ECOLOGICAL ZONATION OP THE DEEP SEA As an introduction to the description of the life in the deeps and abyssal depths of the sea it may be useful to consider the main features of ecological zonation of the oceanic depths. This is especially true because the organisms of even the deepest trenches are dependent on the biological conditions of the overlying water masses, to the very surface of the ocean. As a generalized example we may consider a tropical or subtropical area (Fig. 1). At the surface, light is the dominant factor in the epipelagic zone. The essential ecological feature of the photic zone is that the animals live in the same environment with the primary food source, the phytoplankton, whereas in the lower layers there is no primary production by plants. Although the endemic species of the epipelagic zone either do not migrate or perform only limited vertical migrations, there are many animals which invade the epipelagic zone from deeper layers during the night or pass their early developmental stages in the photic zone. In either case, the abundance of food is the primary factor. The epipelagic zone is a very thin layer. In eastern parts of the oceans, with up-welling and high productivity, it may be less than 100 meters thick, and in clear subtropical areas it may be 200 meters thick. The uppermost aphotic zone is the mesopelagic, inhabited by dark-loving (scotophilic) animals. Whereas temperature is an important factor governing the horizontal distribution of the animals Whereas...