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I am trying to imagine a scenario where, on an abandoned terraformed planet, decapod crustaceans (more specifically, carideans such as shrimp) evolve lungs and colonise the land as insects have done. However, possessing lungs, they can grow to far larger sizes, with the largest growing to the size of moose, (but no larger because of their exoskeleton’s weight limit).

What I am bothered about is this: how could a crustacean evolve lungs? Seeing as they lack swim bladders and therefore having nothing for lungs to evolve from.

user98816
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    How did spiders evolve lungs? How did snails evolve lungs? – AlexP Nov 27 '22 at 21:37
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    @AlexP fun fact: most arthropods have what is known as an Open circulatory system, meaning that air is just absorbed passively by the body and not actively pumped through the body. I should know. – redfrogcrab Nov 28 '22 at 00:05
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    @redfrogcrab: Yes, most arthropods have what is known as an Open circulatory system, meaning that their haemolymph is not used as an oxygen transport system, and they use other means to transport the oxygen to the tissues. (And at least some of them do actively ventilate their tracheae.) The spiders and scorpions which have lyngs, however, do use their blood to carry oxygen, utilizing (AFAIK) haemocyanin for this purpose; and they do have hearts which do pump the blood. – AlexP Nov 28 '22 at 00:14
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    I'm delighted that you received a practical answer. But please be aware, it is not the intent of Stack Exchange to replace the process of education. Explaining how anything could evolve would violate the [help/dont-ask] book rule in spades. You basically asked us to provide, what, two years of education? I suspect this question was better suited for [biology.se]. – JBH Nov 28 '22 at 02:26
  • What's wrong with treating crustaceans as arthropods? – DKNguyen Nov 28 '22 at 18:40
  • @DKNguyen Crustaceans are a kind of arthropod, but the question was about crustaceans specifically, rather than insects or arachnids (who evolved their lungs separately.) –  Nov 29 '22 at 00:11
  • @JBH isn't the point of Stack Exchange to leave answers that can later be found by others looking for similar answers? I just stumbled upon this question and thought it was interesting. Why gatekeep? How is this question any different from this or this in intent? Obviously, neither asker is asking for two years of education. – sleighty Nov 29 '22 at 00:27
  • @sleighty What we have to work with are words and the rules Stack Exchange and the Moderators have placed in the [tour] and the [help]. The first link you suggest isn't as unbounded as you may believe. I do not believe it violates the rules. Your second link differs from this by asking could, not how. It is not our job to read the OP's mind or to assume what he/she meant. We have only what they wrote. People spend their lives trying to undestand evolution, and my goal is to help a new user to use this Stack better. – JBH Nov 29 '22 at 04:33

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Crustacean lungs have already evolved. This is a solved problem, as there are already numerous air breathing crab species, such as coconut and hermit crabs.

Daniel Porteous
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