Fish skeletons

[Notmyrealnam]

My son, who is 8, was looking at a piece of salmon and asked: Why do fish have tiny little bones? I told him that their skeletons don't have to hold them up against the force of gravity. He said (trying this idea out): They don't have to be strong. I said: Salmon do have to be strong enough to swim upstream against the current, so they have to have muscle, but maybe not big bones. He said: But what if they get hit by a rock? I thought he meant a rock falling through the water and explained that rocks don't fall with as much force underwater. But he then asked: What if they get hit against a rock by the current? I told him: I guess their ribs might break, but maybe that bad an accident doesn't happen as often as on land. I didn't get into the idea that some fish bones are cartilaginous (and therefore flexible), and that maybe fish ribs aren't protecting such vital organs as ours (I don't remember much about fish anatomy). Could anyone help me with this continuing conversation?

[lugus]

No fish don't need a skeleton to hold them up but they do need a solid sructure to flex thieir muscles against. Without it they would never have evolved into such efficient swimmers. They do also offer a degree of protection to the internal organs in exactly the same way as ours.