Mindless Wonders: Animals without Brains

Animals without brains

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The brain is the epicenter of our body; it’s where all the action happens and, without it, we’d fail to function. Not only does your brain tell your body when to move, talk, or sleep, it also controls systems without you even thinking about it, such as your heart.

With this in mind, it’s hard to believe that there are lots of animals out there that don’t even have a brain! How do they survive, and how do their bodies function? Let’s dive into the amazing world of these mindless wonders!

How do Brainless Animals Function?

How do brainless animals function?

It’s hard for you and I to comprehend how any living creature could survive without a brain; that’s your brain working overtime right there! But let’s dial it down and think about how many animals exist without one.

Imagine your brain as the control center for your entire body; everything that happens passes through the brain and receives direction from it via your nervous system. Animals that don’t have brains still have basic nervous systems that contain neurons that allow them to respond to external stimuli. It’s just that there’s nobody at the helm.

However, animals such as jellyfish don’t have a nervous system that’s anywhere near the same as ours. They have what is known as a ring nervous system as opposed to a central nervous system. This is made up from nets of nerves around which the neurons can send signals to the muscles and other parts of the animal.

But because this nervous system is not as complex as ours, its function is limited to things like eating, swimming, and reproducing. Animals that don’t have a brain are much less likely to be able to make decisions, be self-aware, or have any other complex cognitive processes. That said, it is thought that some species of jelly, like the box jellyfish, are capable of navigation.

Examples of Animals without Brains

It’s incredible to think that some creatures have been on earth for millions of years and survived without seemingly, the most important organ in the body. I’d love to share some information on some of these fascinating creatures and how they get through life with a much simpler physiology than our own.


Jellyfish have no brain, nor do they have a heart, blood, or bones

Jellyfish are probably the first animals you think of when asked to come up with a creature that doesn’t have a brain. They might appear to float lifelessly around the ocean, getting in the way and stinging when you’re trying to swim, but they’re slightly more complex than we think.

It’s true that they have no brain, nor do they have a heart, blood, or bones. So what do they have exactly? Jellies have two nervous systems; a large net of nerves designed to aid in things like swimming and a smaller net of nerves that they use for everything else. This includes their responses to threats when they’ll curl up into a ball and their eating.

The large system is made up from rhopalia which are attached to the jellyfish’s bell and function in a very similar manner to our inner ear, letting the jellyfish navigate its surroundings by understanding up and down. They also allow them to sense chemicals and light. 

But what I find most interesting about jellies is that they’re the first animal without a brain known to sleep. They even have their own circadian rhythm! But their sleep isn’t the same as ours although they’ve been observed to be in a sleep-like state which usually happens at night. What’s more, they’ve been shown to be less responsive when in this state.

Sea Cucumbers

In addition to having no brain, sea cucumbers have no sensory organs such as ears, eyes, tongues, etc

Sea cucumbers live on the ocean floor and are found all over the world in all types of water. They’re incredibly primitive creatures, and one thing that demonstrates this is their lack of a brain.

Around the mouth of a sea cucumber is a ring of neural tissue which is thought to control signals, via nerves, to the pharynx and the tentacles. But what’s interesting is that, when this ring is removed, the sea cucumber’s movement and function appear to be uninhibited.

In addition to having no brain, these marine animals have no sensory organs such as ears, eyes, tongues, etc. While there are a few exceptions to this, it would seem that the sea cucumber manages perfectly well without them. That system of nerves and neurons I talked about before is what these simplistic creatures use to navigate the world around them.

Sea Stars

Sea stars have no brains but they have a ring of nerves around their mouths as well as one radial nerve which runs along each arm

Have you ever wondered why Patrick Star from the popular cartoon, Spongebob Squarepants, is so, well…dumb? It’s because the creators are trying to portray the fact that sea stars, or starfish as they’re commonly called, don’t have brains.

Despite struggling to concentrate and often seen drooling, Patrick seems to get on just fine. But what about his real-life counterparts? Sea stars are found in oceans around the world and there are more than 1800 species.

Just like the sea cucumbers that I talked about in the last section, sea stars have a ring of nerves around their mouths as well as one radial nerve which runs along each arm. The ring around the mouth is responsible for helping the starfish sense things, as well as with navigation and coordination. With that in mind, it’s worth noting that, while they can navigate, sea stars are incapable of preplanning their movements.

This is complemented by a peripheral nervous system and this is connected to the complex nervous system via the skin. What’s interesting about sense in sea stars is that each arm can detect things independently. For example, if one limb ‘smells’ the scent of food, it will override the rest of the nervous system and become responsible for the movement of the whole animal for the time being.

However, when it comes to movement, it’s not all to do with the brain. Sea stars have lots of tiny feet on the underside of their body, and scientists believe that movement has just as much to do with mechanics. For example, when one foot pushes against the seafloor, this pressure is felt by the others that all follow suit.

Sea Anemones

The sea anemone does not have any sensory organs but instead benefits from sensory cells such as chemoreceptors

The sea anemone is a type of marine invertebrate and, for all intents and purposes, it’s thought of as more of a decorative creature than something functional. While these animals do lack a brain, they still have a nervous system. However, it’s incredibly primitive.

