In this book we focus on the sorts of animal life inhabiting the benthic boundary. Deep-sea sampling world-wide has demonstrated that the evolutionary radiation of life has penetrated to all parts of the world ocean. Animal life has been recovered from the depths of the Arctic Ocean under the polar ice cap to the greatest depths of the deepest ocean trench. Only in anoxic basins where sulphide bacteria flourish are animals virtually excluded.

CATEGORIES OF FAUNA

The fauna of the benthic boundary is comprised of those animals living either on the ocean floor, the benthos, or those associated with the immediately overlying water, the benthopelagic fauna, which comprises swimming or drifting forms, some of which may spend varying amounts of time on, or even buried in, the seabed.

This is not to deny the importance of the other major categories of life which impinge on the seafloor, but normally spend their entire lives swimming (the nekton) or drifting (the plankton) in the overlying water. Biomass of these pelagic communities is highest near the surface, and decreases exponentially with increasing depth until at 4 km depth it is about 1% of that at the surface (Angel & Baker, 1982). However, Wishner (1980a) found, from a net attached to the deeply towed instrument package ‘Deep Tow’ (see Chapter 3), that abundance starts to increase 100 m above the bottom and doubles at 10 m above.