The deep sea is a fascinating world, but it actually consists of many worlds. Deep sea is very broadly defined and composed of the twilight, midnight, abyssal and hadal zones, also called mesopelagic, bathypelagic, abyssopelagic, and hadopelagic respectively. Sometimes you’ll even see parts of the littoral, the coastal ocean that is equivalent to the epipelagic, referred to as deep sea, especially its lowest part, the mesophotic or circalittoral zone. This is where aquanauts, submersibles and remotely operated vehicles study the last remnants of photosynthesis, seemingly impossible kelp forests, coral reefs, rhodolith beds and various coralline, gorgonian and sponge gardens.

As sunlight fades, another source of light becomes important: bioluminescence. It is used to hunt, hide and mate. Speaking of sex, no one can figure out how eels have sex, but it definitely happens in the deep sea. Rachel Carson was fascinated by this and dedicated a third of her first book Under the Sea-Wind to the eel. The deep sea is also a sea of giants. Gigantism is thought to be mostly due to the colder water which can hold more oxygen, which is why polar marine creatures are also huge. In these frigid waters, diverse giant invertebrates dramatically slow down their metabolism to survive.

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