How are octopus classed as fish?

Nope. Taxonomically, both are in the Animal Kingdom, but that's where they diverge. Fish are in the Phylum Chordata (more or less having a spinal cord). Octopi, or pus-fellers as Quarrel called them, are mollusks - no backbone, inter alia. Culinarily, prepping fish vs octopus is very different. read more

Octopuses are not classified as fish. It is a “Cephalopod mollusc” that has eight arms, no skeleton of any kind, a beak and it's mouth is at the center point of the arms. The only thing octopuses have in common with fish is that they live in the ocean. read more