Why did God create reptiles in the rivers, seas, and lakes?

St. Basil the Great says, “Earth had received her [adornment] from her own plants, the heavens had received the flowers of the stars, and, like two eyes, the great luminaries beautified them in concert. It still remained for the waters to receive their adornment. The command was given, and immediately the rivers and lakes becoming fruitful brought forth their natural broods; the sea travailed with all kinds of swimming creatures; not even in mud and marshes did the water remain idle; it took its part in creation … Every creature that swims, whether it skims on the surface of the waters, or cleaves the depths, is of the nature of a moving creature, since it drags itself on the body of the water. Certain aquatic animals have feet and walk; especially amphibia, such as seals, crabs, crocodiles, river horses and frogs; but they are above all gifted with the power of swimming.”⁴⁴⁸