crab vs king crab

crab and king crab both are nouns.

Word	Noun	Adjective	Verb	Adverb
crab	Yes	No	Yes	No
king crab	Yes	No	No	No

As nouns, king crab is a hyponym of crab; that is, king crab is a word with a more specific, narrower meaning than crab:

crab: decapod having eyes on short stalks and a broad flattened carapace with a small abdomen folded under the thorax and pincers
king crab: large edible crab of northern Pacific waters especially along the coasts of Alaska and Japan

As nouns, king crab is a hyponym of crab; that is, king crab is a word with a more specific, narrower meaning than crab:

crab (noun)	king crab (noun)
a stroke of the oar that either misses the water or digs too deeply	large edible crab of northern Pacific waters especially along the coasts of Alaska and Japan
decapod having eyes on short stalks and a broad flattened carapace with a small abdomen folded under the thorax and pincers	large marine arthropod of the Atlantic coast of North America having a domed carapace that is shaped like a horseshoe and a stiff pointed tail; a living fossil related to the wood louse
a louse that infests the pubic region of the human body	a large spider crab of Europe
the edible flesh of any of various crabs	meat of large cold-water crab; mainly leg meat
a quarrelsome grouch

crab (verb)	king crab (verb)
complain
fish for crab
scurry sideways like a crab
direct (an aircraft) into a crosswind