11 Animals Without a Brain (and How They Function)

sea star

We all have a common notion that asserts one can’t function without a brain, but there’s nature and its stupefying mechanisms to prove us wrong. As much as it is a sheer necessity for humans to think rationally and put our mind to use, we tend to think there can not be creatures that can not live without a brain. 

No, we are not talking about the fictional zombies

We’re talking about real-life animals, marine animals, to be precise. Several animals, including the infamous jellyfish, exist without a brain and still function anyway. 

So here’s a question: ‘How do animals without a brain function’? Well, it seems that, to survive, you don’t need a brain (unless you’re a human who has daily chores and a corporate life to attend to). These animals function without a brain since most of them have nerves that react towards stimuli. 

That’s okay, but why do these animals not have a brain in the first place? In this post, most animals that you will see are very simple organisms that lack most other organs and not just the brain.

The major conjecture that we can come to is that these organisms don’t need a brain (that’s it), which is why they abandon the existence of it. One brainless animal, the sea squirt, is originally born with a brain, but it later transforms its bodily composition, eradicating the brain from its body completely.

Now that you know about it let’s get to the post already and learn more about these animals that have abandoned the brain’s complete existence, something we humans can not go without. 



jellyfish pink and purple

A jellyfish is probably the most incredible sea creature with absolutely zilch that makes up for its body composition, except for water. The body of a jellyfish is 95% water, lacking essential organs like the brain, heart, and even blood. 

Since they don’t have a distinct brain, as a replacement, they have a set of nerves that extend radially through their body. The nerves act as sensory organs which help the jellyfish to detect touch, light, temperature and respond reflexively to stimuli.

Jellyfish are generally carnivores who usually feed on plankton, crustaceans, small fish, and even other small or dead jellyfish.

Read: What Do Jellyfish Eat?


Sea Cucumbers

Sea Cucumbers, yellow blood
“This photo” by Roban Kramer is licensed under CC BY-SA 4.0

Sea Cucumbers’ shape is more like a sea apple or a sausage shape, and some of the sea cucumbers resemble caterpillars. They mostly have a soft cylindrical body, which is lengthened, sometimes flattened with any sold appendages. 

Sea Cucumbers don’t have a true brain. Their oval cavity is surrounded by neural tissue, but these nerves aren’t necessary because Sea Cucumbers can move even if the nerve rings are removed. These nerve endings give them a sense of touch and are sensitive to light.

The mouth of Sea Cucumbers is surrounded by several tentacles that can be easily pulled back inside. Sea Cucumbers usually grow for about 10-30cm in length; their body shape ranges from almost spherical to wormlike and lacking arms, unlike the other echinoderms. 

Sea Cucumbers are benthic, meaning that they live on the ocean floor and are found in almost every marine environment. Having their own defense mechanism, which they use when threatened, Sea Cucumbers release sticky threads to ensnare their predators, and some of them can even mutilate their body parts. 

Being omnivores, they feed on tiny particles, including algae and minute aquatic animals; just like earthworms, Sea Cucumbers break down their food into smaller pieces that become fodder for bacteria recycle them back into the ocean ecosystem.


Man O’ Wars

Man O' Wars
“This photo” by 4Neus is licensed under CC BY 4.0

First, these are not jellies! People often confuse these punk-rock creatures with jellyfish, but they are way deadly and dangerous compared to jellyfish. The one similarity (amongst the many) that they share will jellies is they have no brain. Of course, they don’t, or you’d be able to see it through their body! 

Second, a Man O’War isn’t an organism, its ORGANISMS. Meaning? One Man O’War is composed of four zooids (or creatures in simpler terms) that can’t continue to exist or function without each other.

Apart from not having a brain, the Man O’War also lacks other essential body organs. However, it has four polyps and a gas-filled bladder. The bladder or the pneumatophore gives the Man O’War its name since the bladder floats above the water resembling a wrecked or ancient warship sailing across the water. 

Think of it as sharks with fins and Man O’Wars with their pneumatophores!

The pneumatophores are mesmerizingly colored in blue and purple; thus, they are also known as ‘blue bottles.’ 

The Man O’Wars are usually found in warm waters, sailing in a colony of thousands; however, they often wash up on the shores. They feed on small invertebrates, fishes, and crustaceans that can be paralyzed with their venomous sting. 



sea star

Starfish or Sea Star are star-shaped echinoderms having unique defensive anatomy with no brain. Their striking colors help them to scare off potential attackers and camouflage them. To make up for the absence of a brain, the Starfish has a nervous system that is a single nerve ring around the mouth consisting of radial nerves on each arm.

