Flexible Sharks

Q: I once heard it said, "Just because sharks have cartilage doesn't mean they don't have backbones." Does that mean sharks DO have backbones? I thought their "skeletons" were really cartilage and not true bone, so I'm confused. - Lolly Lavee, Las Vegas, Nevada

A: Backbone and bone aren't necessarily the same thing. Backbone also means spine, or a structure running along the back to protect the spinal cord. Since sharks have this structure, they have a "backbone," but it's not made of bone; it's made of the tough, gristly substance known as cartilage.
In sharks, cartilage performs the same skeletal functions as hard bone does in bony fishes. The only real difference between bone and cartilage is that bone is hardened with calcium deposits while cartilage remains more flexible. In fact, the skeletons of all embryos and young vertebrates, including humans, are initially composed mostly of cartilage which later hardens into bone.