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The frog-into-fish is an implausible-seeming animal that I'd like to add to my world

It is born as an aquatic frog. They grow for a while in this form, before metamorphosing into a larger amphibious fish. The fish stage has a skeleton, paired fins, and gills like most other fish. It appears to be a lobe-finned fish, similar to a coelacanth

Could such a metamorphosis take place with normal vertebrate development systems?

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    $\begingroup$ I'm not sure to understand : do they grow directly from tadpoles to fish, or do they go through tadpoles, frog then fish? $\endgroup$
    – Tortliena
    Feb 19, 2022 at 23:15
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    $\begingroup$ Yes why not? Is it possible? Everything is possible! $\endgroup$
    – Drien RPG
    Feb 19, 2022 at 23:17
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    $\begingroup$ Frogs don't have wombs. And fully aquatic amphibians which retain their gills into adulthood do exist, such as the axolotl, Ambystoma mexicanum. (Most fully aquatic amphibians don't have gills as adults, but rather perform gas exchange through their skin.) $\endgroup$
    – AlexP
    Feb 19, 2022 at 23:32
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    $\begingroup$ This seems very similar to mudskippers. $\endgroup$
    – Halfthawed
    Feb 20, 2022 at 0:26
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    $\begingroup$ Are these literal frogs, or are they merely frog-like alien creatures? If the latter it seems not so implausible, you just need to figure out an ecological niche where those two body plans would be advantageous. $\endgroup$
    – N. Virgo
    Feb 20, 2022 at 2:10

2 Answers 2

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No, not in terms of species

Frogs are amphibian animals, fish are not. They are not related at all. A frog will never become a fish, it could behave like a fish.

Neoteny caused by a mutation in the reproductive organs

The frog's offspring resembles a little fish actually. You'd need a frog with neotenic properties. Your frog will remain in the tadpole stadium. A mutation could be the cause: suppose this mutation lets the tadpole mature its reproductive organs. After considerable time, evolution of this new "frog" (it really is still a frog !) will cause further degradation of the limbs and its overall shape and respiration system will become more fish-like.

enter image description here

https://en.wikipedia.org/wiki/Neoteny

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Sort of.

"Normal vertebrate development mechanisms" don't seem to allow much by the way of bone reduction. Bones get bigger, or they get porous, or they break (autotomy suggests an option, but I don't think that counts as a fish rather than an amputated frog). *Note that tadpoles lack not just bone in their tails, but even cartilage, apart from elements that remain in the adult frog. (cite)

I would go with a different sort of 'metamorphosis'. You have a species of direct-developing frogs that somehow retains the plasticity to produce a fish-like form under some circumstances (compare the evolution of axolotls ... nothing I can cite will be very exact, and making it a precise copy of a lobe-finned fish tests the reader's credulity). The frogs are capable of a very resource-intensive pregnancy with just one egg, which is favored in this case. The egg hatches internally, yielding the fish form, which then proceeds to eat its way out of the mother, exiting by the mouth, then consumes the remainder of the carcass.

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