Any biomechanical reason that no creature has evolved to take advantage of lighter-than-air flight?
Sua
I’m guessing because gathering together enough lighter-than-air gasses to pull it off would be very difficult. Plus, it’d be a sitting duck for quick and agile heavier-than-air fliers, like preadatory birds…
Darn, I misread the title and thought this thread was going to be about enormous breasts. 
Well the only lift-gas that seems available would be Hydrogen. This is some explosive stuff to be sure.
We could design a fire-breathing dragon along the lines of that book “The Flight of Dragon.” A dinosaur who had a real bad gas problem such a beast would be large but quite delicate.
Another possibility would be an air-going jellyfish.
Probably for the same reason that heavier-than-air craft displaced lighter-than-air craft in history. The more manueverable airplanes could tear the zeppelins apart. A blimp-creature is a…a…well, not a sitting duck, but an easy target for a predator. You’d have to evolve some sort of defenses that make it impossible for insects, birds, bats, or pterosaurs to make a meal of you. Aside from extreme toxicity, I don’t know what that might be. And of course, you have to consider that the blimps must eat. What do they eat, and how do they catch it? Some sort of filter feeding seems likely, but how does that evolutionary path work? A creature with web to catch small insects isn’t just going to start floating away one day, the intermediate organisms have to make a living too.
I guess the big question is, how do evolutionary novelties arise? It seems to me that there are all kinds of potential adaptations that could never actually evolve, because the intermediate steps don’t exist. It makes much more sense to build bones out of metallic aluminum than calcium phosphate. But organisms learned how to metabolise and use calcium and phosphorus long before they invented skeletons. The skeleton was cobbled together out of already existing metabolic pathways. But there was no reason for our protzoan ancestors to learn to use aluminum, so that whole pathway is closed. Same with wheels, fans, propellers, and anything that requires bearings. I can easily imagine mechanosynthetic “plants” that use windmill-like parts to generate their energy instead of the sun. But how can such creatures evolve?
So, a blimp creature would have to evolve from a creature that was already creating air bladders for some reason other than flight, and there would have to be some reason for the bladders to be filled with hydrogen instead of air, and there would have to be some method for the creatures to feed that is somehow evolved from how they used to feed before they started floating. It seems that the window for such creatures to evolve is very very narrow. At least for flight you can imagine climbing critters learning to glide and evolving into full-fledged flight. But how would floating evolve?
Actually, that’s exactly where I was going. A jellyfish uses energy obtained from food/photosynthesis to “crack” water into H[sub]2[/sub] and O[sub]2[/sub]. Expels the oxygen and diverts the hydrogen to its “bubble.” Floats off to take advantage of a completely open environmental niche - semi-permanent residence in the air.
Sua
Lemur, I think the jellyfish solves many of the “intermediate creatures” issue - filter-feeding, the air sac (though currently filled with that jelly stuff).
In fact, cracking hydrogen would likely be beneficial to the intermediate jellyfish, by increasing water bouyancy of the critter.
Sua
The semi-permanent residence in the air part might pose a problem for the evolution of such creatures. There’s not much easy to catch food up there. Insects and other winged creatures could fly circles around a hungry, non-winged, aerial jellyfish. I suppose sweet smelling, sticky tentacles might catch a few flies.
You don’t need windmills or other rotary parts. Start with a seaweed- your basic alge colony. Give it a few lucky mutations in the elastic fibers that hold it together. Now the action of waves tugging on it generates chemical energy- sort of the reverse of a muscle. Adapt your alge to live on land by the seaweed/ moss/ fern scenerio. You now have a plant that gets its energy from the wind. In a windy enough environment, it can even live and grow in permanently sub-freezing climates.
I don’t agree with some of the above evolutionary reasoning.
We live our lives submerged in an “ocean” of gas.
Go spend your life deep in an ocean of salt water and you’ll see lots of creatures using bouyancy. Many fish have gas-filled “swim bladders,” and when their muscles squeeze them into a small volume, the fish drops like a rock.
Bones and muscles are heavy. Fish use oil as their “balloons”. Archituthes (giant squid) fills it’s muscles with ammonia salts to achive negative buoyancy.
So, why don’t “hydrogen sky-jellyfish” exist? Who knows, maybe they did at one time, but they were wiped out in one ancient disaster or another (along with the whole line of Wheeled Crustaceans, radio-using Nutibranchs, and polar bacteria colonies which keep warm by concentrating U-235.)
I agree with those who said that LTA (lighter than air) flight doesn’t have enough advantages to make it worthwhile. Some parts of the ocean are full of plankton and other organic matter, and you can gather them effectively just by floating around. There isn’t any food in the air. As a means of mobility (hunting or running away from predators) LTA flight is too slow to be effective.
Well, maybe it makes sense as a method of spreading your offspring over a wide area. Many plants have evolved means of scattering seed over a wide area, having them carried by animals or wind. Still it’s a big stretch to evolve tiny hydrogen-filled sacks attached to each seed.