Main category in which fish and shellfish are divided?

Answer 1

Most vertebrate fish -- salmon, trout, tuna and so on -- are of Class Teleostomi, Phylum Chordata -- these are the bony fishes. Shark and rays are of class chondricthyes, in that their skeletal systems aren't bone -- they're cartilage instead. Shellfish generally refers two different phyla: Arthropoda (crabs, lobsters, crawfish, etc -- characterized by a hard exoskeleton) and Mollusca (clams, mussels, abalone, etc.) The primary distinctions are that chordates (fish and shark) have a notochord or spinal cord and an endoskeleton. Arthropods do not have a notochord, have an exoskeleton, and also have striated as well as smooth musculature. Molluscs often have shells (but not all -- consider the octopus or squid), do not have a notochord, and do not have striated muscles.