The deep sea is defined as the part of the ocean below 200 m (656 ft), where light begins to dwindle. Deep-water ecosystems are unlike any other habitat on the planet. Despite being the most extensive habitat on Earth, we know very little about this mysterious world. Even with the challenges of darkness, cold temperatures, and increased pressure, life is able to thrive at these depths, and biodiversity is higher than once believed.

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Deep-sea species have special adaptations to deal with the increased pressure and lack of light.

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Adaptions:

• Bioluminescence (lantern sharks) 

• 6 and 7 gill slits (for more efficient oxygen absorption in a lower oxygen environment) 

• Slow growth and slow metabolism (Greenland shark) 

• Ability to maintain neutral buoyancy (a small amount of positive buoyancy found in two species)