Journey to the bottom of the world

The average depth of the world's oceans is 4 km, and about 75% of their water is at a depth of 1 km or more. These depths include the two most extensive marine habitats on Earth: the deep ocean floor and the water column above it. They are dark, cold and at great pressure, with low concentrations of nutrients and dissolved oxygen. There is no photosynthesis in the absence of light, so oxygen and organic material must descend from above. Despite this, just over 10% of known fish species feed and breed in these habitats.

The fish in the water column at these depths are characterised by sparse populations and little movement. They seize any prey passing by or lured to them by photophores (light-producing organs). They are large-mouthed and sharp-toothed to maximise chances of success, but to conserve energy their bodies have weak muscles, no scales and substantial reductions in many internal organs. Population densities are so low that finding mates is a significant problem. Female anglerfish release pheromones (chemical attractants) to lure males, which then attach to her as ectoparasites. In some species the sex organs of males and females do not mature until an ectoparasitic male is attached. Females then produce millions of eggs.

The fish found on the deep sea floor are more active and often more abundant than those above, feeding on each other, organic material descending from above and invertebrates on the ocean floor.