Scientists have long struggled to explain why bony fishes are so small: The heaviest—the ocean sunfish—is just 2.3 metric tons, but cartilaginous fishes like whale sharks can weigh up to 34 metric tons. Now, a new study of an ancient giant suggests this modern difference is merely an evolutionary accident.
Bony fish, which make up some 95% of all fish species, might be constrained by their metabolism according to one argument. Larger animals generally have to make do with less oxygen per gram of tissue; because bony fish seem to have higher metabolic requirements than sharks, it might simply be impossible for them to grow much larger than the ocean sunfish.
Enter Leedsichthys problematicus. The extinct fish—thought to be the largest on record—lived about 165 million years ago in Europe and South America. It grew to at least 16.5 meters in length and might have weighed 45 metric tons, which means it was larger even than today’s whale shark.
Realizing modern biologists had left ancient fish out of their equation, scientists decided to calculate L. problematicus’s metabolic requirements. They used data from living bony fish as a guide, and they found that it would have not only survived, but thrived: In theory, the giant fish could have cruised along at speeds of 17.8 kilometers per hour while still keeping its tissues adequately oxygenated, they report this month in Palaeontology. For comparison, the speediest living fish probably swim no faster than 30 kilometers per hour.
It’s still a mystery why there are no gigantic bony fish today, but metabolically speaking there’s no reason why they shouldn’t exist, the researchers conclude.