Rashi on Genesis 1:21/רש"י על בראשית א׳:כ״א

1התנינים THE HUGE CREATURES — the large fishes that are in the sea; and according to the statement of the Agada (Bava Batra 74b) it means here the Leviathan and its consort which He created male and female. He, however, killed the female and preserved it in salt for the benefit of the righteous in the time to come, for had they been permitted to be fruitful and to multiply the world could not have endured because of them.

2נפש חיה — that have vitality.