WordNet records no synonym and no antonym for mouthbreeder. 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.
any of various fishes that carry their eggs and their young in their mouths
Synonyms, antonyms and the broader and narrower relations for mouthbreeder come from Princeton University’s WordNet. Full sources and licences.