EDIT: (Important) M. A. Golding commented that the regalecid method of locomotion would be unfit for a constricting predator, so I have completely redone the taxonomy of the sea serpent. They are now eels, closely related to moray eels. The sea serpent taxon has been upgraded to family Serpentimaridae, as opposed to the old subfamily Serpentimaria. I don't have time to draw up a dendogram right now, but they're in the suborder Muraenoidei, in a clade with moray eels.

I think that the current lack of answers is because I have not alluded much information as to the physiology of these animals. Here is a rough sketch I made in Paint (Originally a preparatory draft for a real illustration of sea serpent anatomy):

enter image description here

Note: the reproductive and excretory systems are not shown here, but do not affect their potential size anyway so it shouldn't matter.

So, if the heart needs enlarging, or I should have a bundle of ganglia towards the back as an extension of the CNS, or any other amendment, please let me know. Hopefully, I'll get some answers now that I have clarified a bit more.

This is currently the sixth question about my ongoing worldbuilding project on viable fantasy creatures. I'm too lazy to link all of them here, but the last two were: What kind of animals are my trolls? and How can I explain the evolution of my giants?. You can find links for the rest at the latter.

Now, from giants and trolls we progress to the largest creature in my project, the sea serpent (Subfamily Serpentimaridae). These beasts are not serpents but fish, in the suborder Muraenoidei.

I want these serpents to be massive, at least the length of a blue whale - and upwards from there. Seriously, if the answer is "600 metres", then I'll take that. But, I rather doubt that a fish could grow to a length of 600 metres (Feel free to prove me wrong), so I'll have to find out what's rational and what's not.

Sea serpents are constricting predators, like boas, and use their musculature to crush whales - and, on occasion - ships. They also have pharyngeal jaws, like their close relatives the muraenids, which allow them to swallow a rowboat whole.

I haven't figured out what oceanic zone they'll inhabit, but any is fine for me really. Deeper zones would be convenient as an explanation for why they've remained undiscovered, except for reports by sailors throughout history. They need to be large enough to be able to create waves with lashes of their tail, as well.

To summarise: How big could Muraenoideian sea serpents be?

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    $\begingroup$ to clarify are you asking how big they could be based purely on the biological mechanics of a creature that big or should this creature also be viable evolutionary speaking? i.e. is this a world in which big monsters can exist just because or must they have evolved as darwin intended? $\endgroup$
    – Ummdustry
    Jun 8, 2018 at 18:21
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    $\begingroup$ also regarding boa-constricting blue whales, its worth noting that whales (blue) regularly survive depths of 100 metres which roughly equates to 11 atmospheres of pressure or about fourteen times what a boa constrictor can do. though its still possible this that serpent could ensnare and exhaust the whale (preventing it from rising), poison the whale or perhaps simply "bite it to death.". alternatively it would only attack in the depths using the high pressures there in conjunction with its muscles to constrict the whales. $\endgroup$
    – Ummdustry
    Jun 8, 2018 at 18:29
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    $\begingroup$ @Ummdustry Yeah, I'm asking if this creature could a) exist and survive at the desired size and b) evolve, in our Earth's evolutionary continuum, plausibly. Interesting point about the blue whales, I'll keep that in mind. $\endgroup$
    – SealBoi
    Jun 8, 2018 at 18:46
  • $\begingroup$ @SealBoi - fish of the family Regalecidae have sometimes been mistaken for sea serpents but probably can't move like sea serpents supposedly move, and certainly couldn't kill by constriction. What you need are beings, whether fish or not, with body plans similar to eels or sea snakes and also similar to giant constricting snakes, so they can swim in the sea and constrict like constricting snakes. It may be noted that sperm whales can dive over a kilometer deep and can bite boats in half. Companions of an attacked sperm whale could probably break a giant sea snakes spinal column,.. $\endgroup$ Jun 8, 2018 at 21:15
  • $\begingroup$ ... and crush in its ribs and crush its skill. An ordinary female sperm whale might have a lower jaw maybe 8 feet long and be able to open her mouth 90 degrees, giving a gape of up to 11 feet. That should enable her to bite down on a large part of the thickness of a creature thin and flexible enough to wrap itself around another sperm whale, and bite hard enough to do serious damage to the ribs and spine. $\endgroup$ Jun 8, 2018 at 21:23

1 Answer 1


To answer your question, consider how thick would be the body of the largest animal you intend your sea serpent to prey on by constriction.

