From Are There Patterns in Invertebrate Brains and Intelligence?
Reptiles and fish sometimes show signs of intelligence despite having quite different brains from mammals. But, being exothermic, they don’t do much of anything very often. For example, turtles may rescue each other, but can also spend months in a state of icy torpor with little adverse effect. At one time, it was assumed that the intelligence to rescue would not co-exist with lengthy inertia (the reptilian or triune brain hypothesis). Actually, the two qualities can co-exist, though they wouldn’t be simultaneous.
Invertebrate just means “not a vertebrate,” so there is no single type of invertebrate brain:
Invertebrates have immensely diverse nervous structures and body plans, revealing the variety of solutions evolved by animals living successfully in all kinds of niches.
And that is where things get a bit complicated. Starfish, essentially, do not have a brain or even ganglia, just a nerve ring. Their behavior has accordingly been attributed to “self-organized behavioral patterns” not strictly determined by external stimuli. It would be good to unpack what that implies.
Crayfish seem somewhere in the middle, that is, smarter than we used to think, even though the crustacean brain (a “microbrain” of three fused ganglia) is often studied on account of its comparative simplicity.
We keep learning new complexities of other invertebrate behavior too. For example, mantis shrimp use a polarizing light display to warn their fellows that a hiding place from predators is already taken. More.
See also: Furry, feathery, and finny animals speak their minds
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