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I have seen a lot of headcanon and world building regarding the biology of sea monsters, and as a lover of biology, I thought I would give my own take. What follows is a mixture of analysis, headcanon and straight speculation on the nature of sea monsters. It is split up into several sections, most of which have a central question that they try to address. I hope anyone reading enjoys and maybe even learns something about actual biology in the process.
Evolution
My goal for this section is to first give my best guess as to where on the tree of life sea monsters would fit and then explore what can be inferred about their subsequent evolutionary path.
Broadly, sea monster’s placement on the tree is straightforward. Between the four limbs, tail and ability to breathe air, sea monsters are clearly members of the superclass Tetrapoda which encompasses amphibians, reptiles, birds and mammals. This group evolved from our common ancestor with lungfish which have four bony fins (equivalent to our limbs) and lungs. Sea monsters are not members of the ray finned fish (this is the group you think of when you hear the word “fish”); though, their design clearly takes inspiration from this group.
Within tetrapoda, their placement is less immediately clear. The problem is that tetrapods have a habit of moving back into the ocean. Extant mammals alone have returned to the water three separate times. If sea monsters are an example of a return to water then they could be embedded in multiple places. However, I do not believe this to be the case. Animals that return to the ocean tend not to be able to breathe underwater while sea monsters clearly can.
Instead, my best guess (and personal headcanon) is that sea monsters are not closely related to any other tetrapod and in fact represent the deepest split in the tetrapod lineage. In other words, they are not amphibians, reptiles or mammals but instead their own group that split off even earlier during the transition from sea to land. Aside from still being able to breathe underwater, the clear resemblance between sea monster and fish scales is also evidence for this placement. I suspect that sea monsters maintained their scales from our common ancestor with lungfish while the rest of tetrapoda split off and did away with them. Overall, this would place our common ancestor with sea monsters at about 350+ millions ago when fish were first stepping onto land. Appropriate.
An important consequence of this placement on the tree is that during the many, many millions of years of sea monster evolution, only one species has survived. Compare this to the rest of tetrapoda which has exploded into over 30,000 living species. That is a stark difference; however, it is not unique in evolution. Take the coelacanth for example. It is our nearest cousin after the lungfish, putting its closest common ancestor with anything else at about 420 million years ago. After all that time, only two species of coelacanth are still around.
With no other living references anywhere close, it is difficult to say how sea monsters have evolved between our common ancestor and their modern form. We do not have a fossil record to work with either. Nonetheless, I suspect that sea monsters have been amphibious for their entire evolutionary history. As previously noted, moving permanently onto land would have cost the sea monster’s ability to breathe water upon return. Moving permanently into the ocean on the other hand would have cost them their legs from evolutionary atrophy.
That finishes up my thoughts regarding sea monster evolution. The remaining sections are about modern sea monsters and what can be understood about their biology from observation.
Breathing
Unsurprisingly, a creature that breathes both air and water has an unusually complicated respiratory system. My goal with this section is to answer two principal questions: how do sea monsters breathe air and how do they breathe water? The first one is straightforward; they use lungs like every other tetrapod. The second one however is more complicated.
The most common answer I have seen is that they use gills, and that makes sense at first. Some lungfish use a dual system with gills and lungs; sea monsters have a similar set up deriving from the same ancestral structure. There is just one problem. Sea monsters do not have gill slits. Seriously, go find a picture of one and look at the neck. It is all smooth scales. There is nowhere for slits to be. Even worse, breathing with gills would require the throat of a sea monster to be full of water, but all evidence points to their throats being full of air even when underwater for long periods of time. Notice that whenever a sea monster goes onto land, he or she does not start coughing up water before breathing air. Or recall that Luca was blowing bubbles of air while underwater. Luca had been underwater for at least a day when he did this, and his throat was still full of air!
Fortunately, there is an alternative mechanism for breathing that takes all of these strange facts into account. Even better, it also explains another peculiar feature of sea monster physiology, their “hair.” Sea monsters have external gills. Most animals with gills have internal ones that they pass water over using slits. However, some creatures like the axolotl (which also has fully functional lungs!) evolved to have their gills exposed directly to the water, giving them a structure that looks not unlike a mane. The pads on the back of a sea monster's head and neck fill a similar role. The thin, numerous fronds create a large surface area for gas exchange while also providing some protection for what would obviously be a very sensitive structure. I like this explanation a lot in part because it closely resembles an actual animal and because it incorporates the visual design of the sea monsters.
That answers both of my initial questions, but before I finish out the section, I want to point out that sea monster lungs might be pulling double duty as swim bladders. A swim bladder is a gas filled sack that ray finned fishes use to control buoyancy. It is an incredibly useful organ and is partly why ray finned fish are so prolific. As previously noted, sea monsters are not ray finned fish, but the swim bladder originally evolved from lungs. I do not think it is much of a stretch to infer that sea monsters convergently evolved a similar function. At the bare minimum, they can consciously control their buoyancy by simply breathing out excess air, allowing them to sink faster. They might have an additional ability to increase the air in their lungs via gas exchange with the blood to achieve the opposite effect.
