“Going down, light levels decrease so that at depths of more than a few hundred meters there is no photosynthesis, and, with a few remarkable exceptions, no primary productivity.19 Life in the dark ocean is largely dependent on what comes down from the surface waters. A few hundred meters is also the depth of the oxygen minimum layer, where dissolved oxygen is so scarce that animals with gills—that is most marine animals—are in a bind. Some, like the vampire squid, have evolved physiologies to deal with low oxygen levels, but all must use energy sparingly.”