WordNet records no synonym and no antonym for geoduck. It is the only member of its synset, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a large edible clam found burrowing deeply in sandy mud along the Pacific coast of North America; weighs up to six pounds; has siphons that can extend to several feet and cannot be withdrawn into the shell
Synonyms, antonyms and the broader and narrower relations for geoduck come from Princeton University’s WordNet. Full sources and licences.