Fish, being heavier than water, have a swim bladder to help them stay afloat. These organs are filled with gas and help fish to stay at the depth they want to be in without having to spend energy swimming. However, unlike other (bony) fish, sharks lack a swim bladder.
So how do they do it?
Sharks (as well as rays) have to use other methods to control the depth at which they dwell. Those being:
cartilaginous skeletons
massive livers, which are filled with an oil containing squalene
their fins
First we’ll talk about them bones- or rather, that cartilage. Cartilaginous skeletons make sharks light. At least, considerably lighter than they would be if they had bones. Bone is about twice as dense as cartilage. Their skeletons are also what make them so fast and flexible.
Then there’s those fins. Sharks use a nifty method called the “dynamic lift”. This is when the shark uses its pectoral fins to create a lift beneath them- much like when a bird flaps its wings. But there are two flaws in this method. First, the shark must keep moving (thus spending energy) for it to work. The second catch is that the shark can’t go backwards. Sharks also like to keep moving forward to push water through their gills. (However- derailing for a moment- it’s not true that sharks have to keep moving to breathe. There’s a bit more to it than that. I think I’ll make another post about it soon.)
And finally, the liver! While fish’s swim bladders are filled with gas, a shark’s liver is filled with oil: squalene. All animals produce this compound, and we humans use it for cholesterol, steroid hormones, and vitamin D. But sharks have found an extra use for it. This oily substance is lighter than water. Even with light skeletons, sharks are still heavy and need a lot of this stuff to make them float. Lots of oil means lots of storage space is needed for it, so in steps the liver. And these things are huge. They take up roughly 25 to 30% and a shark’s mass! Big numbers like that are pretty amazing, but you don’t really get just how massive these organs are unless you see it for yourself.
So check out the liver of a great white shark in the video below! (31:37)
ifesbob 