WordNet records no synonym and no antonym for eel. It is the only member of each of its synsets, 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.
the fatty flesh of eel; an elongate fish found in fresh water in Europe and America; large eels are usually smoked or pickled
voracious snakelike marine or freshwater fishes with smooth slimy usually scaleless skin and having a continuous vertical fin but no ventral fins
Synonyms, antonyms and the broader and narrower relations for eel come from Princeton University’s WordNet. Full sources and licences.