Or HaChaim on Genesis 1:24/אור החיים על בראשית א׳:כ״ד

1And God said, etc.: This too was only for that time, as we explained with the creation of the swarms of the waters. And the reason that He did not need to say to them to be fruitful [is that] He implanted in them the ability to reproduce, and they would reproduce on their own. But He did command them not to interbreed, one specie with a different specie; and this is what it states, "according to its specie."