The sea floor is a huge flat area.  It lies at depths of about 4000 to 6000 meters below the surface of the ocean.  It is called the  Abyssal Plain.  Sea floor spreading occurs here, which means that there is volcanic action and a mid-ocean ridge where some of the lava has cooled.  There are also isolated volcanic mountains called sea mounts, and some black smokers, but most of this area is flat and featureless.

    To a surface dweller the abyssal plain is an intimidating environment.  At best, light penetrates the ocean for only about 200 meters  (less than 600 feet).  Below that there is a twilight zone of very faint light, and by 1000 meters the ocean, and the plain below it, is in total darkness.  It is also cold, the temperature being only a little above freezing.  The pressures from the weight of the water above the abyssal plain are enormous, and can easily crush a submarine.

     There are no photosynthesizers on the abyssal plain because no light gets down there.  So what do the animals eat?

    Well, the main problem for the animals living on the abyssal plain is finding something to eat.  There is no light, therefore there are no plants, which cannot photosynthesize in darkness.  Although there are some sulfur using bacteria at the black smoker vents, these vents are rare, and could not support all the animals living on the abyssal plain.

     The life forms on the abyssal plain are dependent on the life forms living in the light far above them.  Many deep sea animals swim up towards the surface at night to find food, and can then become prey for animals that live still deeper.  Many abyssal plain animals stay in the depths and make do with what they can find there.   Tiny particles of organic matter slowly drift downward and nourish the animals living on the abyssal plain.  These animals are able to live on very little -- they have low metabolic rates (they are cold, and chemistry happens slowly in the cold) and may not move around much.  We do not know about their growth and reproduction.

     Food reaches the abyssal plain by drifting down from the sun-lit surface.  As the food drifts past many animals on its way to the ocean floor, much of it gets eaten, and the food that gets to the bottom is made up fine particles, tiny bits of decaying plant matter and tiny scraps of dead animals.  The benthic (bottom dwelling) animals live on and in the fine particles that silently rain down on them.  Sometimes these particles get stirred up: they are tiny and light, and easily wafted up into the water again.  The particles settle very slowly, and so are called "marine snow" when they hang suspended in the water. 

     Occasionally a huge animal, such as a dead whale, may fall down onto the abyssal plain.  The bottom dwelling animals can detect food from a great distance, and they come to eat as much as they can.  Because of their low metabolism, the animals can go a long time between meals, months, perhaps, or even a year or longer.  

     The abyssal plain is surprisingly rich in life forms.  As this environment has changed very little over the history of life, there has been plenty of time for the animals to adapt to the conditions there.  The animals may be widely scattered, but they exhibit a surprising diversity.  The life forms include zooplankton, sponges, sea anemonies, starfish, sea cucumbers, sea pens, octopi and crinoids, as well as many smaller animals, such as copepods and nematodes.  There are many different species, even though the number of individuals in each species may be small.

     The animals in this biome have many unique adaptations.  Many of them are bioluminescent, which means that they can emit light.  Sometimes this is done by providing a sheltered environment for bioluminescent bacteria that live in the animals, sometimes the animals generate the light themselves.  These lights are used for many purposes, such as for attracting mates or becoming a lure for prey.  Some animals have enormous eyes that can detect even the very faintest light, or huge mouths.   Some of these animals have fantastic shapes that are different from the forms of animals that live in the lighted world.

     The animals that live on the abyssal plain are well adapted to their environment.  Could they move to more shallow depths in the ocean?  Remember that the ocean has many inhabitants, like these abyssal plain dwelling rattail fish, and each inhabitant  occupies an environmental niche.  By adapting to a niche an animal develops ways of living that give its species an advantage in eating certain food sources and occupying certain spaces.  This means that the resources in the ocean are being used and newcomers would have to compete for them.  As the bottom dwelling animals have adapted to occupying niches on the ocean floor, they are less fitted to move into other niches which might require new adaptations.

     However, if there were some sort of global catastrophe, such as a huge meteor strike that killed many surface dwellers, some of the abyssal plain animals might move into new or vacant niches.  They are a reservoir of living creatures that might yet inherit the earth.