I'm designing an alien species roughly based on starfish. Starfish in the real world don't have brains or blood, they just let seawater run their nervous and circulatory systems, but I figure a species of any intelligence would need an actual brain. I'm wondering if it would be possible to have a brain that isn't a solid structure but a sort of cerebral fluid in which neurons mingle to form thought. Is that an even slightly viable evolutionary strategy?
In terrestrial biology, brains work by having many neurons, each with many connections to other neurons. It is thought that the particular number and position of the connections between neurons - the synapses - affect the operation of the network and allow learning. The consequence of this is that each neuron exists in a relatively fixed spatial arrangement with other neurons, which enforces a degree of solidity.
If we were to have a liquid brain, in which its constituent cells existed in a fluid and were capable of moving past and around each-other, it would be difficult to form any long-term connections. Having long axons and dendrites to connect to other cells would be impractical, as agitation of the brain would result in cell damage, even more easily than occurs in terrestrial animals' brains. Without these elongated structures, this limits the number of other cells to which any given cell can connect. With fewer connections, the brain becomes less complex. When agitation of the fluid brain can break connections, the brain becomes unreliable.
SO, TL,DR: No, a fluid brain would not be practical, since it could not form a sufficiency of long-term connections to other cells.
The machines we are used to, as well as our brains, rely on parts mounted in stable places with connections between them like axons, wires, or shafts. It is these solid-state connections what gives structure to the machine, what allows one part to move another one and that one move yet another one, instead of just making random noise.
However, nature shows us that there is another approach. When the parts of a machine are sufficiently specific, like enzymes and their substrates or complementary DNA strands, you can just let them float freely instead of keeping them near the parts they need to interact with. A floating enzyme molecule can bump into thousands of incompatible things and nothing happens, but once it meets with its target by chance, they click and perform an operation. The result of that operation then floats on and waits to meet the next operand.
You may think this must be terribly inefficient, and to some extent you would be right. Systems like this can be fast as long as they are small, like cells; Or they can be slow but smart, like the mentioned DNA computers. Either way, it sounds like a viable strategy.
You might want to check out quorum sensing in bacteria. Bacteria can form a colony that behaves like a single bit more intelligent organism. Nothing would really prevent same thing happening in liquid with a bit more loose organization. It's functioning is not going to be as determinate as that of a brain or even that of a colony if the cells float around, but that was not stated as a requirement anyway.
Bacterial communication relies on versatile chemical signaling molecules called autoinducers, which regulate bacterial gene expression in a process known as quorum sensing. Like languages between humans, these signals vary between species. Some bacterial species can interpret many different signals, while others respond to a select few. Quorum-sensing allows individual bacteria within colonies to coordinate and carry out colony-wide functions such as: sporulation, bioluminescence, virulence, conjugation, competence and biofilm formation.
The brains of your hypothetical aliens could be fluidic computational systems. Fluidics is a technology that uses fluid instead of electricity. Turns out this can be used to make computers. Trouble is that electricity powered computers are faster and better. By crude analogy, brains might evolve that work on fluidic principles.
The main problem is that neurological systems based on cellular structures and nerves running on chemistry and electrochemical impulses are far superior to fluidic equivalents.
The other big problem is that fluidic brains will work better at organising the behaviour and cognition of fairly simple creatures like Starfish. It seems unlikely they will work for aliens who can possess civilisations and technology.
There is the not inconsiderable problem of how their memories will work. Although an organism could have a distributed system of cells in its body that act like the neurological structures that are the basis of our memories. Now that's plausible.
For further research, you may wish to look Stanislaw Lem's novel Solaris (1961) which has planetary wide ocean that is an intelligent being. Well, that's the premise of the novel. The story is mainly the difficulties of communicating with it.
Similarly you simply make it a premise of your story that your aliens' nervous system work on the starfish principle of fluid flow (as you described in your question). Unless your story is about research into the neurology of your aliens, don't need to explain in detail how their brains work.
Just to clarify, starfish have water circulatory systems, but they don't use seawater to run their nervous systems. Starfish have nerves just like you do, and even something sort of like a brain, in the form of a central "nerve ring". This ring is different structurally from the ganglion that most cephalized species that lack a true brain have, but it still works essentially the same way, as a control center for the peripheral nerves.
Now, as for the idea of a "liquid brain", this is a cool idea. On the surface, the answer would be no. Brains are dense networks of connected neurons. Even a brain that worked very differently from ours would probably still need to be able to form connections in order to store the patterns that it would need to have things like thoughts and memories, or even to do the basic work of a brain. There are a couple of ways around this though I think.
The first is to have it not really be a liquid, but some sort of semi-fluid slime that still has a densely connected microstructure, like snot or biofilm. This sort of substance could still hold enough of a structure for connections to be stable, while being able to deform and fill its space like a liquid.
If you want a true liquid, there are really only two ways I think it could work.
it's a very still, cold liquid that doesn't get jossled around much. This could work for a sessile species on some freezing planet, but a good smack would probably be enough to give them brain damage, so there would have to be some explanation for how such a fragile brain can exist at all in this environment. Something like a superfluid brain existing in a species that lives in the cold vaccuum of space or something would be cool and might be able to form the connections necessary to work in a liquid medium.
Quantum magic handwavy goodness. Birds can see magnetic field lines via the action of entangled cryptochrome molecules that are connected to other molecules nowhere near them. Plants make use of quantum computing algorithms to ensure that photons always take the most efficient path through the thylakoid. We have no idea how these quantum effects are kept stable in the wet, warm chaos that is a biological system, but they are. I have no idea how such a thing could possibly work, but you could essentially do the same thing as the superfluid brain (connections are formed by quantum entanglement and other assorted handwaving) but in a regular environment, just going "yadda yadda quantum biology yadda yadda" and not worrying too much about the details. There are real quantum systems that maintain their stability in biological environments. We really have no idea how they do it. And so there is a real possibility that a biological liquid quantum computer could exist in story space, even if it's extremely unlikely that one ever really would.
Yes but it would have to be huge due to having to use water flow rather than electrical impulses.
So computers are binary, on/off, you could do the same thing with hydraulics, valves and pumps etc.. and we do for a lot of machinery on a very simple level. But for any sort of reasonable complexity it would have to be enormous.
If they do live in water, then you could say that they have a small liquid brain that acts as an interface between them and the water, and that the water acts like a "shared" brain between them and nature. In that sense they are like a big co-operating organism with huge computational and experiencial power.
An interesting analogy to a "liquid brain" is the adaptive immune system. Our body's B-cells and T-cells really do compute in that they memorize self (to know not to attack it) and memorize pathogens. The immune system is developmentally distant from our neural system (mesoderm derived vs ectoderm), but achieves some similar computational goals.