Every other fish I can think of has bones but sharks do not. Why is this?
In: Biology
Short answer is because they came first. The was a hot moment in time when fish were evolving and one of the first adaptations was cartilage as a structural system – it’s really light, it’s super flexible, and it’s not nutritionally intense to maintain. It wasn’t great at everything though – having squishy teeth and jaws don’t work all that well so a lot of those early fish had beak like mouths and no jaws at all, just a sort of fixed opening. Imagine a sort of underwater Death-Parrot that can’t open or close it’s beak.
Eventually a new thing, called “bones”, started evolving which was strong but heavy and takes a lot of energy to maintain. A lot of fish developed boney heads and teeth and a new-fangled hinge separating their skull from this new thing we now call a ‘jaw’.
That’s where sharks basically spun off, they are speed swimmers who are so good at eating whatever they need to eat with their new jaws and teeth nothing really required them to change. If we’re talking time line, sharks are to dinosaurs as the dinosaurs are to us. *that’s how long sharks have ruled the oceans.*
Other fish started really liking these bone things and started working the bones into their bodies.
Sharks are part of a broader family of fish that never jumped on the bones bandwagon fully and they are called the ‘cartilaginous fish’ which also includes skates, rays, and a variety of shark-like fish you’d probably just thing are baby sharks if you saw them swimming around.
Sharks do have a skeleton much the same as other fish, but most of it is cartilage like your ears and nose. This isn’t unique to just sharks, there’s a whole family of “cartilaginous” fish that includes sharks and rays and a few other oddballs.
Since this tissue isn’t as hard and durable as your bones, it tends to decay/get eaten too and most sharks don’t leave any bones behind except their teeth.
Their “bones” do get tougher and more calcified as they get older so the big ones can leave some skeletal remains when they die.
Now why exactly did they evolve this way? Hard to say. They split off from other fish a *long* time ago way before dinosaurs were even a thought. Their ancestors used cartilage to armor themselves too, but that has disappeared over the millennia in favor of speed and efficiency to chase more nimble prey.
It’s not true that every other fish has bones. There are fish with bones (bony fish) and cartilaginous fish. Other cartilaginous fish include, rays and a kind of fish known as a chimaera. They are estimated to have diverged from what would become bony fish about 420 million years ago.
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