Beginner 8 minutes
Cartilaginous and bony fish: two ways of solving the same problem
Why many sharks must keep swimming and a reef fish can stay perfectly still in the water
A grouper can hang suspended in the water, completely motionless, as if hanging from an invisible thread. Many sharks, on the other hand, sink the moment they stop swimming. Both are fish, both live on the same reef, and they solve the problem of floating in completely different ways.
In this article you'll learn about the two major groups of fish, cartilaginous and bony, how each one solves buoyancy and breathing, and what adaptations they share to orient themselves and survive underwater.
Fish are the animals you'll see the most on any dive. Understanding why one group behaves differently from the other turns passive observation into an actual reading of what's happening in front of you.
Cartilaginous fish: sharks, rays, and chimaeras
Cartilaginous fish (class Chondrichthyes) have a skeleton made of cartilage instead of bone: lighter and more flexible, though less rigid. This group includes sharks, rays, and chimaeras.
Their most visible feature underwater is their exposed gill slits (usually five on each side, though some species have six or seven) without the bony flap that covers the gills of bony fish. Their skin also lacks conventional scales, having dermal denticles instead: tiny structures with the same composition as teeth, which reduce turbulence and give a shark's skin that characteristic rough texture.
None of them have a swim bladder. To compensate, many sharks have an enormous, very fatty liver, rich in an oil called squalene, which is less dense than water and helps reduce their tendency to sink. Even so, most still need to keep swimming to maintain depth: their pectoral fins work like wings, generating lift as they move forward.
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