Crabs are neither fish nor amphibians. They belong to a different category altogether. Crabs are crustaceans, which are a type of arthropod. Arthropods are invertebrates with exoskeletons, segmented bodies, and jointed appendages.
Fish are vertebrates that live in water and breathe through gills. Amphibians, on the other hand, are vertebrates that can live both in water and on land, and they typically undergo metamorphosis from a larval stage to an adult stage.
Crabs, being crustaceans, have a hard exoskeleton, ten legs, and are primarily aquatic, although some species can live on land. They breathe through gills, but their body structure and life cycle are quite different from both fish and amphibians.
In summary, crabs are crustaceans, not fish or amphibians. They have unique characteristics that set them apart from these other groups.