The piranhas are named Tristan and Isolde.
Vito says the beef was good, and Morticia informs him that it wasn't beef, but breast of alligator. Alligator is a white meat that actually tastes like chicken with a slightly fishy taste. By 1964, most states where alligators were indigenous had banned hunting and trading, with the exception of Texas which banned it in 1969. Today, the bans have been lifted, and alligator meat is not that exotic.
First instance of Gomez being excited by Morticia speaking French, which has become an iconic part of the show. In previous episodes it was her calling him Bubala and in one episode croaking like a bullfrog.
Morticia mentions the dinner including a henbane sauce. Henbane is referenced often during the show, usually as a tea. It has a hallucinogenic effect and was historically used, along with mandrake and belladonna (deadly nightshade), as part of a flying ointment often associated with witches. Though poisonous in large doses, it has a number of alkaloids with medicinal properties that are still used in pharmaceuticals today.