sea snake vs ophidian

sea snake and ophidian both are nouns.

Word	Noun	Adjective	Verb	Adverb
sea snake	Yes	No	No	No
ophidian	Yes	No	No	No

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

sea snake: any of numerous venomous aquatic viviparous snakes having a fin-like tail; of warm littoral seas; feed on fish which they immobilize with quick-acting venom
ophidian: limbless scaly elongate reptile; some are venomous

Other hypernyms of sea snake include serpent, snake.

sea snake (noun)	ophidian (noun)
any of numerous venomous aquatic viviparous snakes having a fin-like tail; of warm littoral seas; feed on fish which they immobilize with quick-acting venom	limbless scaly elongate reptile; some are venomous