top of page
  • Writer's pictureanimaleduk

Mysterious Fathoms Below...

A deep sea dragonfish. Image by Jacobs School of Engineering, UC San Diego.

It’s National Marine Week! To celebrate, we’re gonna talk about the final frontier… no, not space – the deep sea!


First of all, let’s cover some basics. As Mr. Ray once said, let’s name the zones, the zones, the zones, let’s name the zones of the open seeeaaa!

Ahem. Anyway. In terms of depth, the ocean is divided into five zones. From shallowest to deepest, those zone are:

  • Epipelagic (photic zone, or ‘sunlight zone’) 0-200m deep

  • Mesopelagic (twilight zone) 200-1,000m deep

  • Bathypelagic (midnight zone) 1,000-4,000m deep

  • Abyssalpelagic (abyss) 4,000-6,000m deep

  • Hadalpelagic (trenches) 6,000-11,000m deep

You may think of trenches when you think of the deep, but ‘deep sea’ actually includes three out of the five zones – bathy, abyssal and hadal! Some sources even include the mesopelagic zone, meaning anything below 200m depth could be considered the deep ocean. We seem to know more about the moon than we know about the deep sea. How strange is that? As you might imagine, living anywhere from 1,000 to 11,000 metres beneath the surface is no picnic! So what challenges do these deep sea creatures face?


High pressure

The pressure underwater increases by 1 atmosphere with every 10m - this means that the hydrostatic pressure in the deep sea ranges from 100-1,100 atmospheres! This level of pressure is enough to crush most organisms. So how do deep sea creatures manage?

Unlike fish that live in the epi or mesopelagic zones, most deep sea fish have reduced (or entirely absent) swim bladders. Deep sea fish also have piezolytes, which work to counteract the pressure on the creature - this is still not entirely understood. Some fish, like the infamous blob fish (Psychrolutes marcidus) are also gelatinous, and less dense than water, preventing them from being crushed by the pressure.


Due to the total lack of sunlight in the deep sea, photosynthesis cannot occur. So where do deep sea organisms get their nutrients from? And how do they find it in the first place?

One way is through 'marine snow', which is the term for organic matter floating down from shallower depths and into the deep sea, providing nutrients. It doesn't have to be small, either - even a decomposing whale carcass could be considered marine snow! In the abyssal and hadalpelagic zones, it is so dark that there is no benefit to having eyes at all, and therefore organism have incredibly reduced eyes, or none at all.

Lack of oxygen

Because there is no photosynthesis occurring, the deep sea also has a lack of oxygen (but is not completely anoxic). Between 500 -1,000m is the area with the least amount of oxygen, as there are more individuals there using it - it is used quickly, but not created due to a lack of photosynthesis, leading to hugely decreased oxygen levels.

As we get deeper, however, and the number of individual creatures decreases, the oxygen is used up less quickly. Downwellings of cold water from the poles constantly replace this oxygen, and cold water holds more oxygen than warm water.

Cold temperature

The deep sea is, predictably, very cold. The water is not warmed by the sun, and cold water is denser than water, therefore sinks to the bottom. Generally, the deep sea is between -1 to +4°C. The exception to this is around hydrothermal vents, which are fissures in the sea floor that emit geothermally heated water, acting like hot springs.

Fewer food sources

There are far fewer organisms overall in the deep sea that in the photic zone. The population density is much lower, and so the chances of encountering other individuals is increased - this must be a real pain in the butt when you're looking for a mate or a meal!

So what are some cool methods these animals use to deal with these challenges and make life a little easier? Well, stay tuned - we'll be writing a part 2 in which we'll talk about jaws, lures, gigantism, bioluminescence and more!


Thanks so much for joining us to learn more about the deep sea. We hope you've come away with a few fun facts about this amazing (and creepy) world! Keep an eye out for part 2!

Thanks again for reading, and remember to always keep learning!

-- Charlie and Tiana

6 views0 comments

Recent Posts

See All


bottom of page