These radial nerves make the Starfish retract from light since they act as light-sensitive eyespots (located on the arms). Though the Starfish don’t have a well-defined brain, it actually saves space in their body, and they don’t need one because they are highly sensitive to touch, light, temperature, and the status of water around them. 

The upper surface of a StarFish is made of calcium carbonate known as ossicles, and the honeycomb structure of ossicles makes their skin tough and hard for hunters to break down. Starfish don’t have blood, and as a replacement, seawater is pumped throughout their body.

The seawater provides the Starfish with all essential nutrients for proper functioning of their organs. Starfish can digest their food outside their body, and they usually feed on coral, sponges, oysters, clams, mussels, and other fishes. These scenic creatures can often be spotted in tidal pools, rocky shores, kelp beds, and sandy seabeds.


Sea Urchins

Do Sea Urchins sleep

Sea urchins have a globoid structure without any limbs trait, and most of the species from sea urchins have needles like projections covering the outer body. Sea urchins don’t have a brain; they monitor their actions by a system called the water vascular system. What’s that? They move their body by alternating the portion of water and pressure within the body. In fact, most of the sea fishes pursue similar movements.

Urchins don’t have observable eyes, but many experts presume their whole body is a compound eye that is very sensitive to light.

Sea urchin is generally found in a wide color combination, patterns, and lengths. Some of them have extremely long spines, and some of them have shorter ones. They can inhabit both polar seas and warm regions. Urchins generally live on algae, sponges, starfish, and several sea animals.

They are self-predatory, eating themselves by chewing and cutting with their five-strong teeth. Urchins are small, spherical creatures that can live at depths ranging from 10 feet to 15,500 feet underneath the water as they cannot swim and live on seabeds. You know why? Because they are spikey (Lame, we know!)


Sea Sponges

Sea sponges
“This photo” by Tam Warner Minton is licensed under CC BY 4.0

Sea Sponges have a jelly-like mesohyl, which is sandwiched between two thin layers of cells, and they don’t have a brain or nervous, digestive, or circulatory system.

Sea Sponges have three types of body symmetry, which includes radially symmetrical, bilaterally symmetrical, and asymmetrical; all these symmetries are well suited to meet their lifestyle’s unique demands. The majority of the Sea Sponge species have asymmetrical symmetry. 

Sea Sponges are among the simplest aquatic animals with dense yet porous skeletons that are highly adaptable to their environment. They are potential filter feeders that are important inhabitants of the Coral reef ecosystems because they filter water and collect bacteria to process carbon, nitrogen, and phosphorus. An integral part of the ocean’s ecosystems, Sea Sponges’ filter-feeding protects reefs from extreme fluctuations in nutrient density, temperature, and light. 

They come in a wide variety of shapes, sizes, and wide color variations, which protects them from the sun’s ultraviolet rays. The sea sponges’ regeneration ability allows them to regenerate from the fragments that were broken off or damaged by the sea currents and predators.

Having adopted a particular habitat that allows them to live on hard, rocky surfaces and also on soft sediments like sand and mud, sea Sponges get their food and require oxygen from the water that filters from their porous exterior.




Are you ‘Happy as a Clam’? The phrase generally considers that the clams are happy at the high tides, but does the phrase have anything to do with a real clam’s mood. Definitely not, because a clam doesn’t have a brain in the first place.

This common phrase is derived from the theory that it is only possible to dig up clams during low tides, which is why clams are happy at high tides because they become impossible to be caught by humans when the water’s high and become a meal to the beachgoers.

They are somewhat circular and oval in shape with equally proportioned shells. Surprisingly, although they have no brain, they have other body organs such as a heart, a pseudo-digestive system, and a circulatory system. These filter feeders survive on phytoplankton and zooplankton, while the large clams, also known as the giant clams, feed on the algal sugar and protein. 

To protect themselves from predators, including humans, clams burrow themselves into the soft substrate of the water they live in. Found in varying depths in freshwater, these creatures are also found deep in the sea, living in a cluster on the seafloor. 



Oyesters don't have a brain

The delectably irresistible oysters – but have you paused for a while and given this a thought ‘Do they feel pain?’ For starters, they don’t’ have a brain, but that’s not where the story ends.

