Both Christopher and Adriana wondered as much in the comments. I suppose it could be slugs. I recently obtained some diatomaceous earth from Territorial Seed in an attempt to halt slug destruction in other parts of my garden; I do have slugs.
But recall if you will that my lettuces are in this tower. The picture above shows the destruction in the midle pot. Does that seem like a slug-friendly attack spot to you? (Is there any such thing as a slug-unfriendly attack spot?) There was not so much destruction in the other pots.
What came immediately to my mind was the lettuce damage documented in this post on my previous blog waaay back in December, 2005. One morning I parted my bedroom curtains, and found this:
I attributed that episode to raccoons which we knew to be frequenting our roof at night. Someone at the Botanical Garden said, "Nah, it's birds. The birds ate your lettuces." I accepted the authority of Botanical Gardeners and have gone with birds ever since. But the thing that troubles me is, don't birds sleep at night? Sometimes in the summer we hear their mating calls all night long, so I guess not. Who knows.
Of course, I am not necessarily attributing the lettuce attack to the same hypothetical creature. But what to do? I was planning to get some bird netting, the sort you'd hang over berries or fruit trees to keep them from getting at your fruit harvest. Should I apply some diatomaceous earth* too? Will it harm the birds? That's the last thing I want to do.
It's easy to envision a bird perching the tower of lettuce pots and muching away.
*Chemists routinely handle a diatomaceous earth product named Celite. I keep wanting to type Celite instead of diatomaceous earth. You can imagine why.