, because it’s got a and it has muscle and connective tissue, and it’s vascularized. So it like has access to the blood supply and a quote, “inner layer sometimes referred to as a pseudoplacenta.”

Tessa 16:49
This sounds a lot more sophisticated than I think I was originally picturing.

Charles 16:53
Right? And so there is vascularized connective tissue so that it has access to the blood supply. There are these specialized cells into which the eggs can implant so they’re not just kind of floating around willy nilly. And there is a pseudoplacenta which probably provides direct nutrition to the eggs as they develop. And also quote, “the brood pouch is formed developmentally by outgrowths of the male abdominal epithelium on the ventral surface, that elongate towards the midline and fuse to form a bag that differentiates into the discreet layers of mature pouch,” meaning that like, evolutionarily, you had a seahorse that just had normal abdominal skin. And then over time, it has developed into this very highly specialized specific.

So it’s not just that there’s like a loose flap of a pouch on the front of a male seahorse. It is a specific structure that is highly specialized in its construction, and its role is only to accept eggs that then implant in the sides and which are fed and taken care of and protected in the pouch of the male, which is fully closed to the external marine environment, as long as they are developing internally. Which is wild, right. Like that’s incredible.

Tessa 18:18
Yeah, yeah.

Charles 18:19
And this is, the pouch develops, particularly basically during like late stages of development. It’s kind of not precisely – ichthyologists don’t come after me – but it is kind of like during seahorse puberty, the males develop the brood pouch to prepare for being reproductively mature, like, Great job buddy. And what is particularly interesting is that the development of this brood pouch is quote “dependent on androgens that are probably of testicular origin,” so…

Tessa 18:59
Fascinating.

Charles 19:00
The development of the brood pouch is dependent on androgens available to the male seahorse, which is like… on a certain level – duh, because it is developmentally part of the like… only male seahorses do this, there are no brood pouches on female seahorses. And if we associate testosterone and other androgens of testicular origin with sexual characteristics that determine sort of reproductively mature maleness, then of course, androgens would lead to the development of the brood pouch because the brood pouch is a part of being a reproductively mature [male] seahorse. But on the other hand, I think that this might be shocking to people because it is so counterintuitive to think of preparation for pregnancy being a specifically masculine physiological trait.

Tessa 20:02
[It’s a] complete inversion of how we normally think about it.

Charles 20:05
Right, exactly. And, and specifically, from 2017 article, quote, “interestingly, both Boisseau and Noumura” – from two different articles in the 60s and 50s – “observed that testosterone administration to females elicited the development of a rudimentary brood pouch.”

Tessa 20:25
holy cow.

Charles 20:28
Right? Sidenote, you can basically just do whatever you wanted in the 50s.

Tessa 20:34
Yeah, no kidding.

Charles 20:34
You can just do whatever you wanted. But if they castrated, if they removed the testicles of non pregnant male seahorses in Boisseau, from 1967, which I think was somebody’s PhD dissertation, that led to like, decrease in the health and development of the brood pouch, and then if they gave testosterone to females that elicited the development of a rudimentary brood pouch. And what I think that this is an interesting, an interesting lesson to learn for no reason in particular, that bodies are very complex, but they’re also very plastic, plastic in the sense of being malleable, where you can administer hormones to even a fully reproductively mature individual of a certain sex, and then observe the development of sexual characteristics basically, across a lot of animals’ full lifespans.

Tessa 21:49
Yep.

Charles 21:50
Where like, also we see sometimes in like sanctuaries, and in zoos, if you have captive male lions, if you for whatever reason have to remove the lion’s testicles, they will often lose their great big manes. And this is kind of what we talk about when we talk about sex being something that is broadly binary, but not totally distinct nor immutable.

