Underwater Creature

Bioluminescence is the ability for an organism to create light through chemical reactions. Creatures use bioluminescence in many ways: to light their way, attract prey, or seduce a mate. Many underwater animals are bioluminescent—from the Viper fish to the Flashlight fish, which is named for its light. Some creatures, such as the angler fish have a concentration photophores in a small limb that protrudes from their bodies, which they use as a lure to catch curious fish. Bioluminescence can also confuse enemies. The chemical process bioluminescence requires at least two chemicals: the light producing chemical called luciferin and the reaction causing chemical called luciferase. The luciferase catalyzes the oxidation the luciferin causing light and resulting in an inactive oxyluciferin. Fresh luciferin must be brought in through the diet or through internal synthesis.
Chemosynthesis
Since at such deep levels, there is little to no sunlight, photosynthesis is impossible as a means energy production, leaving some creatures with the quandary how to produce food for themselves. For the giant tube worm, this answer comes in the form bacteria that live inside it. These bacteria are capable chemosynthesis and live inside the giant tube worm, which lives on hydrothermal vents. These vents spew very high amounts chemicals that these bacteria can transform into energy. These bacteria can also grow freely a host and create mats bacteria on the sea floor around hydrothermal vents, where they serve as food to other creatures. Bacteria are a key energy source in the food chain. This source energy creates large populations in areas around hydrothermal vents, which provides scientists an easy stop for research.