Shirley Henningfield, age 11, of Gary, Ind., for her question:
Why do fish have scales?
Scientists know of at least 30,000 different fishes which is more than all the varieties of birds and amphibians, mammals and reptiles in the world. Most of the fishes have gauzy fins, but not all of them. Most of them have toothy jaws, but a few have no jaws at all. And there are a few true fishes which do not have any scales.
A trout or a mackerel is a graceful animal, streamlined to glide smoothly through his watery world. True, he is very different from the land animal, but he is very handsome and his body is built to cope with life in the water. Instead of arms and legs, the fish has gauzy fins, and instead of lungs, he has gills.
The skin of a fish is covered with slippery slime which helps the water to slide smoothly away from his streamlined sides. Almost every fish wears a scaly coat under his slippery surface. He has two layers of skin, and the glassy round scales grow in the layer which is under the s1ime covered outer layer.
The shiny scales are made frcm material which is somewhat like the material which is used to make the fur and feathers, the nails and claws of other animals. They grow from roots deep in the skin and their edges overlap each other like the tiles on a roof. Birds shed their feathers and furry animals molt, but a fish never sheds his scales. But if his scaly armor is torn, he can repair the damage and grow new scales.
The scalar coat serves the fish as clothing and to some degree it protects him from his enemies. Certainly a sardine could not bite through the scaly armor of a big sailfish. As clothing, the overlapping scales protect the fish's skin from becoming water logged. In sea water, they may protect his skin from the salty chemicals.
The scaly coated fishes developed millions of years ago, and many experts think that they appeared first in fresh waters. At any rate, the ancient seas were made of fresh water, or less salty than they are today. The fresh water would tend to seep through the skin to join the fluids inside the body tissues. The first fishes needed their scales, or the fresh water would sink in and make their bodies swell up and burst.
As a rule, a fish does not stop growing at a certain age. He grows with every season, and the biggest trout is the oldest trout. And his scales grow with him. New material is added, and the glassy disks get bigger with every season. They do their growing during the season when food is most plentiful, and the new sections are added at the outer edges so that there are no gaps in the overlapping armor plating.