I assume this is two statements: one without the parentheses, and one where each parenthesized word replaces the word before it. This is a compact, but borderline unreadable way to write two statements with the same structure. I hate it.
Edit: it makes a lot more sense in a live lecture, where the lecturer writes down the first sentence, then says aloud the second sentence while only replacing the necessary words on the board.
Edit: let me preemptively be a pedant to myself and say that "sub-...-species" is wrong because "bilaterally symmetrical animals" is not a species. Flounder is itself a species AFAIK, not a sub-species of anything. It is a descendant of the common ancestor of all bilaterally symmetrical animals. There, now surely no one will find anything to be pedantic about :D
Have you been listening to the podcast A Problem Squared? This was a topic of the most recent episode (095 = Friday Fears and Disco Spheres). Friday the 13th is very slightly more common than other weekdays for the 13th.
I believe there's a setting for whether it's global or per-window. Personally I prefer global, because I can't keep track of more than one state and I absolutely hate the experience of typing something and getting a different language than you expect.
If I'm honest, I've never heard of elf on a shelf, other than in posts like this.