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11 Types of Animals Without Brains (with Pictures)

The brain is such an essential component of any living animal, it’s hard to think one could survive without one. Sure plants and trees don’t have brains and they’re alive, but animals are much more complex organisms. Regardless, there are many different animals found throughout the world that do not possess a brain yet still thrive in the animal kingdom.

But what classifies something as an animal?

If something falls into one of the following 6 categories, it’s an animal.

  1. Mammals
  2. Birds
  3. Reptiles
  4. Amphibians
  5. Fish
  6. Invertebrates

The large majority of species within all 6 of these animal categories posses a brain, though some don’t. Then again, some animals have multiple brains! Let’s have a look at some of the ones that don’t.

11 Animals Without Brains

1. Jellyfish

jellyfish glowing in the dark underwater

Scientific Name: Scyphozoa

Even though jellyfish don’t have brains, they do have neurons that send various signals all throughout the animal’s body. Jellyfish also do not possess lungs or a heart. They instead absorb oxygen directly through their thin skin.

Since jellyfish don’t have blood, they don’t need a heart. A fun fact about jellyfish is that they have actually been to space. In 1991, scientists put thousands of moonfish jellyfish on the space shuttle Columbia to see how micro-gravity would affect them. What they found was that the jellyfish multiplied and that those creatures that were “born” in space had a problem handling gravity.

2. Sea Cucumbers

giant sea cucumber | image by Neil DeMaster via Flickr | CC BY-ND 2.0

Scientific Name: Holothuroidea

The sea cucumber doesn’t have a brain and instead has an oral cavity that is surrounded by neural tissue, which sends signals to its mouth and tentacles. They move along the seafloor scavenging for food, looking for things like decomposing matter and algae.

Because of the way they feed, they often consume mud and sand, which they will sift through for their food and then expel. Sea cucumbers are a common food source for various sea creatures, including turtles, crabs, and fish.

3. Sea Urchins

Scientific Name: Echinoidea

Sea urchins are another animal without a brain. They instead have a large nerve ring that surrounds their mouth. On average, sea urchins are about 1 to 4 inches in size.

The red sea urchin (Mesocentrotus franciscanus), however, is the largest of the species and can reach sizes of about 7 inches, according to the Alaska Department of Fish and Game. Sea urchins are covered in pointy spines, and the word urchin is actually a 13th century French word that means hedgehog.

4. Starfish

Scientific Name: Asteroidea

Starfish are able to digest food outside of their body even though they don’t have blood or a brain. That, however, isn’t the most interesting aspect of these animals without brains. Starfish have the ability to regenerate their arms, though it can take months or years to fully grow back.

They also have eyes and tiny feet, both of which are located at the end of their arms, and their stomach allows them to digest their food on the outside of their body.

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5. Sea Sponges

Scientific Name: Porifera

Sea sponges are primitive creatures that have no brain and no internal organs. These immobile animals survive by consuming tiny organic particles and organisms that they filter from the water around them.

According to the Sanibel Sea School, some sea sponges can live over 200 years, and fossil records have shown that sea sponges were around 600 million years ago. There are over 5,000 species of sea sponges spread out across the world. While most are found in salt water, about 150 species live in fresh water.

6. Sea Lilies

Feather stars under the sea
Feather stars under the sea | image by Anders Poulsen via Wikimedia Commons | CC BY-SA 3.0

Scientific Name: Crinoidea

Despite their appearance and name, sea lilies are not plants. They are invertebrate marine animals that don’t possess a brain. They are typically found on the sea floor but can float throughout the ocean when looking for a new home.

Sea lilies feed on detritus that drifts through the ocean, and they can reach heights of up to 6 feet. Sea lilies are related to sea urchins and starfish, and they were once thought to be extinct. A group of dense growing sea lilies are beneficial for the ocean’s ecosystem as it provides a home for various other marine life.

7. Corals

Scientific Name: Anthozoa

Corals do not have a brain, but they do have a nervous system known as a nerve net. This system extends from the animal’s mouth down to its tentacles. There are an average of 6,000 species of corals, and they come in various colors, sizes, and shapes.

They are a slow growing animal, growing only about 2 centimeters every year. Corals are also an important part of a balanced ecosystem, and provide a home to nearly 25-percent of all the fish in the sea, according to National Geographic.

8. Clams

Scientific Name: Mercenaria

While clams don’t have brains like humans or mammals, they do have a nervous system that allows them to react to and feel things. Clams are bivalve shellfish that can be found in both salt water and fresh water.

They are filter feeders who consume zooplankton and phytoplankton that they filter from the water. There are over 15,000 species of claims, and the largest species is the giant clam (Tridacna gigas). The giant clam can grow over 4 feet long and can live for about 100 years.

9. Sea Squirts

sea squirts | image by U.S. Geological Survey

Scientific Name: Ascidiacea

Instead of a brain, sea squirts have a group of nerve cells that let them sense touch. These hermaphroditic animals have larvae that are similar in appearance to tadpoles and actually swim freely around the water.

Sea squirts commonly attach themselves to ships, which then transport these unique creatures to different areas throughout the ocean. There are over 3,000 species of sea squirts, and they get their name because they “squirt” water when they are removed from their home.

10. Oysters

examining oysters | image by Alabama Extension

Scientific Name: Ostreidae

While oysters don’t have a central brain, they do have a nervous system, as well as internal organs and a heart. Like many of the animals on our list, oysters are filter feeds and can filter up to 1.3 gallons of water every hour.

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In captivity, an oyster can live up to 20 years, and they have been around for about 15 million years. Even though they don’t possess a brain like many animals, they do have the ability to change their gender, and it is not uncommon for them to change more than once.

11. Sea Anemones

sea anemones | image by Bernard Spragg. NZ

Scientific Name: Actiniaria

Sea anemones are related to corals and jellyfish. And like those animals, sea anemones don’t have a brain but do possess a nervous system, though not a complex one. Their system is more like a nerve net that lets them respond to stimuli.

Because of their colorful plant-like appearance, sea anemones are known as the “flowers of the sea”. But these beautiful creatures are actually predators that prey on various marine life, including crabs, fish, and plankton.

They use their stinging tentacles to entrap and subdue their prey. Once paralyzed, they use their tentacles to move the prey to their mouth to eat.