So why would a crab have a halo? I like halos, I like the idea of drawing attention to something that most people wouldn’t consider an animal like a crab to have. Consciousness, a soul, mental awareness, psychic energies, or whatever. And they actually probably don’t have anywhere near what we, would consider those attributes to be. They’re different. They’re animals like us, but different.
But, it’s fun to play around with the idea that a crab could have a moment in which it might be transcending it’s limited awareness. Where it has a sci-fi moment of psychic connection to a whale, or a revelation from the crab god concerning how one crab should treat another.