Personality might seem like it's something you can only have if you've got complex, human-like intelligence, but that isn't necessarily the case. In fact, the brainless sea creature Actinia equina reveal distinct personalities in how they respond to threats.
The beadlet anemone is related to other cnidarians like jellyfish and coral. In their evolutionary history, they diverged from the more complex organisms that ultimately evolved brains. Instead, these organisms have lots of diffuse nerve networks throughout their body. As far as multicellular organisms go, it's hard to imagine ones that are much more basic or any less intelligent.
At least, that's what you'd assume. University of Plymouth researcher Mark Briffa has been able to demonstrate that these anemones have distinct personalities. Although that term has a more specific meaning in a human context, in a broader sense all it really means is that an organism shows a consistent set of behaviors that are not universal to the species as a whole - in other words, one anemone will respond to a situation in a different way than another, depending on differences in their personalities.
To test this out, Briffa and his research team threated the anemones by squirting them with water. He did this to 65 anemones three times each. All the anemones closed up the opening on their surface that serves as a combination mouth and anus - again, these are not complicated creatures - and retracted their tentacles. After a certain amount of time, the anemones would return to normal, but how long it took them depending on the specific anemone, and the time it took each to relax was highly consistent across the three trials.
So that's evidence of a very basic form of personality. What about complex decision-making? Well, whenever two anemones meet, the naturally solitary creatures will fight for territory. They can use special cells called nematocytes to shoot toxin into an opponent, or they can rely on brute size to win the day. If two non-stinging anemones fight, then the bigger will generally win, while if only one stings another, then the size of the former's nematocytes will determine who wins, and if both sting each other, then it depends on how many successful blows each anemone can land on the other.
The point is that there are a number of different outcomes, and more importantly there are a number of different approaches the anemones can take to a fight. In 82 staged fights between anemones, the researchers saw every single permutation, indicating that the anemones were actually choosing how to engage with each other depending, it would seem, on whether they considered it "worth it" to fight.
Obviously, the actual decisions being made here are very basic, but we are talking about organisms without brains - I think it's OK if we grade on a bit of a curve. In fact, this is the first time researchers have been able to demonstrate this level of decision-making in such a simple organism, suggesting that this isn't just the preserve of more complex animals like mammals or birds. It appears some of our most prized cognitive abilities are found very deep in our evolutionary history - even existing long before the emergence of unimportant little things like, say, brains.