Abyssopelagic Zone (Abyssal Zone)

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Conventionally about 4,000 m to 6,000 m, overlying the abyssal plains that make up the largest single habitat type on the planet by area. Permanently dark, near-freezing, and food-limited: the abyssal seafloor lives almost entirely on the small fraction of surface primary production that survives the fall as marine snow. The 4,000 m and 6,000 m limits are agreed conventions rather than measured discontinuities; the 6,000 m figure in particular is a working definition chosen because trench faunas below it start to look distinct.