What's the most common vertebrate on the planet?

Fish are the most species rich group of vertebrates and there might be over 200,000 species once we're done exploring. But what species is the most abundant. If you look around you it can't be seen. You need to look in the oceans but even a boat won't help. The most abundant vertebrate is a tiny fish deep in the ocean. Here's an excellent story in the NYT about them and deep ocean research.