Why is an octopus considered a mollusk instead of a fish?

The easiest separation point between mollusc/mollusk and fish is whether it is a vertebrate. Vertebrate means if it's an animal that possesses a skeletal structure, vertebral column or backbones, regardless if it's made of bones or cartilage. Another separate term is craniate which means an animal with a skull. read more

The other main reason why they are not considered fish is because all fish are vertebrates. All mollusks, including octopuses, are invertebrates. They evolved separately from fish long ago. read more

An octopus does not have a spine or a hard skull. So even though it shares gills, aquatic lifestyle, and a lack of fingers, it cannot be a fish. An octopus is a mollusk (like a clam), class Cephalopoda. Cephalopods can be confirmed as mollusks through DNA testing and are characterized by symmetry, a big head, and tentacles. read more

The octopus is considered a mollusk because it has a mantle that holds the organs it uses for breathing and excretion and has a rasping tongue, or radula. The octopus also shares features of its nervous system with other mollusks. read more