Which animals do not have a brain?

You need a brain to live, right?  Nope!  Well, you do, but there are a fair number of Animal Kingdom cousins who don’t.  Let me introduce you to some of them!

Photo by Daniel Battershell

 

If I only had a brain…

First, a quick definition of what kind of thinking organ we’re looking for. 

The thinking that happens in our brain (conscious and otherwise) is carried out by neurons – nerve cells that process information by sending signals wherever they need to go.  A bunch of neurons gathered together directing traffic as part of the central nervous system is considered to be a brain.

A bunch of nerve cells clustered together is called a ganglion, and if it’s part of the peripheral nervous system (as opposed to central) then it’s not officially a brain.  (As a note, part of our brain is called the basal ganglia, but it’s argued that this region should be called the basal nuclei to be less confused with non-brain ganglia.)

Regardless, while some of the animals on our list may have ganglia controlling some of their functions, most of the animals here don’t even have any ganglia at all!

 

Tunicate

Commonly known as the sea squirt (that’s the cutest name!), this marine filter feeder looks and functions like a blobby straw.  It’s been around since the Cambrian Period, so it has done well for itself without a brain.

But get this – only the adult sea squirt has no brain.  A baby squirt, which is a tadpole-looking larva, actually has a tiny brain and one eye, and it can swim around but it can’t eat.  When the juvenile gets hungry enough to become a grown-up, it finds a place on the ocean floor to settle in for a filter-feeding and stationary adult life.  Once rooted, the baby squirt grows and absorbs all the parts it no longer needs, including its tail, eye, and brain!  These useless bits turn into new parts as the sea squirt becomes fully grown…and brainless.

Trichoplax adhaerenes

This creature is only a millimeter wide and it sucks up food with its underside, so we’re going to give it a break and totally excuse it for not having a brain.

This creature doesn’t have a cute nickname as of yet, but the phylum name (placozoa) means “flat animals.”  So far, trichoplax is the only species in the phylum, but we may discover more species in there as we look more closely down the road.

Trichoplax looks like a teeny, grayish, almost transparent, shapeshifting pancake.  It also needs a cute name.  Squirmy Cake, perhaps?



Photo by Bernd Schierwater …of a squirmy cake

Echinoderms

These are our good friends the sea stars, urchins, sea lilies, and sea cucumbers.  A few have ganglia, but nobody here has an actual brain.  There’s no planning ahead in the echinoderm’s daily life.

Sea lilies are rooted to the ocean floor and gather food via their five pairs of feathery arms, no thinking necessary.  The others, like the urchins, creep around looking for their food.

Sea stars have no ganglia at all, yet they have some sense of touch, smell, sight, and so forth.  Apparently, if one of the sea star’s arms smells something good, it stages a coup over the other arms’ initiatives and starts pulling the creature towards the food source. 

Sea cucumbers may be brainless, but their defense mechanisms are genius.  They can disgorge their guts and internal organs, startling and grossing out a would-be attacker.  They also can eject long sticky tubes from their anus which can ensnare and permanently disable a predator.  Disgusting but effective, which seems pretty smart to me.

Jellyfish

Instead of having a brain or even ganglia, jellyfish manage to get their business done by virtue of a neural net – a system of connected neurons interwoven around the animal’s body.

Like the humble squirmy cake, jellies can be 1 mm wide …but the big ones can get you with 100-foot-long tentacles, and some of the little ones can kill you with relative ease, so brain or not, it’s best not mess with that whole phylum.

Corals and Anemones

Like the jellies, corals and anemones lack a centralized nervous system and instead have a neural net of sorts initiating movement around the body as needed.

By the way, sea anemones, corals, and jellyfish have all digestive chambers with a single opening, which serves as both the mouth and the anus.  Just thought I’d share that.  (And put these on the list of things I’d rather not be reincarnated as.) 

Sea Sponge

Perhaps the most famous for not having a brain, the sea sponge doesn’t even have a digestive, nervous or circulatory system.  Instead, it has a bunch of unspecialized cells that can migrate around the animal’s body and transform into whatever type of cell is needed at the time.  How cool is that? 

And check it out – sponges can sneeze.  And while our human sneezes are fleeting, a sea sponge sneeze can last for 30 to 60 minutes!  This impressive feat is explained in an article here which notes, “Sponges are the only multicellular animals without a nervous system. They do not have any nerve cells or sensory cells. However, touch or pressure to the outside of a sponge will cause a local contraction of its body.”

Bivalves

These are your clams, oysters and mussels, which don’t have brains but do have ganglia, so in the company of all these other brainless creatures we can go ahead and give the bivalves some little graduation caps.

Honorable Mention for Slime

Slime mold is not a member of the Animal Kingdom and so can’t be included in this list, but it deserves mention in the annals of brainless function because, according to a study by the University of Sydney, this mold has memory!

This single cell organism leaves a trail of slime to tell where it’s been, and research has shown that slime mold is capable of anticipating periodic events and even solving mazes.  Biologist Chris Reid admits, "I, for one, welcome our new gelatinous overlords."