You would think it would be easy to count the number of glowing (bioluminescent) animals in the ocean, just by looking at videos or photographs taken at different depths. Unfortunately, very few cameras are sensitive enough to show the pale glow of many marine animals. Below 300 meters (1,000 feet) the ocean is essentially pitch black, so animals don’t need to glow very brightly. Also most animals don’t glow continuously because making light takes extra energy and can attract predators.
Because of the difficulty in counting glowing animals at depth, most previous estimates of the proportion of glowing animals were based on qualitative observations made by researchers peering out the windows of submersibles. Martini and Haddock’s study is the first ever quantitative analysis of the numbers and types of individual glowing animals at different depths.