Tessa 22:19
Yeah, I think we can definitely rule out immutable at this point. Yeah, certain ideologues might want you to believe otherwise…

Charles 22:27
[Jokingly] For no reason in particular… both that biological sex in terms of the overall phenotypic pattern of your body is something that is determined genetically, in those organisms which have a genetic component to sex determination, and not all of them do. So it is determined first chromosomally. But then secondly, during development, where the developing embryo responds to the genetic code that is available to it. And as we saw in the episodes that we did in March, there are a number of situations where those two singular developmental pathways are not fully diverging, where because of the presence or absence of this gene, or the presence or absence of that gene, there can be complexity as to how that developmental pathway plays out in individuals. And then your biological sex, as in sort of the apparent phenotypic distinction, you know, between different patterns is also developed, again, when you go through the process of puberty, and developing towards adulthood and reproductive maturity. And then the appearance and presence of the sexual traits are maintained throughout the rest of your life by the presence or absence of different for instance, hormones. So I think it is much more useful to think of biological sex, not as a binary where there is a single on off switch, and then you’re set for the rest of forever, but as something flexible and fluid and malleable, that is determined again, and again and again, until you’re dead in the ground.

Tessa 24:17
Right, which I mean, from an evolutionary point of view makes sense because it would be advantageous in biology to have that degree of flexibility and redundancy too.

Charles 24:27
Yeah, it’s… we know that there are a lot of fish for instance, that… the term that will appear in the literature, which I’m going to say in case people are curious and they want to look it up more, but which I think we should move away from because of the cultural implications of the word “hermaphrodite.” But the term that you will see in the scientific literature is “sequential hermaphroditism,” where an individual will begin life as… and to reiterate the basically the working definition of sex that we use in biology as a term that we can apply across, the term sex that we can use as a as a tool to apply and compare of life across, you know multicellular organisms is the distinction between two different kinds of gametes, right?

You have the small, relatively motile gametes, sperm, and then you have the much larger and sessile gametes, ova, also known as eggs. And so there are a lot of fish that begin life as one sex producing sperm, for instance. And then at some point in their life, they change and they start producing the other one, or I think there are probably at least a couple of examples of individuals producing both individuals producing both at the same time are largely invertebrates, and particularly a lot of mollusks is how I know about them.

So for instance, the parrot fish is very famous because there was that book that was one of the first YA books that talked about trans boys, Parrotfish, and then there are also like clownfish. So like, in Finding Nemo, Nemo, his dad actually should have become a female fish after his wife died. But… so I just wanted to bring up that point about testosterone, I thought it was fascinating. And I was correct.

From that same article, quote, “the pseudoplacenta is rich and blood vessels suggesting father to embryo interaction through epithelia on the lumen, similar to mother to embryo interaction in mammalian placenta,” which is also fascinating that like these two very broadly, evolutionarily separated groups have developed totally separate forms of pregnancy. But the method of caretaking for eggs for the developing embryo does actually have a lot of similarity, which is again a lesson about patterns that we see in evolution, and how constrained sort of the genetic and subsequent physiological mechanisms organisms have.

Or to put that in a way that might be comprehensible to somebody who has who hasn’t spent the last decade of their life in post secondary biology courses, that there are kind of these underlying… there are no, like, laws in biology, which might be philosophically contentious thing to say. But listen, I’m not afraid of you, philosophers. I am afraid of the icthyologists – ichthyologists don’t come after me, philosophers do what you must. But I would say broadly that we do not have laws in biology the same way that we have, like laws in physics, right? Because so much of evolution is contingent, meaning that the way things evolved is not the way things always would have evolved. But they’re very dependent on the context, geologically, geographically, chronologically, environmentally. So much of evolution is driven by the environment in which it happens, versus just the sort of intrinsic properties of organic life.

However, because of the limitations of physical laws and chemical laws, there are only so many ways that life can develop. And in fact, evolution is also very constrained, future evolution is constrained by the evolution that has already happened. The reason that, like, humans are not going to sprout membranous wings tomorrow is that we don’t have kind of the raw genetic material to do that from, whereas if you have an insect that evolved from a lineage of winged insects, and then they lost their wings, there was kind of always the possibility that they might re evolve wings, because they kind of have that potential to work with.

Tessa 29:24
Right.

