What defines a brain?

A side effect of my bafflement yesterday over how lobsters became some sort of strange right-wing analogy for the rightness of there being winners and losers (or something) was getting into a discussion about whether lobsters have brains.

That decapod crustaceans are brainless is a claim I have seen repeated many times, often in the service of the claim that lobsters cannot feel pain. This article, refuting Jordan Peterson, said:

(L)obsters don’t even have a brain, just an aglomerate of nerve endings called ganglia.

This is a bad description of ganglia. It makes it sound like there are no cell bodies in ganglia, where there usually are. Here are some. This is from the abdominal ganglion of Louisiana red swamp crayfish (Procambarus clarkii):

These show cell bodies of leg motor neurons from several species (sand crabs and crayfish, I think; these pics go back to my doctoral work).

These are neurons in a ganglion from a slipper lobster (Ibacus peronii), where those big black cell bodies are very easy to see:

And these are leg motor neurons in slipper lobster:

And there is substantial structure within that alleged “not a brain” in the front:

And we’re know this for well over a century, as this drawing from 1890 by master neuroanatomist Gustav Retzius shows:

So ganglia are more than “nerve endings.” So putting that aside, are there other features that make brains, brains?

Intuitively, when I think about brains, I think of a few main features. Two anatomical, and one functional:

1. Brains are big, single cluster of neurons. Even though there may be many neurons in, say, the digestive system (and there are not as many as some people claim), it’s so diffuse that nobody would call it a brain.
2. It’s in the head, near lots of sensory organs. In humans, our brain is right next door to our eyes, ears, nose, and mouth, which covers a lot of the old-fashioned senses.
3. It’s a major coordinating center for behaviour.

Decapod crustaceans (not to mention many other invertebrates) meet all those criteria. Sure, the proportion of neurons in the decapod crustacean brain may be smaller than vertebrates, but I have never seen a generally agreed upon amount of neural tissue that something must have to be a brain instead of a “ganglion in the front of the animal.”

I have a sneaking suspicion that some people will argue that only vertebrates can have brains because we are vertebrates, and vertebrates must be special, because we are vertebrates. That is, people will define brains in a way to stroke human egos.
 And, as I implied above, some people make the “no brains” claim out of self-interest. I don’t think it’s any accident that I see “lobsters don’t have brains” coming from institutes that have close ties to commercial lobster fisheries.

I suppose that some could argue that limiting the word “brain” to vertebrates is a way of bringing recognizing that vertebrate and invertebrate nervous systems are structured very differently. They are, but why only do this for one part of the nervous system? This is a little bit like saying “invertebrates don’t have eyes,” because they have compound eyes instead of our camera-style eyes. We routinely give things in invertebrates and vertebrates the same names if they have the same functions.

And in practice, I see people referring to octopus brains all the time. They do so even though, like other invertebrates, a large proportion of the nervous system sits outside the brain. From memory, roughly half the neurons in an octopus reside in its arms.

In practice, I am far from the only person that calls the clump of neurons at the front end of decapod crustaceans, “brains.” From this page:

So, fellow neuroscientists, if you don’t think invertebrates can have brains, why not? What is your dividing line?

Hat tip to Hilary Gerstein.