Amoebas are microscopic one-celled organisms that were first shown in the episode "Patty Hype."
When Mr. Krabs wasn't receiving business in the Krusty Krab, he was watching a show about them in his office. They also appeared when SpongeBob turned Plankton into a baby (as seen in the episode "Goo Goo Gas"). He was shrunken down after being hit by the Goo Goo Gas to microscopic smallness. The Amoebas were 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.