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Do all animals have brains?

Well, you probably figured out by now, I have an inquiring mind. This time I used it to investigate if animals, other than mammals, fish and birds, used typical brain power to learn and function. I always presumed there must be some brain or at least a coordinating mechanism within every animal to allow it to breathe, move, reproduce, eat, etc.


It looks like I was correct, but I have also learned many animals don’t have the same central brain system we have. In fact, several animals don’t have a brain at all.

As it turns out, many of these are marine organisms, and of those, many are sedentary, spending their entire lives anchored to a rock or coral reef. Central brains are replaced by sensitive tentacles, ganglia and modified decentralized nervous systems in these animals.

Starfish have neither brains nor hearts, yet they can reproduce, hunt and excrete waste. The brain is replaced by a nervous plexus, which, simply stated, is a network of intersecting nerves which control specific bodily functions, such as respiration or circulation. In essence, the central brain we utilize is replaced by a series of mini-brains throughout the body of the starfish.

Comb jellies, likewise, don’t have hearts, brains or a system which controls the excretion of wastes, but again rely on a decentralized nerve-based system to control these functions.

Sea cucumbers use a different mechanism to survive. At the end of each tentacle and in their throat, they have various nerve endings which allow them to send and receive stimuli, based on the needs of the animal. Interestingly, since they, too, don’t have hearts, they rely on a vascular system which transports water, potassium and proteins throughout the body, to ensure the animal survives.

Sponges go one step further, as they utilize totipotent cells. These are essentially undifferentiated embryonic cells which have the potential to develop into any type of cell. This means the animal can adapt virtually any of its cells to perform essential bodily functions. As a result, sponges do not require specific organs or a defined nervous system, since all their vital activity is performed at the cellular level.

Jellyfish have neither brains nor nervous systems, so they rely on what seems to be a random distribution of individual nerves which are interconnected. In actuality, this system effectively takes a touch and light stimuli, received by the animal, and converts those into life-sustaining responses. Although a Portuguese man-of-war looks like a jellyfish (which is a single organism), it is, in fact, a large colony of individual animals, called hydrozoans, each of which provides essential functions to keep the entire colony alive.

Although corals look like tiny plants, they are animals as well! A vast array of individual organisms live as a single colony which functions to preserve each individual, while keeping the colony intact. Their means of survival are, again, slightly different, as they don’t have hearts, brains or a nervous system. These are replaced by a network of polyps with small appendages featuring sophisticated nerve endings. These allow the individuals to catch prey and perform the necessary life-sustaining functions.

Moving to land-based animals: most have a brain and organs to keep them alive, but earthworms don’t. In fact, an earthworm essentially only has a mouth, anus, and muscles encased in a pliable exoskeleton. Again they don’t have a brain and rely on sensory stimuli, such as vibrations, to detect threats.

Leeches also have no brain but do have neural networks which are distributed throughout the body, as in several marine organisms.

So there you have it. Nature is so incredibly complex and ensures the survival of individual organisms through many diverse mechanisms, not just one, as many might presume. While we’re on the subject: does a large brain make you smarter, and does having no brain make you less intelligent? Apparently not. The size of the brain has nothing to do with intelligence. Each of its components facilitates learning and memory. Even tiny organisms with no brains at all can learn to find food and avoid predators.


Geoff Carpentier is a published author, expedition guide and environmental consultant. Visit Geoff on-line on LinkedIn, Facebook and Instagram.

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