Aaron Slee of Balclutha Primary School asks :-

Why do sharks have cartilage rather than bones?

Steve Dawson, a marine zoologist at Otago University, responded.

With the exception of squids, which can sprint at high speeds over short distances, all fast-moving marine creatures have some kind of "skeleton" to support their muscles. They don't need to support their own weight, as land animals do, because the water does that.

Muscles work via contracting, and, for efficiency, they need something to contract against. A bony or cartilaginous skeleton provides this, and hence greatly improves the efficiency of swimming. That does not explain why sharks have cartilage rather than bone, and I don't know if anyone can answer that for certain. Evolution often comes up with different answers to be same problem, and modern fish show that either bone or cartilage will do. Fish with bony skeletons have been very successful in terms of number of species, so in that sense bones are better. There are about 1,100 species of sharks and rays, and at least 21,000 species of bony fish.

Probably cartilage was simply an alternative solution to bone, and provided some muscular support in the ancestors of sharks and rays. Those individuals with better skeletal support could swim more efficiently, and hence feed, breed and avoid predators better. So, the ancestors of those individuals, the modern sharks and rays, have inherited that solution.