Starting with volume 3, the Moyashimon people decided to get a little silly. By printing little black bars at the bottom of the pages, they were able to create designs visible when you look at the bottom of the book. Not all of these designs are easily recognizable, or explained in later volumes.
vol. 3 edge microbe
vol. 4 edge UFO
vol. 3 t-shirt (for reference).
Vol. 5 chef's hat?
Vol. 6 rows of grapes?
Bottom of vol. 7. Soy sauce bottle.
Top and bottom of vol. 8. Can't be sure of what because of the page misalignment. Maybe something beer-related.
Top and bottom of vol. 9. "Drama" and sideways "9".
Bottom of vol. 10. Microbe.