יִשְׁרְצ֣וּ הַמַּ֔יִם שֶׁ֖רֶץ נֶ֣פֶשׁ חַיָּ֑ה
Waters, swarm with living swarms
The fifth day God continues to define His creation by its continuing testimony. First, the sprouts sprouted; then the lights lit. And as animal life (Hebrew: נֶ֣פֶשׁ חַיָּ֑ה -- nephesh khavah ) was created, the lower forms of aquatic life (Hebrew: שֶׁ֖רֶץ sherets, swarming or creeping things) began to swarm, or teem (שָׁרַץ sharats) in the water.
There was no need to give a lot of detail. God created things in general categories that any observer could relate to. If it moved about in large groups, it was a "swarm".
It is notable that the "soulish life" is first mentioned in the lowest creatures. This kind of life is reserved for the "animal kingdom" from plankton to pachyderm, and beyond. It came first in the water, and, as we see in the next segment, the air.