Darlene Astles, age 13, of Oromocto, New Brunswick, Canada, for her question:

Why aren't ocean fish salty?

The family history of the fishes dates back millions of years to times when the ancient seas were less salty than they are today. The early fishes were adapted to this fresh or fairly fresh water. But through the ages, their sea water gradually grew saltier as rivers dissolved chemicals from the land and dumped them into the oceans. Meantime the bodies of fishes changed to keep pace with the times.

They developed tough scales to protect their skins from the salty chemicals around them. The chemical composition of their body fluids changed very little and they did not drink the salty water. Since fish dine on other fish, their meals were unsalted. This is why fish food is fresh meat and we need a helping of salt when we eat fishes from the salty sea.