Fish gills, AS Biology

When first hatched, the young species of fish are less than 2mm long. Explain how these young fish get enough oxygen to their cells without having gills.

Ik the answer is about SA:V, but without gills how is oxygen taken in/seperated from water?

It is probably to do with diffusion across a membrane down the gradient of partial pressures of gases. So if the partial pressure of oxygen is higher in the water than in the fish, it diffuses through. This is only efficient when you have a high SA to volume area, hence why you need gills in bigger fish

Yeah, it's the same as the why transport mechanisms are unneeded in certain organisms, like flatworms. As they have a high surface area to volume ratio, each cell is close to the surface, thus, oxygen can directly diffuse down concentration gradient, into the cells.

But, where is this oxygen? Like, water is H2O, how does fish take the oxygen away without gills? (or im being stupid and missing something basic?)

Aah, well, I suspect it'd be the oxygen dissolved in the water. Maybe there are certain enzymes on the fish's surface which extract the oxygen from the water. LOL, tbh, you're not being stupid, it's a confusing question. But i THINK the expected answer would be the diffusion and SA:V thing. Just write that, and a few things about the cells being close to the surface and the thin, flat shape of the fish, and I think you'll get the marks.

Source: I did a similar question a few days ago. But, on flatworms, which, again, live in water.

That's right, but no enzymes just simple diffusion of dissolved gases in a medium. When gases dissolve in a liquid you express it a partial pressure of the gas instead of concentration, so when the partial pressure in one medium (water) exceeds the partial pressure in another (fish) you just get net movement of the gas across into the fish cells where it remains dissolved into the cytoplasm.

So, in water, there are actual O2 molecules too, not just H2O ones?

That's right, but no enzymes just simple diffusion of dissolved gases in a medium. When gases dissolve in a liquid you express it a partial pressure of the gas instead of concentration, so when the partial pressure in one medium (water) exceeds the partial pressure in another (fish) you just get net movement of the gas across into the fish cells where it remains dissolved into the cytoplasm.