Here's the Discover magazine article Some of the first animals to venture onto land commandeered empty seashells for protection. Why isn't there a sea otter that uses a shell to store air as it sinks to the bottom? :bugeye:
First Ground Animals Borrows Shells
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Fraggle Rocker
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If he had a shell with enough air in it to make a serious difference in his ability to stay under water longer, it would by so buoyant that he wouldn't be able to descend very far. Remember, water pressure increases tremendously as you go down to lower levels, so the upward pressure on the shell would keep getting stronger as he went lower.
Then of course there's the question of just exactly which seashell is shaped so perfectly that it would hold a significant amount of air?
No. It gets weaker as the gas compresses.
The more air becomes compressed, the less buoyant it is -while air becomes slightly more dense as it becomes compressed, it displaces significantly less water - so overall, depth has a negative effect upon buoyancy of a fixed gas volume (at surface pressure).
That's why SCUBA divers wear Buoyancy Compensation Devices (BCDs) - kind of like life-jackets which pump air directly into them via a direct feed from the cylinder to compensate for the compression air spaces in things like the neoprene in their westuits.
Maintenance of buoyancy for novice divers is somewhat counter-intuitive - you put air in when you go down and let it out as you go up
(the goal is to use the BCD to maintain neutral buoyancy at all times - adding air on ascent can cause a runaway acsent - the shallower you go, the more the gas expands, the faster you ascend, the gas expands more quickly, becomes harder to control............. and the quicker you end up in "The Pot" (a decompression chamber in a hospital)
been there
done that
teaching licence revoked ( re-instated after investigation by PADI)
ps - this gives us a clue to the answer to the OP's question
I'll let someone else figure it out, but I'll supply a few clues
consider energetics
consider that marine otters are tool-users
consider their diets
consider how heavy objects are more easily lifted in water than in air
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Fraggle Rocker
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Well sure. But the shell itself is rigid and if it could be made airtight it would withstand the pressure of a dive to a certain distance before it cracked, and up to that point the air would not compress. I figured he was postulating otter engineers who would invent some clever way to seal it around a tiny little mouthpiece.
The shells would be fine for the depths of kelp forests (around 100m I think). The otters are adept at using seaweed to anchor themselves, they could simply anchor the air-filled shell to the seaweed strands! Kelp forest. Fraggle, I just had a thought, maybe the otter could fill the seabed shell with air from the seaweed air-filled bladders!! It wouldn't need to sink with it!Well sure. But the shell itself is rigid and if it could be made airtight it would withstand the pressure of a dive to a certain distance before it cracked, and up to that point the air would not compress. I figured he was postulating otter engineers who would invent some clever way to seal it around a tiny little mouthpiece.
Doubtful (and I'm being kind when I say that).
10m of water has equivalent pressure to 1 amosphere of air - so even at that depth the volume of the air would half that of the surface, because your at twice atmospheric pressure.
So at 100m the air would be 11 times as compressed - so the otter would have to struggle with a shell with 11 times its lung volume just to take one breath.
Thats the equivalent of an average human (with a lung volume of around 6L) trying to drag about 8-9 basketballs underwater.
I'd wager you'd find it difficult enough to swim down to the bottom of a 3m swimming pool carying just one.
Otters are good swimmers - but not that good.
The energy expended by the otter to submerge something with that much buoyancy just to gain a single breath would be huge (and would probably use more oxygen in the effort than it would get back) - so there is no energetic payoff for the animal to use that strategy.
There is a far greater payoff for the animal to pick up a heavy stone from the bottom and carry it to the surface to crack open the shells of the shellfish they eat - the stone is lighter in water so easier to lift when its in water - and less energy is expended in getting to the food inside the shelled animal.
In that way the water is working with the otter - not against it.
and funnily enough thats exactly what marine otters do
If you were seeking common-sense, you've just found some.
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Well sure. But the shell itself is rigid and if it could be made airtight it would withstand the pressure of a dive to a certain distance before it cracked, and up to that point the air would not compress. I figured he was postulating otter engineers who would invent some clever way to seal it around a tiny little mouthpiece.
but even then, as soon as the otter breathed it it would revert to ambient pressure - and it would make it more difficult to submerge as it would retain fixed bouyancy all the way down
Okay, but what about the scenario of an otter using the air-filled bladders to fill a large shell already at the sandy bottom and tieing it down with seaweed strands?
takes some serious thinking and thumbs. not for otters...
No, I disagree. The otters would simply find a den to eat the seaweed. When the bladders burst after being bitten, the den naturally fills with air!
What advantage is there for the otter to spend time that it could spend foraging for food, forgaging for seaweed gas bladders (they arent all filled with air) instead?
It would mean that it could stay hidden with an underwater den. It's pups would be safe from black-back gulls and other predators such as man. The sea otter was hunted mercilessly in the past for their pelts, an elusive seaweed eating otter would remani undetected!
Enmos
Valued Senior Member
What do Sea-otters have to do with the article?
It was just a curiosity that crossed my mind..that's all..
Enmos
Valued Senior Member
Why Sea-otters? Sea-otters are mammals while the article is about early arthropods.
Why aren't you wondering why Sea-otters don't have more pairs of legs?
This'll make you lot laugh. I reckon it's the best suggestion yet for the $1million dollar prize Mermaid Fever Makes A Splash In Israel
But Otters are carnivores.
Mod note: No, I'm not laughing. Sea otters have nothing to do with the article in question, and now you’re linking to stories about mermaids. Thread closed until such time that you can develop a coherent train of thought.
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