Zones of the Sea: Benthic and Pelagic

In science classes starting even in elementary school, most of us learned about the layers of the Earth (core, mesosphere, asthenosphere, and lithosphere [core, mantle, and crust]) or the atmosphere (troposphere, stratosphere, mesosphere, thermosphere, and exosphere [aka space]). Well, the ocean has layers as well (like an ogre or an onion… but not a parfait, Donkey). There is a bit of a difference.

If you think about it, pretty much every form of life on land lives in a very small zone right on the surface. Even birds, though they can fly, live in trees, bushes, or on the ground. The ocean is a truly 3-D environment; there are critters living at every level, right at the surface, on and beneath the bottom, and anywhere in between.

Ocean zones are divided based on distance from shore, depth, and how far light reaches. The two basic zones are the benthic zone and the pelagic zone.zones

Benthic refers to the bottom of the ocean, from where the waves meet the beach to the deepest part of the ocean. Animals that live in this zone are called benthos and can range from crabs scuttling through the surf or just-discovered creatures at the bottom of the Mariana Trench. This zone covers a huge range, the entire ocean floor, and is divided into the littoral/sublittoral (intertidal/continental shelf), bathyal (continental slope), abyssal (abyssal plain), and hadal (ocean trenches) . Some examples from the deep sea are: hydrothermal vents, cold seeps, hydrothermal seeps, and brine lakes (yep, lakes on the bottom of the sea).

vent

 Hydrothermal vent

Pelagic – the word literally means ‘open sea’, and that’s exactly what this zone is; all of the water from the surface down to the ocean floor. The pelagic zone can be divided further into the photic zone (where light reaches) and the aphotic zone (with little to no visible light). 90% of the ocean’s life lives in the photic zone (also called epipelagic [pronounced epi- (like epi-pen) – pelagic]) and this is also where the ocean’s primary producers live and photosynthesize. When there is no longer enough light for these organisms, like algae and phytoplankton, to photosynthesize you have reached beginning of the aphotic zone. Less than 1% of sunlight reaches the upper layers of this zone and as you travel down to the bottom of the ocean you have now crossed over into…twilight zone

… The twilight zone begins the strange creatures of the deep with big eyes, scary teeth, strange bodies, and – the coolest part – bioluminescence (organisms making their own light). Things just get weirder the further down you go. You start with the mesopelagic, bathypelagic, abyssopelagic, and finally the hadopelagic. As you travel through these zones, organisms have to deal with no light, intense pressure that would crush a human, and wide expanses with little food or prey. These challenges lead to some pretty cool adaptations – check them out below.

Barreleye fish - the green orbs are the eyes and yes, the head is transparent

Barreleye fish – the green orbs are the eyes and yes, the head is transparent. Large eyes capture even the tiniest amount of light, helping this fish see.

deep sea angler fish

Deep sea angler fish. This fish uses the fleshy lure on it’s head to entice prey towards it’s mouth. The tip of the lure is bioluminescent (glows or flashes). Recall Dory and Marlin’s adventures looking for the dive mask?

Aptly named dumbo octopus

The aptly named dumbo octopus. This octopus is specially adapted to live in the deep ocean, able to survive intense pressure and cold temperatures. The flaps or “ears” are used for movement instead of the siphons of other octopuses.

Bioluminescent fish. Light is produced through a chemical reaction in the organism's cells. This adaptation is often used to hide (by breaking up the animal's silhouette or matching light from above) or to startle predators or lure prey.

Bioluminescent fish. Light is produced through a chemical reaction in the organism’s cells. This adaptation is often used to hide (by breaking up the animal’s silhouette or matching light from above) or to startle predators or lure prey.

For more information visit Extreme Science and Marine Bio

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3 responses to “Zones of the Sea: Benthic and Pelagic

  1. This is what I loved about _The Blue Planet_: it reallys is a different world down there. I liked your point that unlike the air, creatures live in every layer of the ocean. I guess there’s no food in the upper atmosphere.

    • Thanks! Another way I like to picture it is that if the atmosphere was inhabited like the ocean, there would be all sorts of critters living from the ground all the way up to space (which would make it very hard for planes!)

  2. Pingback: Shark Week: The Unknowns | E-sea·

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