I have to agree with Kas as well, I observe behaviors of my fish being happy, playing chase, exploring the plants, etc. I spoke to someone before at a reptile & invertebrate shop that said reptiles, fish & inverts didn't feel pain or emotions at all. He also thought of them as conversation pieces/ decor' and shouldn't ever be handled. Excuse me, but what a pompus-***! He shouldn't even be working there.
I even have a gecko, she shows when she's not feeling well, or if she's excited about hunting her live crickets.

She even is a bit curious of new things in or outside of her terrarium. I believe every creature, big and small, invert or not, have a mind and their own individualities.
I've seen the snails in my tank bully eachother, ride eacothers' back, and walk upside down on the top of the water to get food. I don't think it's the size of the brain counts, it's all about how it's used. In light of many creatures' small stature, they can certainly make up for with their crafty-ness IMO.

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