Patrick Star and Gary the Snail are related.

In the season four episode “Rule of Dumb,” it is revealed that Gary’s father, Sluggo, is Patrick’s uncle, which makes them cousins. The plot of the episode is that Patrick discovers he has inherited royalty in the form of a crown, and declares himself king. 

After he realizes being a king makes him miserable, he returns the crown and it is discovered that Gary is on the other side of his family tree, and that he is the true heir to the crown. The ending may be a reference to how snails and starfish share the same biological kingdom, but it’s still quite a crazy reveal. Then again, what else should we expect from the show we’ve come to know and love?