Catfishes and sharks are not covered with neat scales, as most other fishes are. The sharks have hard little bumps embedded in their skins and so do some of the catfish. They eat almost anything in sight and take better care of their children than most other fish. Certainly the sharks and catfishes have a lot of things in common. It is natural to think that they might be related. But they are not.
The mighty sharks have a family tree that goes back 350 million years. They are the oldest group of living fishes and the bulky 50 foot whale shark is the largest fish in the sea. They are related to the bat winged ray fishes and their small cousins are the dogfishes. All these creatures have skeletons made of tough, rubbery cartilage. This is the main reason why they are classed in a minority group all by themselves.
There are other groups of fishes, but all of them have skeletons made of true bones. One group includes the delicious catfish that live in our lakes and rivers. If you have sampled one for dinner, you know that he has his share of brittle bones. This makes him a member of the majority group called the bony fishes. The catfish has the usual round, fishy gills where you would expect to find his ears. A shark's gills look like a row of slits along each side of where you would expect to find his neck and shoulders. Of, course, both the round catfish gills and the sharky gill slits take in dissolved oxygen from the water.
The catfish's scaleless skin makes him very different from other bony fishes. But it does not make him related to the sharks. Experts tell us that the world's first teeth belonged to the sharks and they had simple little teeth all over their bodies. Later they grew lots of bigger ones in their haws, but they kept the remnants of their old family teeth embedded in their tough skins. A few catfishes have bumps embedded in their tough leathery skins. But these little flakes are made of true bony material.
Most fishes lay lots of eggs and then desert them in the hungry sea. Many of the sharks give birth to live babies, big enough to take care of themselves. For example, the mother black tip shark is about five feet long, and her four or five newborn babies erasure about 18 inches. Other sharks lay eggs protected in hard cases.
All the catfishes lay eggs, but many go to a lot of trouble to take care of them. Those that live in our streams dig hollow nests among the stones and water weeds. The parents guard the eggs until they hatch. When enemies come near, they fight like fierce little sharks, even though they are not related to the sharks.
The banjo catfish looks like a banjo with ears and a long tapering tail. He belongs to South America. The mother banjo pastes her eggs onto her chest until they hatch. The father gaff topsail catfish gathers the newlaid eggs into his big mouth to keep them safe from harm.