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Im working on a project where I'm designing a theoretically possible planet with life and I had an idea for a massive sea animal (500-550 meters in length). Water animals get help from buoyancy but something of this size likely has too low of a surface area-volume ratio but I want to know if its possible, not likely. Food and space wont be a problem however gravity still will be. Even on my planet with 0.55x earth's gravity it may still have problems, is there anyway I could get around them? I've thought of using the material limpet teeth are made of for bones but I don't think that will be enough. If I cant make it this big, how big could it get? I've done some other stuff to increase size limits like adding an extra 3% oxygen to the atmosphere with a density comparable to earth, 1.06 bar.

I've already thought of work arounds for the problems that would come for something of this size and for the planet (An example being the magnetic field) so all I need to know is whether or not it could get this big.

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    $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – Community Bot
    Commented Feb 8, 2023 at 20:04
  • $\begingroup$ Welcome to worldbuilding! "Massive sea animal" is a bit vague. Must it even have bones? Must it feed in a particular way? How do you feel about jellies and worms? $\endgroup$
    – PipperChip
    Commented Feb 8, 2023 at 20:22
  • $\begingroup$ I think the easiest way to get something really huge is to make it not really one organism, but a colony of multiple organisms perhaps from several species working together. Something like a mobile coral reef. Is that sufficient for what you want? $\endgroup$
    – Fhnuzoag
    Commented Feb 8, 2023 at 20:29
  • $\begingroup$ As per @Fhnuzoag's comment, see big siphonophores. I suspect that one of your limiting factors would be what can eat them. $\endgroup$ Commented Feb 8, 2023 at 20:42
  • $\begingroup$ @PipperChip I did want it to have bones, I thought to use the same stuff limpet teeth use (goethite nanofobers wrapped in proteins) to have stronger bones that can hold more weight. The way it feeds doesn't really matter much, though I did think that it could filter feed like whales but not need the filters at all because of it's size, eating anything that comes it's way, possibly sucking them in like a grouper. $\endgroup$ Commented Feb 8, 2023 at 21:43

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If the sea creature spends all its life suspended in water, it doesn't matter what the surface gravity of the planet is. In that case you merely have to make sure that the escape velocity of the planet is high enough to retain an atmosphere for geological eras of time. Not the surface gravity, but the escape velocity.

And if Earth humans visit or live on the planet, you need to keep the surface gravity below about 1.25 to 1.5 Earth gravity, so the humans are comfortable and healthy enough.

If the sea creature ever comes on land, perhaps devastating cities like in a monster movie, than the surface gravity of the planet will be very important in determining its maximum possible size.

The largest land animals on Earth, with a surface gravity of 1 g, were sauropod dinosaurs.

One of the tallest and heaviest dinosaurs known from good skeletons is Giraffatitan brancai (previously classified as a species of Brachiosaurus). Its remains were discovered in Tanzania between 1907 and 1912. Bones from several similar-sized individuals were incorporated into the skeleton now mounted and on display at the Museum für Naturkunde Berlin;[16] this mount is 12–13.27 metres (39.4–43.5 ft) tall and 21.8–22.5 metres (72–74 ft) long,[17][18][19] and would have belonged to an animal that weighed between 30,000 to 60,000 kilograms (66,000 to 132,000 lb).

it would have weighed about 33 to 66 short tons.

Most of the largest herbivorous specimens on record were discovered in the 1970s or later, and include the massive titanosaur Argentinosaurus huinculensis, which is the largest dinosaur known from uncontroversial and relatively substantial evidence, estimated to have been 70–80 t (77–88 short tons) and 36 m (118 ft) long.[21][7]

https://en.wikipedia.org/wiki/Dinosaur_size#Sauropodomorphs

The legendary lost fossil of Maraapunisaurus fragilisimus, originally known as Amphicoelias fragilisimus has been estimated to have been up to 200 feet long and weigh up to 150 long tons or 170 short tons, though more recent estimates are smaller.