Charles 29:25
And so what I’m, what I’m trying to say is that it seems like there is some like phenomenon in you know, the genomes of vertebrates, that if given the situation where it is very advantageous to sort of develop pregnancy, even wildly independently from mammals, working from that same raw vertebrate material. They’re basically developing, not exact but equivalent, very similar structures and function. Does that make sense at all?

Tessa 30:01
Yeah, yeah.

Charles 30:03
So we’re in the brood pouch their pseudoplacenta, basically, the brood pouch is doing everything that it can for these little guys, you know, they’re getting osmoregulation so that doesn’t get too salty in there; waste extraction; they are providing nutrients – they’re having a great time. And then from another paper speaking, you know about the family as a whole, the, quote, “the interactions between the male body and the developing embryos are inversely proportional to the degree of egg exposure to the external environment, and directly proportional to the anatomical complexity of the pouch and the specialization of its skin,” which is basically just saying that in Hippocampus, where they are fully enclosed in the pouch, no exposure to the surrounding water, there is a very highly developed high degree of interaction between the male body itself and the developing embryo.

So it’s not just like the male, seahorse, as we’ve already said, accepted the eggs and then they’re just kind of hanging out in there, and then eventually they’ll hatch, and then they’ll be gone – the male body is functionally going through pregnancy. From another article that I read during periods of like, reproductive availability, so like when females are producing eggs, male seahorses can go through multiple pregnancies, but each time they do, the cost of the pregnancy is higher. Think about how tired a human person is after they give birth. And then if they immediately have to give birth, again, it’s… the whole thing is probably going to be worse for them, because they didn’t have any kind of recovery time. And similarly for seahorses, so it is not a totally low cost situation where they’re just kind of hanging out. The male seahorse is investing a lot into the health and safety and development of these eggs. And isn’t that isn’t that sweet? Happy Father’s Day.

And so embryos can hatch from the eggs before they emerge from the pouch. So there may be a period of time where they’re literally free swimming inside of the pouch. Which if you thought a baby kickin was uncomfortable, you know, I mean…

Tessa 32:25
A whole school of them like swimming around in there, basically.

Charles 32:29
And then eventually they hatch. And you know, and then the world keeps turning, and the whole thing happens again. And so that is the story, the physical story of male pregnancy in Hippocampus fish.

And then the, the final thing that I kind of wanted to touch on is the sort of evolutionary questions that this brings up, because it is fascinating, obviously. And particularly the reason that unusual reproductive strategies get so much attention is, (a), because scientists – and you can tell them I said this – are nosy perverts.

Tessa 33:12
Yeah, no argument here.

Charles 33:14
No argument. If… the MINUTE we find evidence of alien life, the minute there are going to be people asking how they reproduce. And that’s humans. That’s what it means to be human. But because we are interested in these instances of unusual reproductive strategies, is also because it brings up the question of, if this is different from all of these other things that are happening, why? Like, why did this happen at all? And we’ve got a couple of ideas.

So one thing that is very interesting is that in bony fishes, usually the sort of the classic image of fish reproduction is one of very low investment post egg stage, right? Where basically, you put all of your parental energy into developing a bunch of eggs, or developing a bunch of sperm, and then you spawn into the water. And you’re basically like, Go with God, my children.

Tessa 34:18
Yep, yep. They’re on their own after that.

Charles 34:20
They’re on their own. And of course, there are exceptions to this along a wide spectrum of parental involvement. Going back to one that we mentioned earlier with clown fish as in Finding Nemo, you know, you do have parental overview. So there are fish that will watch over eggs afterwards but have no like direct caretaking necessarily, and then you get to seahorses, where it is all investment all the time. However, in those teleost fishes that have a degree of parental involvement, it is much more common that the parental care is from the male fish rather than the female fish. So it is I think, in one article that I read, it was like 61% that have parental involvement, it is exclusively paternal care. Happy Father’s Day.

Tessa 35:20
Huh.

Charles 35:20
From that perspective, it is not that unusual that syngnathids would have developed this form of highly invested male paternal care of, of male egg care through brooding, either on their tails or on their abdomen, given that most of their, you know, taxonomic fellows also follow a higher male investment than female investment pattern. But obviously, most fish are not experiencing male pregnancy, or else we would not be so excited about the seahorses all the time.

