swarming creatures: the small water, winged, and ground animals that breed and spread in overwhelming numbers
At its heart this word is about numbers, not motion. English Bibles often say "creeping things," but the picture is not slow crawling: it is life that breeds and spreads until it cannot be counted. The related verb (שָׁרַץ) means to teem or swarm, and the noun names the creatures that do it, the small things that fill the water, the air, and the ground in overwhelming numbers.
The same word carries two very different moods. When the opening of Genesis says the waters teem with these swarming creatures, this abundant life is part of what God makes and calls good. Far more often, though, the word appears in Israel's food laws, where these same small water, flying, and ground creatures form a class that may not be eaten, and where the ground-dwellers even make a person unclean if their dead bodies are touched. One word thus stretches from blessed abundance to something to be shunned.
In the laws about clean and unclean food, the word is set right beside the near-rhyming word for "detestable," so that the very sound of the sentence ties these teeming creatures to the verdict "loathsome." Scholars still debate why they are singled out: some point to how they blur the tidy categories of creation, filling any element and moving in no one clear way; others to a concern for restraint and for life; others to the way a community draws lines around itself. They overlap with another word for the small things that move along the ground (רֶמֶשׂ), though that word stresses low, gliding movement while this one stresses sheer numbers. What all agree on is that these small, numerous, ground-close creatures sit at the edge of the ordered world.