• Brocken Inaglory
  • A fish

Leo Smith, who’s been an assistant curator of zoology at the Field Museum since 2007, has spent the last several months working on the upcoming exhibit “Creatures of Light: Nature’s Bioluminescence,” which opens this Thursday. Smith studies the evolutionary biology of fishes, particularly venomous and bioluminescent ones. He talked to me recently about his research and the upcoming exhibit; part one of the discussion focused on the “Creatures of Light” exhibition, the difference between bioluminescence and biofluorescence, and how male anglerfish become parasitic when they meet females. In this second part, he talks about what defines a fish, the ocean’s “no-bone zone,” and the first species he ever described. Smith, in his own words:

What people think of as fish fall into three main categories. The natural groups are the cartilaginous fishes, the sharks and rays; the lobe-fin fishes are the lungfish and the coelacanth. And then the ray-fin fishes, which is what everyone thinks about other than sharks. In order to say what is a fish you have to basically either include things like mammals, birds, amphibians, or you have to make some sort of distinction somewhere. You can do like, vertebrate that lives in water, but then you got whales and other things.

From an evolutionary standpoint, the fish is probably best thought of as including us and anything that crawled up on land. But it’s complicated. I’m more closely related to a lionfish or a cichlid or rainbow fish or a trout than a trout is to a shark. We shared a more recent common ancestor with a trout than a trout did with a shark. A shark is like a second cousin once removed from a trout, and we’re like brothers or something.