From a wonderful review article that I read, which I will of course link in the show notes, there are basically four hypotheses. And here they are. One is the order of gamete release hypothesis, meaning that you cannot fertilize eggs, unless there are eggs there to fertilize. So by that logic, the male is the most likely to be left behind with the eggs, and thus most likely to have the parental responsibility. Another is the association hypothesis, in which external fertilizers are more likely to be associated with offspring than internal fertilizers, which kind of makes sense. If you think about spawning were like, if you think about internal fertilization, like in birds, like in mammals, basically, after the point of fertilization, the male individual can just hop along and never know what happens versus if you’re spawning, and you’re both already there.

Tessa 37:01
Right.

Charles 37:01
Well, you know. The cost hypothesis in which quote, caring males experienced lower costs to future fecundity than carrying females, meaning that sperm are pretty…

Tessa 37:13
Yeah.

Charles 37:13
They are relatively easy to produce in a high number.

Tessa 37:16
That makes sense because like, in some respects, the trade off is the female produces eggs, which are very energy intensive. And then the male takes care of them as they gestate, which is also very energy intensive. And, you know, it makes sense for them to split that rather than having the female do all of it.

Charles 37:37
Yeah, particularly if you were thinking of monogamous couples, where it’s kind of a, you’re in this together strategy. And then the certainty of paternity hypothesis, in which caring males have increased paternity. To contextualize these hypotheses, we have to think about fitness and evolution, where take emotions out of it, don’t think about this emotionally, right? There is this kind of constant, the win condition of evolution, right, from a certain perspective, is that you have as many offspring as possible, so that your genes are replicated as many times as they can be. That’s like, that’s when you get the trophy, right? Where the trophy is, your genes live on and somebody else’s don’t.

So you often get these sort of conversations of… conflict in reproduction, where you have male choice, female choice, male competition, etc, etc, etc, we’re all that comes back down to balancing the investment you are putting in and the yield that you are getting out of it, right. Often when we talk about paternity, we are talking about the conflict of control, and, and basically being cucked, right, where… there are multiple fish that have like a single sort of phenotype typical of females, and then multiple common phenotypes of males, that correspond to very different reproductive strategies. So for instance, there are those where a male fish will have like a group of females that live all around it, and they’ve theoretically fertilized all those eggs, but sometimes there will be another phenotype of male where they look like the typical female, and they sneak in, and they sneakily fertilize, and it’s like these different strategies that are available of you can be the big male, and you can theoretically fertilize a lot of different females eggs, or you can be the little male and you can’t get a bunch of females, but you can get much closer access to eggs without having to deal with male male competition.

Tessa 39:57
You see this in mammals as well.

Charles 40:00
Yeah, you know, mammals…

Tessa 40:02
Boring.

Charles 40:04
And so then thinking about it, given everything that we know about fish, and about male competition and paternity and investment, aka cost versus yield, how do we square that with the idea of males who are normally the sort of gadabout town, low investment individuals, then evolving to have not just high investment, but the the highest investment in sort of their own personal costs to reproducing in this certain way? And the thing is, I don’t know, I don’t think we have a specific answer. I am because, listen, I’m in a philosophy program. I’m just here to ask questions, my dudes. We’re just having a good time. And it’s, I’m not going to get too much further into this, because we’ve actually been recording for quite a period of time. And to get into all of it very, very specifically, I think would take a lot of time that we don’t necessarily have, but what I will do is I will link relevant articles that talk about this, and I’ll just I’ll set you loose the audience.

Yeah, I think as a as an ending note on this conversation. There is another phenomenon that is very, very interesting to me, where another family, Solenostomidae, known as the ghost pipe fish are regularly returned as the sister group to Syngnathidae in a phylogeny – just meaning that like, in the same way that our closest living relatives are, I think chimpanzees and bonobos are now considered a clade. So chimpanzees, the clade made of chimpanzees and bonobos, the closest living relatives of the syngnathids are the ghost pipefish, in this other family Solenostomidae.

