Turtle Island Size and ecology

Question

Disclaimer

I've had a look and the other Large Turtle posts seems to be world based or assuming the turtle already exists.

Question

My question is how large could an island sized turtle reasonably get and what ecology would reasonably be needed, eg large amounts of surface algae/plant life.

What have I done?

Based off of a mostly wild guess the turtle would need to be around the size of 1-2 city blocks if we presume the "locals" can tame and somewhat influence the turtle to make landfall periodically. This is where the locals would gather supplies (wood for fire/repairs and the like). I'm imagining that the locals diet would consist of a large amount of fish and sea-based foods which can be acquired from the turtle-back.

Presumptions/Goals:

• The turtle will naturally want to swim on the surface, this won't be a "trained" thing the locals teach it.
• It should be large enough to "house" a "small tribal settlement"
• The "locals" know it's a turtle
• The settlement are happy to work symbiotically to keep the turtle alive, even as much as "taming" it to cause it to make landfall periodically for "supplies"
• Ideally some form of tropical habitat, but not required if another habitat makes more sense for reasons
• Locals have tribal or at most bronze age level technology with farming being little to none due to being primarily turtle based and no real desire to "settle" on land.
• Presume everything is earth like regarding planet size and maths assumptions

Answer 1

Any animals with classical shapes as we know them could probably not grow much larger than the blue whale in water, or the largest dinosaurs on land. That's because those creatures are/were already pushing the envelope when it comes to the Square-Cube law.

You can't put a city block atop a blue whale, and I am tempted to stop here and just leave a quote from my own profile in this site:

Your fictional creature is too big to stay alive. Just handwave it!

However, a giant tortoise might have some advantages when it comes to large sizes. For starters, they could evolve a more efficient respiratory system than mammals. The reason being that they could copy mammalian lungs given enough time to evolve, and on top of that they have already mastered the ninja art of breathing through the anus. In this way they could have an extra pair of lungs if they can get their rear end out of the water, and they can also take oxygen from the water itself.

Another way this could work is that the turtle could use its shell as a natural flotation device. Since it should only feed on plankton there is not much need for it to be a diver like whales. This allows it to be somewhat hollow, thus for the same mass as a whale, it would be considerably larger. Notice that since your villagers will put structures atop the shell, the turtle might sink a little - they may have to compensate by adding flotation devices of their own to the shell.

As for how much they need to eat, let's nerd this out. A blue whale (let's assume a 100 tons one) eats about three point six metric tons of krill per day during feeding season. The amount of calories in krill varies; By googling, I found that some sources have them at about 1 calorie per gram, others mention up to 6 calories per gram. Being pessimistic, that means a blue whale eats three million, six hundred thousand calories per day.

So how large a turtle could be to have those caloric needs, so that it can feed where a whale could? According to this handy table, a turtle's caloric needs seems to grow almost linearly with its weight. A 100kg turtle needs 2,275 calories a day. Mathing it out, a blue whale diet could feed a 158 metric tons turtle, or around 50% more massive than a blue whale. Let's round it down to 150 for now to facilitate calculations.

Again, we want the turtle to be kinda like a buoy. Whales are very fusiform. The fictional turtle here more likely flat like a couple frisbees connected to each other. A whale is as dense as water, which means that a 100 tons blue whale has a volume of 100m³. The equivalent turtle will displace 150 m³ (remember, it is about 50% heavier). Suppose it is flattish, and the shell is two meters high on average - for a very circular shell, its radius would be... about five meters (about 16 feet in freedom measurements). That is not impressive at all... That means about 78 m² of surface area. That's not the area of a city block - that is the area of a regular apartment in some areas of cities such as London and Tokyo (YMMV).

We need to amp up this turtle. Make it twice as thick (doubles the weight), and double the radius too (quadruples the weight). We end up with a turtle that weights a thousand and two hundred tons, thus requiring 28,800,000 calories per day, and it will have a surface area of about 314 m². That is still far from a city block size (in Chicago for example a city block may typically be 100m $\times$ 200m, so 20,000 m²) and you are going to have a lot of problems feeding it. You may stop here, and substitute a fleet of turtles for a really large one. They may have coevolved with humanity, eating part of the humans' catch as well as their sewage in order to meet their caloric needs (in exchange for the shelter and transportation they provide). Also notice this creature is waterbound - it's enough of a Kaiju that it can't walk on land.

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So far we have only been dealing with caloric needs and weight. We have not even nerded out the circulatory and nervous systems. You could handwave the latter by taking inspiration from the stegosaur, which was larger than a SUV with a peanut-sized brain. But the more detailed look you put into this creature, the more outlandish it becomes, until you simply have to stop looking for hard realism and start handwaving some bits in order to keep things fun.