Metamorphosis
The need for metamorphosis is clear when the relative size of eggs and adults is understood. A mature Atlantic cod can reach two metres long, and weigh over 200 pounds (96 kg). The female will produce over 100,000 eggs in a single spawning, and the resulting larvae are quite tiny. There is no way they could live like the adults, which are more or less top predators. The larvae cannot even swim against the current. Therefore, they must live a different kind of life before they can live as the adults do.