The Medaka, occassionally refered to as (with tongue in cheek) the "Japanese Guppy". Of course we love these fish! They're a pretty decent lab animal, producing lots of offspring relatively quickly and with many interesting mutant strains available, including one with nearly-transparent tissues (fun with skeletons).
There are numerous research groups around the world focussing on this species, including this one in Japan. The above picture came from a site with entirely Japanese text (Kanji?), so I'm going to return to my habit of claiming that if I can't read a copyright notice, I don't worry about it.