On the fifth day, God creates all those things which live in the water and all birds.

Genesis 1:20-23  And God said, Let the waters bring forth abundantly the moving creature that hath life, and fowl that may fly above the earth in the open firmament of heaven.  21 And God created great whales, and every living creature that moveth, which the waters brought forth abundantly, after their kind, and every winged fowl after his kind: and God saw that it was good.  22 And God blessed them, saying, Be fruitful, and multiply, and fill the waters in the seas, and let fowl multiply in the earth.  23 And the evening and the morning were the fifth day.

Notice that God does not say “fish” or “crustaceans” or anything specific.  He simply commands that the waters “bring forth abundantly the moving creature”.  The Hebrew verb translated “bring forth abundantly” is a cognate of the Hebrew word translated “moving creature”.  They both have to do with teeming or swarming.  Certainly the rivers, lakes, seas, and oceans are teeming with life of all kinds.  The word translated “whales” is a generic term which means “sea monster” or “dragon” or “serpent”.  It likely included all large creatures living in the waters, which is why it is described with the word “great”.

“Fowl” is also a generic term and refers to any flying creatures including birds, winged insects, and mammals such as bats.  We shouldn’t be alarmed that God does not group animals the way we do.  He chose to group them more by function and environment.  That’s His prerogative.

All of these creatures were to “be fruitful and multiply”.  This ended the fifth day.