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.