If, for example, the prey has a thickness of 1 foot and is perfectly cylindrical, it will have a circumference of 3.14 feet. If the sea serpent wraps around the victim three times and has length left over, it must more than 9.4 feet long. And the giant snakes known to science would usually be 10 to 20 feet long if they attacked prey a foot thick.

If, for example, the prey has a thickness of 10 feet and is perfectly cylindrical, it will have a circumference of 31.4 feet. If the sea serpent wraps around the victim three times and has length left over, it must more than 94 feet long. It could easily be 150 to 200 feet long accounting for the extra neck and tail lengths.

I point out that the longest prehistoric or contemporary animals known to science had or have lengths in the region of 100 to 200 feet. And some scientists find such lengths amazing because of various factors including the time it takes for signals to travel the length of the body along nerves which sometimes are single immensely long cells. Some scientists would claim the longest animal proven to have existed must have been the longest animal possible, while others would say that longer animals might be possible.


How thick can the body of the constrictor be if it is capable of bending enough to constrict around the body of the prey? Depending on the flexibility of the constrictor, what is the maximum possible relative thickness of the constrictor's body compared to the prey's body?

If a sea serpent is flexible enough to constrict a victim with a diameter of 10 feet, will the sea serpent be flexible enough to constrict prey with diameters of 5 feet or 1 foot, or will those smaller animals simply escape out of the oversized loops of the constrictor because it can't constrict tightly enough?

It seems to me that a growing sea constrictor will only be able to constrict animals within a certain range of body thicknesses. It will be unable to constrict thinner or thicker animals.

Once you have decided upon the body thickness necessary for your constrictor to constrict prey within the desired thickness range, you must then decide how your constrictor is proportioned. The length of your constrictor would range from X times its thickness to X plus Y times its thickness.

And what would be plausible relative proportions for your sea constrictor? You can study the proportions of various snakes that constrict. You could make your sea constrictor proportioned between the stubbiest snake that constricts and the most elongated snake that constricts. Or you can make a more generalized study of the relative proportions of all the long, narrow creatures in the world.

Or, if you decide that your sea constrictor should have a certain width, you can calculate what its body mass would be at various lengths. If it is carnivorous it shouldn't be much more massive than the largest known present day or prehistoric sea carnivores or it probably won't be able to feed itself.

Or, you can decide what is the largest mass sea creature it can swallow whole. Since it will probably usually eat several smaller sea creatures for a meal instead of only one of the largest it can eat, you might decide that the largest creature it can swallow whole is as large as the largest meal it needs, and then decide what body mass a creature would need for that amount of flesh to make a meal.

I think that a combination of those methods should enable you to decide what size your sea constrictors usually are, though of course there could be freak exceptionally small (boring) or large (terrifying) specimens.

I can't help thinking that even the smallest monster you might decide upon would be horrifyingly large.

  • $\begingroup$ Okay, I've worked out that the desired prey has a diameter of 13 feet, which would give it a circumference of about 40 feet. That'll be about 120 feet to wrap around, with 30 feet left over, so the sea serpent will be 150 feet long. I believe that Shastasaurus, the largest marine predator ever found, had a mass of about 65,000 kg. I've also worked out that the width of a boa constrictor is about 1 40th of its length. Therefore, my serpent is 1.25 m long. But I have no idea how to find out how dense it is, which is what I need to calculate its mass. $\endgroup$
    – SealBoi
    Jun 10, 2018 at 11:05
  • $\begingroup$ Edit: that should be 1.25 metres wide in the second-last sentence $\endgroup$
    – SealBoi
    Jun 10, 2018 at 12:31
  • $\begingroup$ Titanoboa is a little less than half that length so you might want to use it as a base. interestingly titanoboa was a fish eater so it might even serve as an origin for your creature. $\endgroup$
    – John
    Jun 10, 2018 at 13:26

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