Inspired by the original novel version of Fantastic Voyage, the Super Friends take a "Journey Through Inner Space" when Aquaman (Bill Calloway, ex-Love, American Style) is mutated into a prehistoric shark-like creature.
The implication here is that the King of the Seven Seas evolved from such fish. Bollocks to that, it's just a lame plot contrivance. Most of the cast also play additional characters.
Yes, this was weak due to the plot contrivance, but it also set the stage for the literary-inspired World's Greatest Super Friends the next season.