Located in the gastrodermis and the epidermis are two nerve nets that are connected at the pharynx and other locations around the sea anemone’s body. Since these animals do not have the complex muscles of other, more developed creatures, they cannot move in the same way. That said, there are fibers that, when stimulated, can contract the body of the sea anemone, allowing it to bring its tentacles in.

The sea anemone does not have any sensory organs but instead benefits from sensory cells such as chemoreceptors.

Despite being one of the most primitive types of creatures in terms of its nervous system, the sea anemone is actually proving to be very beneficial to humans. That’s because it’s been used to look back at how the heads of more developed animals evolved. Despite being significantly different, it’s thought that there are similarities in the genes of animals like a fully developed head and those of the sea anemone.

What’s more, since these creatures are known to be able to regenerate their nerve cells, scientists are hoping that they could provide us with clues on how to regrow human brain tissue. Just imagine what this could mean for the treatment of neurological diseases!


Sea sponges are brainless animals

I’ll go back to my Spongebob reference from earlier; while poor old Patrick is given the dunce hat, the main man Spongebob seems to be the image of intelligence. However, if he were a real person (or sponge), he probably wouldn’t be able to work in a greasy spoon or take boating lessons.

That’s because sea sponges are also brainless animals and, unlike many of the other creatures I have discussed here, sponges don’t even have a nervous system! But that doesn’t mean that they aren’t capable of surviving. In fact, they’re pretty good at sensing what’s going on around them and even responding to it.

It’s known that if you touch a sea sponge, it will contract in response, and one scientist even caught a sponge sneezing; gesundheit! It’s thought that these types of responses are due to cilia; structures that allow the animal to sense.

It’s thought that sea sponges have been on earth for more than 600 million years, and they don’t seem to have evolved much in that time, having no organs, no circulation; no nothing! However, they could provide clues as to how complex brains evolved. They do have 18 different types of cells, and it’s believed that creatures like sponges were the building blocks for the evolution of nervous systems as we know them today.

Sea Squirts

Sea squirts are of the world’s most primitive animals and it doesn’t have a brain

The sea squirt; that’s a cute name, right? Well, it’s certainly fitting for such a small creature that kind of looks like a tadpole with one eye, a tail, and a spinal cord. It’s another of the world’s most primitive animals and of course, it doesn’t have a brain.

But it’s not because they weren’t born with a brain; it’s because sea squirts eat their own brains after reproducing!

These creatures, after entering their tadpole-like stage, will bury themselves head down into the seafloor. Their gonads, which contain both eggs and sperm, point upwards and squirt out cells in order to reproduce; it’s not hard to see where the creature got its name.

After this, it’s stuck in this inverted position forever, and as chemicals and metals enter the gonads, rendering the animal lifeless. It’s at this point that it feasts on its own brain as a final source of energy. At this point, the nervous system degrades entirely and the creature wafts in the ocean current.


While coral don’t have brains, they do have nerve nets

It isn’t hard to imagine that coral is brainless. After all, it’s a seemingly inanimate animal that some people even confuse for a plant (it’s not because it doesn’t produce its own food via photosynthesis.) But while these marine creatures don’t have brains, they do have nerve nets, just like a lot of the animals I have already talked about.

Corals, and there are more than six thousand species, have nerve nets that stretch from the mouth to the tentacles. While they don’t actually have any sensory organs, they’re still capable of sensing the world around them thanks to chemoreceptors. One of the main benefits of this is that it helps the coral to detect prey.

What’s more, the nerve endings within the nerve net allow the coral polyps to experience the world around them. This isn’t in the same way as you and I, for example, coral cannot feel pain. On top of this, since the nerve net is not a complex system, coral will respond to physical touch with the same motor responses as other creatures with a similar physiology.

Bivalves (Mussels, Clams etc)

Instead of a brain acting as the control center, bivalves have a nerve network and three pairs of ganglia which are connected by one nerve cord

Animals like oysters, clams, and mussels have a unique nervous system that’s vastly different to our own. Instead of a brain acting as the control center, these simplistic creatures have a nerve network and three pairs of ganglia which are connected by one nerve cord.

Amazingly, while their nervous systems are not very developed, a single clam is thought to possess around 6000 neurons.

The three pairs of ganglia are responsible for controlling different parts of the animal. For example, the cerebral ganglia control the bivalve’s senses while the pedal ganglia are in control of the foot. The final pair is the visceral ganglia which are responsible for swimming.

While bivalves certainly aren’t capable of complex cognitive functions, there has been evidence to suggest that their nervous systems can respond to hormonal stimuli such as the release of serotonin and dopamine.

Sea Urchins

Sea urchins lack a brain

Our final mindless creature is the humble sea urchin. A delicacy in many places around the world, these spiny creatures are a part of a family containing around 950 species. But what they all have in common is the lack of a brain.

That doesn’t mean to say that they can’t survive though and that’s thanks to a neural ring around the mouth which is the center of its nervous system. From here, there are five radial nerves that connect to a series of smaller nerves around the body of the sea urchin.

Amazingly, sea urchins are able to sense the world around them thanks to sensory cells in the feet, mouth, and spines. They’re incredibly sensitive to light and touch but what’s really interesting is that, despite the lack of eyes as we know them, sea urchins’ entire bodies are thought to function as a compound eye. It’s also been noted that they have photoreceptor cells in their feet that further allow them to ‘see’ and navigate.

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