The twilight zone
The twilight, or mesopelagic, zone, and extends from 200m to 1,000m down. Animals living here have various adaptations for living in the dimly lit waters. Some species have enormous eyes to find food. To avoid being eaten, many are transparent, including squid and crustaceans. Some fish have silvery reflective scales to help make them 'invisible'.
The dark depths
Almost no light penetrates below 1,000m. The water is cold, reaching 3ºC, and contains very little oxygen. The pressure is enormous, up to 1,000 times that on the surface. These dark waters include the bathypelagic (1,000-4,000m), abyssopelagic (4,000m to the ocean floor) and hadopelagic (water in ocean trenches) zones.
Aliens in our midst
Despite the fact that 60% of our planet is covered by water over 1,600m deep, we know very little about pelagic and other deep-sea life. However, the few expeditions to these depths have revealed strange, almost alien-looking animals - such as fish with giant teeth and gaping jaws, long tentacle-like feelers, and even glowing patches.
The deep sea is much poorer in productivity than shallower regions. The fish tend to be much smaller than on the surface, with minimal bone structure and more jelly-like flesh. They are therefore slower and less agile than fish living near the surface. They also tend to grow much more slowly than surface fish. Some take many years to reach sexual maturity.