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I already made a similar question searching for possible features which can make this fictional specie biologically functional, so this question can be considered a second part of this. I would recomend read the previous question but anyways here a little description

Schistosoma is a flat worm specie in which the male is notorious bigger than the female, and this have a curvated chanel in which takes the female for give her protection, this causes a constant copulation while are together, they bond themselves by the presence of a suction cup. Females can go out from this chanel and find other males, but the males compete for evite this. With a little more of context, the answer of my previous question proposes traumatic insemination in the uterus with genital stabbing like a water bug and fusion of the circulatory and nervous system as an enhanced version of what a monkfish would do.

At that time I had the idea of the concept, but not about the characteristics and biological mechanisms that would make this work, so this is what I must build the general anatomy of this species, therefore I have some problems.

First to make this design work, How much difference in size should the male and female have and how much is realistically possible?

What possible anatomical characteristics should the male and female have for this mechanism to work?

With this I mean, in one of my design tries the females is like a little centauroid (more similar to a naga or "amphisbaenia centaur") while the male is giant hexapod gorilla.

Previously I wanted to take start this from a terrestrial specie, but now I am open to make this an alien or gentically created specie (my point is just applye the strange Schistosoma like feature).

And finally a little aclaration, maybe this could sound to other similar questions about sexual dimorfism and how much different male and female can be, but in this case is specifically to work as the described way.

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    $\begingroup$ I think you need to give some more specificity in the question about your biological goal. Once you open up alien as an option, you can pretty much do whatever you like. This is a site that regularly discusses mecha and kaiju, so sky's the limit. I actually answered a biomecha question that might be an answer, here: worldbuilding.stackexchange.com/questions/181423/… $\endgroup$
    – DWKraus
    Apr 10 '21 at 11:55
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    $\begingroup$ @Drakio-X The previous question was different, but the same general approach could apply, with your males becoming glorified replaceable robots for the females. I'd consider something marsupial-esque, where the babies are birthed/hatched by females but the male cares for them/carries in a pouch. It would help allow the females to be even smaller (not much more than a brain with ovaries), especially if the females have a way to share blood/tissue without rejection problems. Maybe the females could be conjoined to a brother, but need a separate male for fertilization? Functional hermaphrodite. $\endgroup$
    – DWKraus
    Apr 11 '21 at 5:46
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    $\begingroup$ your biggest problem is it takes a while to learn to control a body, the more self control a species has the longer it takes, so the male will spend years learning to control its body then the female will take years to learn to control the male body. you also have the issue of how it feeds itself during its second "infant level of control" stage $\endgroup$
    – John
    Aug 24 '21 at 12:40
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    $\begingroup$ @Drakio-X absorbtion without rejection already occurs in nature, see the angler fish. $\endgroup$
    – John
    Aug 24 '21 at 12:40
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    $\begingroup$ @Drakio-X they lost the part of their immune system that causes tissue rejection, which is wild. royalsocietypublishing.org/doi/full/10.1098/rsbl.2019.0594 $\endgroup$
    – John
    Aug 24 '21 at 22:14
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Anatomical considerations

If you’re looking at quite deep integration (fused circulation/nervous system) then you are constrained by the fact that the brain issuing motor orders must be somewhat proportionate to the body executing them. Assuming you don’t just want a big female head on a male body, you have a couple of ideas you could consider:

  • Not all animals use an explicit spatial map-like representation for their motor control (a somatotopic map); the octopus delegates this fine topography to large neural bundles in each arm, while the central brain only issues high-order commands. This means that the brain can remain relatively small (only 1/3 of an octopus’ neurons are in the central “brain”) and somewhat detached from the practicalities of how movement is mechanically achieved. Your female may have a similar role, with the bulk of pattern-to-muscle details entrusted to the male, so that her brain would not need to scale with the male’s body size
  • You may not need or want directly fused nervous systems anyway. The way in which the female directs the male could be by compulsion, guidance or persuasion. Imagine that the pair bonding process releases powerful hormones that make the male deeply invested in keeping the female happy, or satisfying her wishes. This relationship might be closer to the commitment of social insects to the hive: a forager bee isn’t following commands from the queen, she just really really wants to collect pollen to take home.

But why does the male not just think for himself? One option is that stupidity is used by this species as a sexually selected trait. Putting yourself (and your genes) at unnecessary risk is pretty much the point of the male sex: if you can survive despite being this dumb, you must be really strong, fast, resilient etc. As long as the hobble trait (stupidity) does not manifest in females, it’s a perfect tool of hyperselection. So imagine a teenage phase in which unpaired males just out-idiot each other by swallowing Tide pods and taking selfies next to predators and what have you, which for some reason the ladies find irresistible. After a boy finds a girl, she keeps him straight with good advice and potentially a bit of octopodean mind control.

Reproduction

I can only respect an answer that brings up hypodermic traumatic insemination, but I really don’t think it makes any sense in this scenario. This is a last-resort solution in some promiscuous species with high male-male competition and very adversarial sexual selection practices, and is detrimental to females (as you might imagine), which fits poorly with a symbiotic relationship with stable pairs. The most obvious solution seems to me just boring old external fertilisation, where females release eggs after pair bonding and males inseminate them on their own time after collecting them. Seahorses had been brought up and are probably the best example.

If creating eggs makes the female really particularly vulnerable, this may explain why they developed a relationship so dependent on the male, and why they would evolve a mechanism to select a male who commits physically to protecting them through the next “pre-pregnancy”; the male, in turn, by bonding to the female, ensures that the egg she finally produces is available to be inseminated by him and not a random other male she fancies.

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  • $\begingroup$ giving up control is not unprecedented, angler fish do that, of course they do it by dissolving their own nervous system. $\endgroup$
    – John
    Aug 24 '21 at 12:37
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    $\begingroup$ That’s true, but the anglerfish male doesn’t need a functioning motor control system, while this chap does. Either he retains his motor cortex, or she finds a way to map hers to his muscles (very hard post-development!) $\endgroup$
    – Guest
    Aug 24 '21 at 12:43
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    $\begingroup$ actually remapping neurons can be done, insects do it, you just loose function while it is being done. $\endgroup$
    – John
    Aug 24 '21 at 12:48
  • $\begingroup$ This is a really good answer. $\endgroup$
    – Drakio-X
    Aug 24 '21 at 22:06

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