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

The lost fossil of Bruhathkayosaurus Matleyi has been interpreted as showing a length up to 44.1 meters (145 feet) and weights up to 175-220 long tons or 193 to 243 short tons, though much smaller estimates have been made.

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

The blue Whale Balaenoptera musculus is generally considered to be the largest species of animals on Earth.

The blue whale is the largest known animal to have ever existed.[35][36][37] The International Whaling Commission (IWC) whaling database reports 88 individuals longer than 30 meters (98 ft), including one of 33 meters (108 ft), but problems with how the measurements were taken suggest that any longer than 30.5 meters (100 ft) are suspect.[38] The Discovery Committee reported lengths up to 31 meters (102 ft);[39] however, the longest scientifically measured individual blue whale was 30 meters (98 ft) from rostrum tip to tail notch.[40] Female blue whales are larger than males.[9][41] Hydrodynamic models suggest a blue whale could not exceed 108 ft (33 m) because of metabolic and energy constraints.[42]

The record-holder blue whale was recorded at 173 tonnes (190 short tons),[47] with estimates of up to 199 tonnes (220 short tons).[48]

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

Let us suppose that under some conditions of life style and food availability some sea creatures on Earth could grow to weigh up to 199 tonnes or 220 short ones.

An animal 500 meters long would be 1,640 feet long. An animal 550 meters long would be 1,804 feet long. Most sea animals weigh approximately as much per unit volume as water does.

A cubic meter of water would weigh 1,000 kilograms or one tonne. A creature weighing 199 tonnes would have a volume of 199 cubic meters. A cylinder with a length of 500 meters would have a radius of 0.356 meters to have a volume of 199.08 cubic meters. A cylinder with a length of 550 meters would have a radius of 0.3395 meters to have a volume of 199.16 cubic meters.

So if your creature has a mass that would make it weigh 199 tonnes in Earth gravity it would be about 702.247 to 810.0147 times as long as its diameter.

the bootlace worm Lineus longissimus is very narrow. It is usually only 5 to 10 millimeters in diameter, but specimens have been observed up to 90 meters long, or 3,000 to 6,000 times its usual diameter.

In 1864, William M'Intosh described a specimen that had washed ashore in the aftermath of a severe storm by St Andrews, Scotland, which was more than 55 m (180 ft) long,[6] longer than the longest known Lion's mane jellyfish, the animal which is often considered to be the longest in the world. However, records of extreme length should be taken with caution, because the bodies of nemerteans are flexible and can easily stretch to much more than their usual length.[citation needed]

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

So some bootlace worms might reach 5,500 to 11,000 times their normal diameter.

A bootlace worm 10 millimeters in diameter and 30 meters long would have a volume of 0.00235619 cubic meters and thus weigh about 0.00235619 tonnes. So a bootlace worm weighing 199 tonnes in Earth gravity would have 84,458 times the volume and thus have 43.8747 times the dimensions, being about 0.438747 meters or 1.436 feet in diameter, and about 1,316.241 meters or 4,318 feet long.

I don't know what the life style of a creature like such a vastly enlarged bootlace worm would be.

Of course 199 tonnes or 220 short tons doesn't have to be maximum possible mass of a sea creature on Earth. Maybe creatures much more massive could live in the oceans of Earth if they managed to evolve a suitable life style for growing that large. Maybe sea creatures on your alien planet could grow much larger than on Earth for some reason.

If your creature is cylindrical, it should have a maximum possible diameter to avoid overheating. I doubt if any known animal on Earth with a perfectly round cross section has had a diameter of more than about 5 meters or 16.4 feet.

There are claims - accurate or not - that the largest male sperm whales have to live in only one hemisphere because they can't cross equatorial waters because of overheating - and they have diameters less than 5 meters.

Possibly your creature could live in very cold waters to avoid overheating.

And possibly it might have a oval or rectangular cross section of 5 by 10 meters or 5 by 20 meters, or something, to help it cool off.

Or maybe it is ectothermic, and doesn't have insulating blubber like marine mammals and so can cool off much easier than marine mammals can.

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