No Bones About Em
Cartilage, the stuff most people associate with bendable ears and nosesor with career-ending injuries in professional athletesseems a poor choice of material for the skeletons of the worlds most formidable fishes. Nevertheless, sharks, skates, and rays have entirely cartilaginous skeletonseven their jaws are made of the soft material. Such scaffolding must work well, because the group boasts the largest and some of the fastest species of fishes in the sea.
Bone would have been a stronger and, one would think, more useful skeletal building block for sharks. In fact, bone predates them, and it played a role in the skeletons of some ancient, now-extinct sharks. So why did the ancestors of todays sharks and other cartilaginous fishes abandon bone in favor of a skeletal material that other large animals use only sparingly? I have recently come to think that cartilage gives sharks at least one important selective advantage: they can grow much bigger than bony fishes.
Now anyone who knows a fisherman also knows how hard it can be to pin down the size of fishes. Most people would agree that the gentle, planktivorous whale shark ranks as the largest fish in the sea. Some might even guess that second place goes to another plankton eater, the basking shark. Next in line, the scourge of Amityville, is the great white shark. After that, the ranking gets more nebulous and more prone to fishing lore. Still, the strongest contenders for numbers four through eight are all cartilaginous fishes: the Greenland shark, the manta ray, the sawfish, the six-gill shark, and the tiger shark (not necessarily in that order!).
All of them, each classified in a separate family, are larger than the two largest fishes classified as bony: the beluga sturgeon and the ocean sunfish. And yet the sturgeon, which has reliably been weighed at just about a metric ton, and the ocean sunfish have skeletons made almost entirely of cartilage. The marlin takes the prize for being the largest bony fish that actually has a skeleton made entirely of bone, but its weight is less than 80 percent that of the sturgeons.
Patricia Hernandez of George Washington University in Washington, D.C., James A. Strother at the University of California, Irvine, and I have constructed a model that attempts to explain why cartilage became the skeletal material of choice for the giant fishes of the ocean. Picture a fish swimming at a constant speed. The swimming muscles generate thrust that exactly offsets the drag of the body as it moves through the water. (If the offset were inexact, the fish would speed up or slow down until a new constant speed was reached.) Similarly, as long as the fish moves forward, the fins and body generate lift that exactly counteracts the fishs tendency to sink [see illustration below].
So consider what would happen to the fishs physical dimensions if, magically, it doubled in size. Three aspects come to mind: length, area (the shadow cast by a light shining from above), and weight. Double the fishs length, and, presumably, the animal also doubles in width. The new fish has not twice its former area, but four times as much. As for weight, it is tied to volume, which is equal to the length times the width times the height. When length and, presumably, width and height are doubled, weight goes up eightfold.
So what would happen to the negative buoyancy of a fish as it increases its length? Well, the weight of the skeleton should directly correlate with the weight of the fish, so the skeletons negative buoyancy should cube when length is doubled. But the lift force needed to counteract the negative buoyancy scales quite differently. The key to lift is the profile of the lifting surfaceanother way of describing the shadow cast by the fish. Lift, therefore, should grow with the square of the length rather than the cube.
That sets up a fundamental problem. Because lift grows more slowly than the negative buoyancy acting against it, a growing fish species eventually runs out of lift; thereafter, it would be doomed to a life of squirming along the bottom. Of course, cartilaginous and bony fishes are in the same bind, but the cartilaginous skeleton weighs considerably less per foot of fish than the bony one. The propensity to sink kicks in at a longer length and a greater weight. It seems that one advantage of the cartilaginous skeleton is something of a reprieve from a size limit [see illustration below].
The worlds largest fishes, as seen from the perspective of a snorkeler looking down, all have enormous profiles. Each profile and the lift generated by each of the creatures forward swimming motion would grow fourfold if the creatures doubled in size, but their volumes (see inset above) and their weights would grow eightfold. Because lift grows more slowly than weight, the biggest fishes conserve weight with skeletons made of cartilage, not bone.
The skeptical reader is no doubt wondering how whalesnot to mention extinct animals such as the plesiosaurs and Stellers sea cowavoided the sinking trap. Those animals are (or were) all larger than the largest bony fish; in fact, the blue whale is the largest animal on the planet. But they have one thing in common: they all come from lineages that have returned to the sea after an evolutionary stint on land. They no longer swim as fishes, gliding through the middle depths in dynamic equilibrium. Instead, their lungs act as flotation devices, and they remain tied to the surface by positive buoyancy, which, to descend, they must swim against.
The motto of the early sharks that wanted to get bigger was not bone stinks but rather bone sinks.
Adam Summers (email@example.com) is an assistant professor at the University of California, Irvine. This column is his fiftieth for Natural History.
Copyright © Natural History Magazine, Inc., 2007