Why are fish brains so small?

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 expensive tissue hypothesis. We hypothesize that mesopelagic fishes at lower trophic levels may be investing more in neural tissue related to the detection of small prey items in a low-light environment.

In other words, there are metabolic constraints at lower ocean depths over and above the temperature-dependence. And interestingly, fish that are lower on the food chain (trophic levels) have relatively larger brains; possibly because it requires more difficult sensory/etc computations to find their prey in a sensory-deficient environment:

Although encephalization in marine fishes of the mesopelagic was partially explained by trophic level (Tables 2 and 3), this finding disagrees with expectations under the expensive tissue hypothesis. Rather than finding an increase in encephalization at higher trophic positions, our analysis supported an inverse relationship. This trend of increased brain size relative to body size at lower trophic positions may be partially explained by the increased sensory needs of planktonic feeders at depths below 200 m… Plankton feeders in particular tend to have greater eye and lateral line modifications in order to detect more minute prey quantities (Bleckmann, 1986; Coombs et al., 1988). While changes in brain morphology have been associated with epipelagic fishes living in turbid water…

(ht Neuroskeptic)

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