The largest fishes in the sea have cartilaginous skeletons. There are a number of hypotheses as to why this might be true, though none explain the diversity and number of very large cartilaginous fishes. Patricia Hernandez and I have propsed a functional limitation on size that may be a general explanation for the skewed size distribution.
As a fish grows, its weight and the negative buoyancy of its skeleton grows as the third power of length. At the same time the thrust and lift generated by the fins grow with the square of length. At some length, there will be insufficient lift to counter the sinking force of the skeleton.
A cartilaginous skeleton weighs less than a bony skeleton of the same length, so this theoretical maximal size is larger for the cartilaginous fish. This may provide an explanation for why the eight largest fishes are cartilaginous and the two largest bony fishes have cartilaginous skeletons. We have shown that cartilaginous fishes do have lighter skeletons per unit length than do bony fishes and that the heaviest cartilaginous skeletons are from benthic fishes.
Both marlins (Istiophorids) and mako sharks (Lamnids), two very large, pelagic fishes, can burst swim at very high speed. Swimming quickly requires a stiff body to minimize the wasteful pushing of water to the sides.
The marlin has a bony skeleton and it is stiffened by the simple of expedient of having very few vertebrae and tying those vertebrae together with bony bridges (zygapophyses) and connective tissue. The mako on the other hand has a cartilaginous skeleton and a great many simple, disk-like vertebrae. We have confirmed that sharks stiffen their body by increasing their internal pressure when they swim fast. The thick, inextensible skin must then resist any expansion and would make the body stiffer.
Gabby Martinez (Blinks Fellow), Eliot Drucker and I tested this hypothesis at Friday Harbor Labs using spiny dogfish as a model system. Jaquan Horton (Blinks Fellow), Eliot and I then tested whether bony fishes can pressurize their body. They do, but the net pressure does not increase with increasing speed.