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Animals in the Deep Sea

Instructor: Taormina Lepore

Taormina has taught advanced high school biology, is a science museum educator, and has a Master's degree in museum paleontology.

In the depths of the ocean live many wild and diverse sea animals. From special adaptations to unique modes of life, this lesson will explore a few examples of the animals in the deep sea.

The Deep Sea

Some people say we know more about deep space than we do about the deep sea. Our deep oceans may be on Earth, but the creatures that live within the light-starved depths of the oceans are all too alien.

There are many deep sea animals, and for this lesson we're going to focus on just a few examples of fish, which are vertebrates - animals with backbones, and a few examples of invertebrates - animals without backbones. Each is uniquely adapted to an often harsh environment, and the alien depths of the oceans into which light rarely penetrates.

The zones of the ocean
ocean zones

Before we begin our discussion of deep sea animals, let's define a few terms that relate to the zones of the ocean. These zones will help us understand the levels of the deep sea. The top, sunlit layer of the ocean is known as the epipelagic zone. Plants and photosynthetic organisms can live here. Beneath this zone, from 200 to 1000 meters, is the mesopelagic zone, also known (seriously!) as the twilight zone. Some light can reach this zone, but not enough for plants to grow. Beneath the mesopelagic zone, from 1000 to 4000 meters, is the midnight zone, or bathypelagic zone, so called because no light reaches it. The deepest depths of the ocean are called the abyssal zone, where water is freezing cold and a great amount of pressure is exerted on life that dares to live there, and the hadal zone, where the ocean is extended down and down into enormously deep trenches.

What kinds of animals would be able to survive in such frigid, dark places in the deepest sea?

Deep Sea Fishes

Many of the strangest and most fascinating organisms of the deep sea are the animals that jump straight to mind when you think of the ocean: fish, of course! Highly specialized bony fish, or teleosts, have been around for 300 million years. While many fish live in the deep, including many teleost relatives, we'll explore two bony fish species in particular that paint a bright picture in the gloom.

Lanternfish

A lanternfish with characteristic photophores
lanternfish

These fish are aptly named, because they have special phosphorescent, or glowing, organs called photophores on the sides of their bodies that make them light up and shimmer in the dark. Think of these organs as a kind of fish head light! The pale green or yellow light helps the lanternfish attract mates and spot prey in the dark.

Lanternfish are a mesopelagic species, so they hang about in the twilight of the deep, but rarely venture any further into the depths. It's estimated that lanternfish are so numerous, they make up around 65% of all the deep sea fishes. When a living organism produces a glowing light, it is also known as bioluminescence. We'll look at another fish that exhibits bioluminescence in the deep.

Anglerfish

The humpback anglerfish
anglerfish

Anglerfish are a bathypelagic fish species, seeking their prey by the light of their ghostly angler, which projects from the front of their head. This glowing lure is known as an illicium, Latin for fishing rod. Another bioluminescent species, the anglerfish take on a nightmarish appearance due to their sharp teeth. The illicium can be used both to lure prey and to attract mates, and the source of the light comes from a cooperative relationship with luminescent bacteria.

Deep Sea Invertebrates

There are an untold number of diverse invertebrates; animals without backbones, within our oceans. The deep sea is no exception, and to describe the strange array of beautiful and mysterious invertebrate creatures would take several lessons! Here are two examples of the charismatic invertebrates of the deep.

Vampire Squid

The cloak-like vampire squid
vampire squid

Even the name sounds like something from a horror movie, but vampire squid are very real creatures. They live in any aphotic zone of the deep, meaning any zone without light. Vampire squid live at a depth where oxygen is minimal, yet they can breathe and thrive using the large surface area of their gills.

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