WordNet records no synonym and no antonym for tentacle. 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.
something that acts like a tentacle in its ability to grasp and hold
“caught in the tentacles of organized crime”
any of various elongated tactile or prehensile flexible organs that occur on the head or near the mouth in many animals; used for feeling or grasping or locomotion
Synonyms, antonyms and the broader and narrower relations for tentacle come from Princeton University’s WordNet. Full sources and licences.