Fascinating speculation about fish brain size
I’ll take “questions I didn’t realize I was interested in”. The deeper you go in the ocean, the smaller brains get. From the abstract:
Here, we test three hypotheses of brain size evolution using marine teleost fishes: the direct metabolic constraints hypothesis (DMCH), the expensive tissue hypothesis and the temperature-dependent hypothesis. Our analyses indicate that there is a robust positive correlation between encephalization and basal metabolic rate (BMR) that spans the full range of depths occupied by teleosts from the epipelagic (< 200 m), mesopelagic (200-1000 m) and bathypelagic (> 4000 m). Our results disentangle the effects of temperature and metabolic rate on teleost brain size evolution, supporting the DMCH. Our results agree with previous findings that teleost brain size decreases with depth; however, we also recover a negative correlation between trophic level and encephalization within the mesopelagic zone, a result that runs counter to the expectations of the…
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peakmemory 
It’s amazing such tiny brains work at all
Even more amazing are the brains of honey bees. They engage in some very complex information processing with only a few hundred neurons.
Spiders too