The Change
Allow me to get the obvious out of the way. The change uses magic, and there is no way around this fact. I mean their tails literally disappear. Forget biology, that does not even make physical sense. As a result, I am not going to try explaining the mechanisms underlying the change and simply accept that it happens. Instead, the main question of this section is not how sea monsters change but what about them changes? How much of a sea monster’s body is affected? Is it purely external or does it go deeper? While the evidence available is not complete, the vast majority points toward the former. The change is external.
First and foremost, we have excellent evidence that the change leaves the nervous system almost completely unaffected. Personality, memories, emotion, motor function: none of this is different between forms. The change does not even seem to make the nervous system stutter. Sea monsters can continue moving and speaking like nothing is happening. The only part of the nervous system that does change seems to be the nerves inside of appearing and disappearing body parts like the tail. Otherwise, observation suggests that it is entirely conserved.
The fact that the nervous system does not change is by itself strong evidence that the change does not affect the internal body. The nervous system runs the entire body in a deep way, and suddenly changing the organs without changing the brain is a recipe for illness at least initially. Luca does not become sick during his first day on land, so his brain has not been thrown off by suddenly having different organs to work with.
Nervous system aside, there is a lot of other evidence that the internal organs and systems are unaffected. For example, sea monster lungs work just fine in aquatic form; there is no need for them to be different in human form. Or consider the digestive system. Sea monsters have different teeth depending on form, suggesting a different diet above and below water. However, if the digestive system changed to accommodate the change in food, this would lead to problems whenever a sea monster switched forms. Food does not digest instantly, so food ingested in one form needs to be processable by both. The best way to do this is to just have the same gut in both forms. Finally, the skeleton and musculature are for the most part visually unaffected (excepting the tail and other minor features). Sea monsters have essentially the same shape in both forms, and the way their joints and muscles move does not appear different either. It also just makes sense that muscles and bones would not change as that would seem like a recipe for a face plant.
The evidence that I have described does not cover every biological system that a sea monster must have; however, it points uniformly to the change being external. Consequently, I am going to conclude that the entire internal physiology of a sea monster is unaffected. Their hormones, metabolism, immune system and other organs do not change between forms. This requires substantially less magic, and I also think that it is more interesting for them to differ from actual humans even when disguised. It helps keep them more of a distinct species instead of just some humans with alternative color palettes.
Miscellaneous Observations and Conclusion
This last section does not have an overarching question or theme tying it together. It is just for all the observations and ideas that were too small to get a section.
First up is reproduction, starting with the question of eggs. My placement on the tree puts sea monsters before the evolution of hard shelled eggs. Hard shells evolved to keep eggs from drying out on land, a problem that an amphibious creature would not have. Consequently, I believe that sea monster eggs are soft like fish eggs, assuming of course that they do lay eggs. Live birth has been convergently evolved many times. Sticking with the idea of eggs though, they are probably quite big—maybe the biggest soft shelled eggs on earth—and they probably only come in batches of one or two. Sea monsters are complex, expensive and long lived, all of which means small brood size.
Moving a little forward in development, it is now time to talk about babies. Luca lived his entire first decade underwater, so I think it is safe to say that sea monsters spend their first few years underwater. Consequently, I suspect that they barely even have limbs when first born; they just look like tadpoles with little nubs. Full arms and legs come later. I am also completely confident that juvenile sea monsters are as completely helpless and goofy as human babies. Sea monsters clearly fill the same culture driven, highly adaptive niche that humans occupy on land, and that requires having children that are not hardwired to do anything but can learn to do everything. The consequence is babies that probably cannot even swim correctly when born. I would not be surprised if parents have to help children get out of their eggs. The child is also definitely not capable of eating by itself, and instead the parents have to help. Think birds regurgitating chewed food.
Onto something completely different: phantom tail. Phantom tail is a direct result of the central nervous system remaining the same during the change. The brain is still wired to expect a tail even if it is not there, and so it feels like it is (Fun fact: the vast majority of our experience is cooked up by the brain and does not come from external sensory data. The brain is just really good at predicting reality). Conversely, a sea monster that first does the change would not be able to tell the difference between his or her middle and ring fingers. The two fingers have only ever been one and learning the difference takes time and neurological adaptation. The sea monster will eventually be able to tell the difference of course. He or she might even get phantom fingers in sea monster form, but that would require staying exclusively on land for at least months, possibly years.
Sea monsters probably do not sweat, at least nothing like humans do. Obviously, sweating makes no sense underwater, but even on land, humans are the only species out of thousands that use sweat to cool down. I doubt that sea monsters would convergently evolve that, especially when they can just jump in the water if it is too hot.
Finally, it is clear that the Paguro’s reluctance to go on land is a very recent development, likely a result of increasing human encroachment. Sea monsters have far too many traits that are clearly designed for living on land (i.e. legs) not to have spent significant time above the water in the last few hundred years. Alberto’s lifestyle is almost certainly closer to how sea monsters have lived for the last few million years. They are amphibious tetrapods, and there is no getting around this fact.
Well, that wraps all of my thoughts. This was quite fun to think about, and I hope anyone who read this far learned something and enjoyed doing it!