Oysters have a nervous system that can detect damage on the skin; however, asserting that they feel pain will be going too far, and we do not have any scientific research to back the statement, so enjoy your oysters with a dash of lemon because they are too good to not be eaten.

Oysters are bivalve mollusks that live in both marine or brackish habitats. In most Oysters species, the valves are highly calcified with an irregularity in shape. Oysters are filter feeders that trap planktons and other particles in their gill’s mucus and transport them to their mouth for digestion. Oysters have three-chambered hearts that lie under the adductor muscle, which pumps colorless blood to all parts of their body. 

Oysters don’t really have a brain, but their system consists of pairs of nerve cords and ganglia. Oysters are a good source of zinc, iron, calcium, and Vitamin A, and they can be simply eaten by opening their shell and everything that it contains, and they are also used in a variety of drinks.


Sea Anemones

Sea Anemones no brain

The Sea Anemones are marine animals who are named after a terrestrial flowering plant anemone due to their colorful appearance. Sea Anemones don’t have discrete brains; rather, they have a primitive nervous system that is involved in maintaining their biochemical and physical responses to various stimuli.

Sea Anemones are capable of changing their body shape dramatically by contracting, lengthening, twisting, and bending; the majority of them are attached to the hard surface for about a week or month, and they can also be seen creeping around on their bases. 

The Sea Anemones have an oral disc that features a ring of tentacles armed with cnidocytes acting as both defense and predatory mechanisms. Sea Anemones use the toxins present in the small vessels of each nematocyst to paralyze the prey to easily move it to the mouth for digestion.

The diet of Sea Anemones usually consists of plankton, crabs, fish; some sea anemones can be greedy and brave at the same time, feeding on starfish or jellyfish. Sea Anemones can easily be found deep into the Oceans and in shallow coastal waters worldwide.


Sea Squirts

Sea Squirts
“This photo” by Prilfish is licensed under CC BY 4.0

Obviously, they are named squirts! With their sac-like structure, you can possibly not find a better name! Sea Squirts have got their names so because when they are removed from the water, they expel water violently from their upper surface like squirting.

These invertebrate filter feeders have a cylindrical structure with a size varying from 0.4 to 12cm, and their one end is always attached to a hard solid surface, with some of them have root-like extensions that help them to grip on to the surface. 

Squirts settle to the bottom and get attached to the surface for the rest of their lifetime. When they have fixated themselves on the sea-ground, it is then that they need to feed. The fantastic transformation begins when the sea squirts absorb all the tadpole parts, including the gills, the twitching tools, and lastly, their little brain, which they feel (not really) they don’t have use of.


Sea Lilies

Sea Lilies are probably the least known animals of the ocean. The basic body structure of Sea Lilies resembles a plant with a stem and a crown, which consists of a cup-like central body with a set of five arms, usually branched resembling feathery stars. 

Though Sea Lily’s don’t have a brain, they have a nervous system consisting of a central nerve ring that surrounds the mouth and radial nerve branched into the arms; these radial nerves serve as sensory nerves.

The mouth of a Sea lily is located on its upper surface and is surrounded by five or more feeding arms. Sea Lily uses their feathery arms to collect the plankton from the water. They can regenerate their body parts that they lost by adverse environmental conditions or by the attacks of predators, and this ability helps them survive attacks by predator fish. 

Sea Lilies inhabit areas of high current flow, living on the substratum. They usually live in clumps and hide their central mass, which helps them to keep their vital body parts safe from injuries. 

Are there Terrestrial Animals that do not have a brain?

No, there are no such known terrestrial animals that do not have a brain. Why? Terrestrial animals seem to have to do a lot to survive the adversity on the land and the plights that haunt their existence. 

To survive, land animals have to save themselves from predators sitting at the apex of the food chain and have to search for food. Unlike aquatic animals who readily get their food floating in the sea (most animals without a brain bump into food instead of searching for it), terrestrial animals have to search for their prey or food, which requires the use of the brain.

Apart from that, aquatic animals, on the other hand, have a medium (the water); thus, the brainless aquatic animals do not really have to move but ‘go with the flow.’ On the other hand, Terrestrial animals have to walk and show locomotory features without which they would not be able to survive for long, which requires decision-making and coordination of nerves and the brain at the same time. 

Read More

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  5. Which Animals Have The Biggest Brain?


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