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On a planet like the earth with its same gravity and ecosystem what is the biggest a boney sea creature could get both in terms of size and weight. I don't mean biologically but literally, since this creature would be fed by humans not going out to hunt by itself.

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    $\begingroup$ Are you thinking a boneless sea creature can be bigger? Why the emphasis on boned sea creature and not just sea creature in general? $\endgroup$
    – Nelson
    Commented Dec 15, 2021 at 1:51
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    $\begingroup$ Like cattle ? "Humans feed them" ? Whales are hunted down for their meat, but there is no way they can be kept and fed in captivity, they don't do well in captivity changeforanimals.org/whales-and-dolphins-in-captivity $\endgroup$
    – Goodies
    Commented Dec 15, 2021 at 2:01
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    $\begingroup$ Reference dinosaurs and blue whales. They can get "quite big". $\endgroup$
    – Kyle B
    Commented Dec 15, 2021 at 5:38
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    $\begingroup$ This is a good question: straight up vertebrate physiology. All discussions that I found about on constraints on whale size center around feeding strategy and food availability. If you take those constraints away, what is the maximum size the whale body plan can support? I took a look and this is not easily answered. $\endgroup$
    – Willk
    Commented Dec 15, 2021 at 14:51
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    $\begingroup$ Ultimate limit I would expect would depend on thermal dissipation. That is, how large can it be before it can't transfer sufficient heat to avoid handle the rate of denaturing/cooking due to thermal temperature increases as side affect of metabolic activity. $\endgroup$ Commented Dec 17, 2021 at 19:01

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I will honestly say that the limiting factor here is not a skeletal frame, but the size of the sea that they live in, that sea's ability to handle a huge output of feces, and the availability of food. Many or most creatures in the sea have figured out neutral buoyancy. Once that is established, if you creature doesn't have to move, then it can be merely a blob of flesh with a vestigial skeleton doing no other function than protecting a brain, or operating a jaw. The creature doesn't have to hunt, so it doesn't require mobility at all. What you will have is an enormous jellyfish with a few bones.

There can be an argument that respiration can be a limiting factor if there is a central gill/lung organ center, but your creature did not put this limitation. Oxygen must be able to reach every cell in the body, even if it is miles away. That won't be practical without multiple sources of air/water (whichever it breathes). A central heart should be able to pump the whole system eventually no matter how many miles of veins they have to go through, but the blood must have oxygen when it gets to the cells far away from the heart. Multiple hearts are also an option. So, as I said, the circulation system has to be able to replenish oxygen many times along its path through the circulation system.

This same limitation applies to sustinance, so your organism will need many "mouths" and stomachs to keep the blood nutrient-rich.

There is an animal which already fits this description: a sponge. It breathes and eats through thousands of pores, all supported on an amorphous skeleton. It doesn't have or need muscles or mobility either, the sea brings food to it. It doesn't have or need a nervous system, but there's it should be possible for your creature to support one. So, your question really is; how large can a sponge physically get? I don't know that there is a limit as long as the sea can feed it, provide oxygen, and remove waste.

Lymph, antibody, and other hormonal systems will also need to be duplicated in order for natural biological functions to operate very far from the organism's center.

Heat dissipation would limit most terrestrial animals however if the creature had a natural endothermic process within its body metabolic heat can be reduced to a waste chemical and carried away in feces, where likely some other symbiotic organism consumes it and returns it to food. Based on a comment below, this deserves an example. As suggested by IndigoFenix, there will need to be a fractal-like division of conduits for resources beginning with larger cavers, then propagating down to smaller pores. Here is why: Assume your animal carries away heat by the common water + ammonium nitrate reaction we all know from medical cold packs. The animal will naturally produce ammonia, and through respiration it will oxidize this ammonia into nitric acid HNO$\ _3$, which will then be neutralized again with more ammonia: $$\ NH_3 (g) + HNO_3 (aq) \to NH_4 HNO_3 (s) + 145.1 kJ \text{ energy.}$$ This heat energy must be near a place where it can be carried away, close to the dermis. So the larger pores—or caves—will be heated, and must be in an environment which favors heat removal by winds or ocean currents, or their human host. Possibly the human society uses this natural heat source in some symbiotic way, to run engines? But as you see this creates ammonium nitrate, which can now be carried through the system into the animal's core body, or wherever heat removal needs to happen. Ammonium nitrate is combined with water to remove internal heat through the reaction $$\ NH_{4}NO_{3}(s) \stackrel{H_{2}O(l)}\longrightarrow NH_{4}^{+} + NO_{3}^{-} $$

This reaction is endothermic and absorbs heat in the production of a solution of ammonium and nitric acid in water, called the energy of solution.:

$$ \Delta\textbf{H}_{\text{sol}}=\sum_{}^{}\text{(products)}-\sum\text{(reactants)}$$

The heat it absorbs however is less than the heat generated by forming the products, because we look to a table and find the Enthalpy of Formation of our reactants and products, then do the math:

$$\ \begin{array}{|c|c|} \hline \textbf{Compound} & {\Delta\textbf{H°}_{\text{f}}} \\ \hline \text{NH}_4^+ & -132.8 \text{ kJ/mol} \\ \hline \text{NH}_4\text{NO}_3 & -365.1 \text{ kJ/mol} \\ \hline \text{NO}_3^- & -206.6 \text{ kJ/mol} \\ \hline \end{array} $$ So $\text{-132.8 + (-206.6) - (-365.1) = 25.7 kJ/mol} $.

Life metabolic processes ultimately produce a net positive heat, but it is very possible to generate the heat near the body surface, carry the products to another area, and remove heat somewhere else. This accomplishes the thermal regulation brought up in another post.

As to weight, the creature would need to be near neutral buoyancy so its mass will equal the mass of the water it displaces. Structural support by the skeleton is almost irrelevant. Whatever the weight of that mass of water is, is the weight of your creature.

Consider this: your creature could have its heat-producing reactions primarily happening near the arctic and antarctic sea, then carry the ammonium nitrate thousands of miles away to the tropics, where the endothermic reaction takes place. A single organism could be manipulating global climate this way. There is no logical reason it could not.

Almost all life has some sort of symbiotic relationship, and this creature will likely need many. For example, a cow can not digest its main food source, grass. It is a rumnant, and it would die without the bacteria and other organisms living inside its rumen. The reason cows and other ruminants are constantly "chewing their cud" is because they are actually processing their grass back-and-forth between their mouth and stomach. It's an amazing and complex process that requires the animal to be specially designed so it can help the bacteria which break down the green plants for it. All the amazing details on rumination can be studied in simple terms at the US FDA here.

It is very easy to see a human population having a symbiotic relationship with this living sea as well, because it has an explosive and fertilizer in its thermal regulation system. Controlled harvesting of this would greatly benefit any agricultural society.

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  • $\begingroup$ Nice thinking. Remember that the "pores" will need to have substantial differences in size, with enormous open caverns branching into smaller tunnels all the way down to the microscopic level. If it gets big enough what you're going to wind up with is a living underwater city, with people carrying nutrients and waste in and out to sustain it. $\endgroup$ Commented Feb 14, 2022 at 8:37

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