sea louse vs isopod

sea louse and isopod both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea louse	Yes	No	No	No
isopod	Yes	No	No	No

As nouns, isopod is a hypernym of sea louse; that is, isopod is a word with a broader meaning than sea louse:

sea louse: marine isopod crustacean
isopod: any of various small terrestrial or aquatic crustaceans with seven pairs of legs adapted for crawling

sea louse (noun)	isopod (noun)
marine isopod crustacean	any of various small terrestrial or aquatic crustaceans with seven pairs of legs adapted for crawling