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How do animals breathe underwater?

Hundreds of millions of years ago our ancestors had this ability to breathe in water, but it was lost in the course of evolution.

How do different types of animals breathe?
How do different types of animals breathe?

How do aquatic animals breathe beneath the water?

Hundreds of millions of years ago the very, very distant ancestors of humans, and of all the land animals with spines and four limbs, had this ability to breathe underwater, but it was lost after the first air-breathing creatures began to live on earth full-time.

🔹 Today, humans can only breathe in water with special equipment or in fiction with unique underwater abilities, like Aquaman.

The comics explain how the hybrid Aquaman, half human, half Atlantean, and all his human-like Atlantean cousins, can breathe in the depths of the ocean thanks to "gills" that are not visible, so the details are left to the imagination of the reader or viewer. 

But how exactly do real-world creatures breathe in their watery environments?

There is a great deal of dissolved oxygen in most of the planet's seas, lakes and rivers, although our air-breathing lungs simply cannot process it. But the inhabitants of the aquatic world have developed other methods to access the oxygen in the water.

Do all aquatic animals breathe oxygen dissolved in water?

Some animals, such as jellyfish, absorb oxygen from the water directly through their skin. A gastrovascular cavity within their bodies serves a dual purpose: digesting food and transporting oxygen and carbon dioxide.

In fact, the first microbial life forms on Earth that used oxygen obtained it in the same way. This form of respiration probably appeared about 2.8 billion years ago.

Breathing through diffusion of oxygen over the body surface is also found in echinoderms, a group of sea animals that includes starfish, sea urchins, and sea cucumbers.

Starfish absorb oxygen as water flows through bumps on their skin called papules, and through grooves in other structures called tubular feet. However, some types of shallow-water sea cucumbers have a different type of specialized breathing adaptation: a breathing "tree" structure located in the body cavity near the anus. 

As the cucumber's rectal opening sucks water into its body, the respiratory tree draws in oxygen and expels carbon dioxide (yes, that's just what you're thinking: breathe through the butt).

How do fish breathe underwater?

In fish, the gills have proven to be a successful system for breathing, using a network of blood vessels to extract oxygen from flowing water and diffuse it through the gill membranes, according to the National Oceanic and Atmospheric Administration (NOAA).

In most fish, the gills have this same pathway. However, gills are not exactly a one-size-fits-all utility. Their structure may vary from species to species to accommodate oxygen needs. Indeed, active predators have different gills for their higher oxygen demand.

The shape of the gills may even vary between members of the same species, depending on the oxygen conditions in the water where they live. 

Studies have shown that fish can adapt their gill morphology when their aquatic habitat becomes polluted. Over time, their gill filaments become more condensed to resist contaminants in the water.