Ambiguous Situation: A number of tantalizing details are left out. We aren't told, for instance, how or why everyone dies, nor whether or not that includes the barnacle itself. Similarly, the barnacle's relationships with the rest of the cast are quite vague.
All Just a Dream: Sort of. In the end, it turns out to to be a story Patrick was telling to SpongeBob himself.
The Aloner: If the barnacle didn't die in the end, he probably became one of these.
And Man Grew Proud: One interpretation of the barnacle, of how it became ugly enough to kill everyone and of how, and why, we still tell its harrowing tale.
Crapsack World and/or Crapsaccharine World: In the Barnacleverse, the existence of a single unattractive crustacean is enough to trigger the Apocalypse. Despite this, Patrick narrates the tale like a bedtime story, and originally told it as an attempt to cheer upSpongebob.
Eyeless Face: Real Life Barnacles do not have eyes, or even any kind of identifiable facial features, so the reader can assume that the same applies to The Ugly Barnacle, and may be part of what makes it ugly enough to kill everyone.
Grey and Gray Morality: We're never told whether the Barnacle was good or evil, nor are we told the allegiance of Everybody. This means that depending on how you look at it, it could be a great victory, a great tragedy, or neither that Everybody died.
Mind Screw: After the ending, we're left with a great big pile of unanswered questions. Just how did the Barnacle's ugliness kill everyone? Did the Barnacle himself die? Literally nothing is ever explained.
Starfish Aliens: Perhaps the reason why the barnacle was so ugly was that it was from a different planet? Everyone can be quick to assume that anything not like them is ugly. Also the storyteller is a starfish.
Sword of Damocles: The threat of the barnacle is enough to stop anyone from doing anything. Ever.
Thirteen Is Unlucky: The actual story, discounting the "The End" tag, is exactly thirteen words long, and you can't get much unluckier than having everyone die.
Truth in Television: In Real Life, Barnacles arequite ugly◊. Not enough to kill everyone, but their repulsive anatomy note They have prehensile penises and lack of any identifiable features, including eyes or even a face, were obviously a fitting inspiration for something as exceedingly hideous as The Ugly Barnacle.
Twist Ending: Be honest: Were you expecting the barnacle to kill everyone?