What was the last message of the Dolphins in "The Hitchhiker's Guide to the Galaxy"?

In "The Hitchhiker's Guide to the Galaxy," the last message of the dolphins to humanity is "So long, and thanks for all the fish." This phrase signifies the dolphins' final farewell and gratitude towards humans for the fish, as dolphins were the second-most intelligent species on Earth and knew of the impending destruction of the planet to make way for a hyperspace bypass. Essentially, this message represents their acknowledgment of the good times shared with humans and a farewell before leaving Earth, playing on the dolphins' reputation for intelligence and their strong connection with the sea.