Marine biologists discover a whalefish -- an incredibly rare deep-sea fish -- swimming in the ocean in this video. Because it is so dark where they live, females have well-developed sensory pores, called the lateral line system, running up and down the sides of their bodies to sense vibrations in the water. Males, however, are less developed; they do not have mouths at all! They look so different from females that they were once categorized as a separate species. Learn more about how scientists solved this mystery at our Meet the Suspects story.