No, clams. Like some of it would be my own weird sort of "How To" but sometimes I'd like to just write an article on something I've made for my world.
Consider: the Neru, a melon-sized clam that eats fish in a manner similar to that of a carnivorous plant. (Think Venus fly trap.)
It's a fairly small part of the story, but I know a lot about it because one of my characters is named after it. The reason his parents chose the name is related to how it's viewed by the various cultures. Which is related to its habitat and so forth.
I have dozens of things like this, and then there's mythology and deep backstory. You know, lots of stuff.
I mean, I could also do some things with clan-based culture since The Book of Worlds has a lot of that. Similarities and differences and such. But going into ecology is great fun too.