One of the most fascinating biological environments on Earth is found on the floor of the ocean. Kilometers below the surface, no light reaches these areas, so for life to exist, it must have a different energy source. This entire ecosystem is living off the energy of the Earth.
Rocks deep inside the earth contain elements in reduced form; they haven’t reacted with oxygen because there is no free oxygen inside the Earth. At places like mid-ocean ridges, the Earth itself brings these elements up to the surface and exposes them to the ocean. Life in these settings can take up some of these elements, like iron, react it with oxygen in the water, and use that energy to sustain itself. The scientist’s term for this life would be chemosynthetic – using chemistry to gain energy (contrast that term with photosynthetic, as plants are).
This entire seafloor is covered with white crabs (10 cm scalebar). There’s no reason for any of the organisms here to waste energy by coloring themselves; there’s no sunlight so nothing can be seen anyway. This is an area rich with a type of life very different from anything we’re familiar with in our lives up at the surface, almost 2500 meters above.
"Dense mass of anomuran crab Kiwa around deep-sea hydrothermal vent" by A. D. Rogers et al. - A. D. Rogers et al. in PLoS Biology. Licensed under Creative Commons Attribution 2.5 via Wikimedia Commons -http://commons.wikimedia.org/wiki/File:Dense_mass_of_anomuran_crab_Kiwa_around_deep-sea_hydrothermal_vent.jpg#mediaviewer/File:Dense_mass_of_anomuran_crab_Kiwa_around_deep-sea_hydrothermal_vent.jpg