[chris (OP)]

Christian has been in touch to ask:

When humans spend an extended period of time in zero-gravity, they lose muscle mass and bone density. Would aquatic animals such as fish or whales be immune to this effect, since they are already used to the buoyancy of the water?

What do you think?

[chris (OP)]

What a fantastic question. I'll need to have a think about this one...

[Colin2B]

They are not immune, but in water they don't experience microgravity.
The buoyancy lifts the animal's body, but the internal organs, bones etc are still affected by full gravity and push down against the other organs.
Much of our maintainance of bone mass comes from muscles working to move limbs agains resistance and the marine animals will still get that when swimming and that is how they have evolved so there is very little change. The problem for astronauts is that we have developed bone mass to support our weight so they need added exercise against resistance to maintain that mass.

[chris (OP)]

Definitely an @atrox or @SquarishTriangle question this one!

[Colin2B]

Worth noting that the largest bones of a fish are in the spine where the strongest swim muscles are attached. If the fish is allowed to swim it can still exercise these muscles.

[SquarishTriangle]

My understanding of physics is pretty basic but I agree with what Colin2B has said. There should still be gravity acting on animals in water. The difference between water and land is that there is an additional effect of gravity acting downward on the animal (weight of water above the animal) and a buoyancy force that is acting upwards (or downwards).

I am finding it a little difficult to imagine that a large water column with an oxygenated atmosphere could exist in a zero-gravity environment. Wouldn't the water all just randomly float away, leaving behind a dry whale? Or are we talking about a giant bowl containing water, oxygen and a whale? Either way, I don't think this animal is going to live long enough to experience bone density issues.

[chris (OP)]

In humans the stimulus to bone growth is impact loading. Standing, walking, running and jumping all apply forces along the bones, which respond by maintaining or even increasing mineralisation. A human in orbit experiences "weightlessness", analogous to a life spent swimming in the sea. The absence of bone-loading leads to rapid atrophy of both bones and muscles. However, I think this is compensated in aquatic animals by the fact that water is a very dense medium, so the work done to move through it probably provides adequate stimulus to the bones to maintain their strength.

[alancalverd]

As an alternative to strenuous exercise (which loads the water and CO2 processing systems of space stations, and makes the place smell sweaty) the Russians experimented with vibration therapy to impact-load cosmonauts' skeletons. Not sure why they have gone back to treadmills.

It's interesting to compare radiographs of policemen and soldiers. Pretty much the same sort of person, doing the same sort of job, but constant slow walking gives coppers a characteristic heavily mineralised pelvis and  femur, whilst soldiers crawl, run or drive, and have rather more "normal" bone density.