The first occurrence.
And God Elohiym said, “Let the waters swarm with swarms of living creatures, and let winged creatures fly above the earth across the face of the firmament of the heavens.” Gen 1:20
The primitive root.
shin ש = two front teeth, thus sharp, press, eat, two, again
resh ר = head of man, thus head, first, top, beginning, man
tsadey צ ץ = trail, thus a man concealed, journey, chase, hunt
The story: The two front teeth, shin, can convey the idea of plurality and multiplication, as anything x2 increases its number. So, multiplied (shin) heads (resh) which creep (tsadey, as a hunter does along a trail); i.e., the swarming of creeping things.