And what’s interesting here is that they brood eggs also on Integumentary outgrowths inside fuse pelvic fins. So I think I saw them referred to as “armpit brooders” where there is like a little space underneath their pelvic fin. And that’s where they put their eggs. It’s, it’s basically like, what if you had special extra bones in your armpits, and you put eggs in there. And also you were a fish and you were swimming around.

But interestingly, the solenostomids, the ghost pipefish, they are female skin brooders, they’re… they, the males do not brood in that group, the females do, but as multiple articles I read emphasized, there are no instances of male brooding evolving from a lineage where the ancestral trait is female brooding. So basically, there are no examples that we know of in the evolution of these groups, where, you know, a group of fish will start out at some point in evolution with female brooding, and then switch it up. And it turns out, the males end up being the exclusive brooders. Thus, the conclusion that brooding is very unlikely to be an ancestral trait in the clade made up of solenostomids and syngnathids. So the ghost pipefish, and then our guys… does this make sense if you are not deeply embedded in systematics?

Tessa 43:24
I think so, but then again, I also know a lot about systematics.

Charles 43:28
I’ll, I’ll try to put it as simply as like, basically, ancestral trait is just a trait that belongs to an ancestor, that then we… and this is very useful in constructing phylogenies, because if we think about the ancestral trait, and it’s also present in all of these other descendant groups, then we can be like, Oh, that tells us something about the evolution of these groups. Because unfortunately, slash fortunately, time travele is not real. We cannot go back to hundreds of millions of years ago in geologic time and be like, that’s when that happened, and that’s when that happened. And you know, fortunately, unfortunately, time travel probably isn’t ever going to be real, from what I know of what physics…

Tessa 44:11
Yeah, that’s a whole other episode of the podcast.

Charles 44:13
That’s a whole other episode. That’s many episodes. Anyway… But basically, it is very, very likely that the ancestor to both of these groups that ghost pipefish, as well as the pipefish- pipefish, seahorses and sea dragons, was not a brooder that like the ancestral trait was non brooding. And then brooding evolved many times independently in these different descendant groups. And here’s why this is interesting to me. Because the ancestral trait was probably, based on what we know about the evolution of these groups and the development of these behaviors and adaptations, etcetera, etcetera – brooding was not present in their common ancestor, but it is present in both of them now. And I think this gets to an interesting phenomenon that we see, you know, littered across evolution of organisms of very closely related groups convergently evolving very similar traits, even when, like a proto version of that trait was not even present in their common ancestor. And I just think that’s kind of a fascinating open question in the evolution of organisms, you know what I mean?

So that is our episode on seahorses and on syngnathids. Overall, I hope you enjoyed this journey together. I learned a lot. I… my whole preconceived notions about seahorses and how I thought they probably worked aretotally overturned.

Tessa 45:50
Yep, Likewise.

Charles 45:52
What a wonderful journey. If you want to find me online… on’t find me, just go to the podcast’s Twitter @ASABpod and talk to me about animorphs.

Tessa 46:04
You can find me on Twitter @spacermase, you can also check out my stuff on my website, tessafisher.com.

Charles 46:12
You can find us on Twitter @ASABpod or at our website where we post show notes and transcripts for every episode – on these episodes where we do independent research the show notes are a party and I really encourage you to go look at them – asabpodcast.com. I also I realized we should probably start saying at the end of this, thank you to Nicole Petkovich for our intro music. She made that and then she was a guest on this show.

Tessa 46:40
Yay Nicole.

Charles 46:40
And yeah, if you would, if you are a trans and or non binary scientist, science worker, science journalist, science fiction author, member of the cockroach pet trade, and you would like to be a guest on that on the show, we have a contact form on our website and we also have a Google form that you can fill out that is linked on our Twitter and our website. So if you would like to be a guest, we are always looking for guests. And if you liked the show, please tell people about it, because word of mouth is the most reliable way for podcasts to grow.

Tessa 47:14
And until next time, keep on science-ing.

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