Johnsen entered biology with backgrounds in math, physics, and art and has since used all three fields to investigate the visual ecology of oceanic zooplankton. After a frustrating graduate career, in which he studied the vision and behavior of animals with neither eyes nor brains, he completed postdoctoral fellowships at Harbor Branch Oceanographic Institution and WHOI. After a year as an assistant scientist at WHOI, he accepted a position in the Biology Department at Duke University. He is interested in all aspects of vision in oceanic species, with a particular emphasis on strategies for camouflage. The camouflage research has involved investigations into UV vision, whole-body transparency, cryptic coloration, and counterillumination using techniques ranging from blue water diving to protein biochemistry to Monte Carlo modeling of photon trajectories. He gives numerous talks to the general public and his work has been featured in newspapers, magazines and a John Updike poem (pdf).