Amoebas are microscopic one-celled organisms that first appear in the episode "Patty Hype."
When Mr. Krabs is not receiving business at the Krusty Krab, he watches a show about them in his office. They also appear when SpongeBob turns Plankton into a baby (as seen in the episode "Goo Goo Gas"). He is shrunken down after being hit by the Goo Goo Gas to microscopic smallness. The Amoebas are shown to say that Plankton's baby size is microscopic.
Plankton also gets a pet amoeba in "Plankton's Pet" and names him Spot. Spot looks much different than the amoebas in "Goo Goo Gas." Spot makes another appearance in "Spot Returns" and has several children who are amoebas.