Costa Rican ID

I'm dodging the below-zero weather in Minneapolis and just scooped a few of these out of a stream in central Costa Rica. I'm guessing they're livebearers but too immature to show male features... would anyone care to take a guess about genus (or family, even?)



They look quite a bit like the fish in this forum post (http://aquadico.com/forum/6332/xenophallus-umbratilis-lago-arenal/) but don't otherwise resemble other photos of Xenophallus that I can find online.