Over on Sandwalk, Larry reposts a list of euphemisms for stupidity (“Not the sharpest knife in the drawer”, “One neuron short of a synapse”, so on).
There are some on the list that are biological, but I think we could benefit from more. I will start things off with one of my own:
He’s not the brightest bioluminescent bacterium in the photophore.
For some reason, one that my brother sometimes says (with a requisite straight face) cracks me up:
He’s not the smartest guy in the world, if you know what I’m saying.