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I haven't been able to find much on this question. I am making a world that is fairly cold compared to ours, not quite a snowball planet but enough that the highest average temperature is 66 degrees Fahrenheit at the equator, with most regions drastically colder (43 Fahrenheit or so is the average). It also has slightly lower gravity than our planet, not by a drastic amount but around 80% of Earth's gravity.

Now, I am going through this by making a biosphere that progressively evolves for fun, and I reached a debate when looking at one of my creatures. Little needs to be known about this creature as all you need to know is that it uses hemocyanin instead of hemoglobin. This is because this creature lived in the deep ocean where it is generally cold, and it has now reached land and colonized it, currently being in the warmer areas of the planet.

Now, from my research, it seems to be that hemocyanin is much more inefficient than hemoglobin is, and there isn't any massive lifeforms on our planet that have hemocyanin blood, as most vertebras have hemoglobin blood. Minor handwavium is acceptable, but it may break suspension of disbelief if I have a brontosaurus-sized creature with much less efficient blood. I am also not asking for square cube laws, I am perfectly aware of them, I just want to know if hemocyanin provides size limitations and if so, what kinds and how to bend or fix these limitations with evolution.

So, in conclusion, what is the largest feasible size a creature using hemocyanin instead of hemoglobin could reach?

King of the Hounds
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  • For the readers less well acquainted with ye olde traditionalle units of measurement, 66 degrees Fahrenheit is about 15 degrees Réaumur. (And 66 °F or 19 °C is not so very far from the 80 °F or 30 °C which is the actual average temperature at the Equator on Earth.) – AlexP Nov 04 '21 at 02:35
  • I don't understand why it has to be hemocyanin when you're postulating a different planet and a different evolution. There are many proteins that could evolve to carry oxygen. Also, many deep ocean organisms have used hemoglobin at hydrothermal vents or at cold seeps and whale carcasses. – Mike Serfas Nov 04 '21 at 17:49
  • @MikeSerfas Well, I chose it to be hemocyanin and it made the most sense to me, so it is. And I also am not that well-versed in biology, I get my enjoyment from writing the evolutionary tree and niches the creatures occupy, making a new type of blood would be draining, no pun intended. – King of the Hounds Nov 04 '21 at 18:16

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Giant squids are pretty big.

giant squid! reallly!

https://www.nationalgeographic.com/science/article/140110-giant-squid-picture-hoax-ocean-animal-science

Ok, maybe not that big. But 700 kg is not chump change! And they have got blue blood full of life giving hemocyanin. And in the UK, haemocyanin.

Go ahead and have your big critters have blue blood. Hemoglobins vary hugely in O2 binding efficiency and they are all red. You could have hemocyanins do what you want them to do. Or assert that the blue moiety is vanadium, or cobalt. Or nickel.

Prior art: Would it be possible for mammals to evolve blue blood?

How could blood be purple and reasons for it?

AlexP
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Willk
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  • You Brits with your stuck-up fancy spelling, just gotta be different from the rest of us. (/s, obviously) – In Hoc Signo Nov 04 '21 at 02:42
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    To my knowledge, weight is much easier to deal with in an ocean than on land, isn't it? I thought perhaps oxygen requirements were different either. – King of the Hounds Nov 04 '21 at 03:53
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The square-cube law dictates that the size of an animal increases, it's metabolic rate per unit mass decreases linearly.

This means that if haemocyanin can support the needs of a small animal's metabolism, scaling up that animal will only make it easier for its oxygen transport mechanism to meet its needs.

This means that using haemocyanin instead of haemoglobin would not be a limiting factor in the upper size attainable by animals that use it. Maximum size would be dependent only on other factors such as bone strength.

Monty Wild
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There really shouldn't be any limit due to blood composition. If hemocyanin is less efficient than hemoglobin, you just use more of it, circulate it a bit faster, or have a slower metabolism - that is, your large creature is more like a sloth than a shrew.

Size limits of earthly creatures with hemocyanin blood aren't related to the blood (AFAIK, anyway), they're because the creatures are invertebrates, and so have the size limits imposed by exoskeletons (or no skeletons, as with molluscs), and lack of real circulatory systems.

jamesqf
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