Plankton (singular plankter) are any organisms that live in the water column and are incapable of swimming against a current. They provide a crucial source of food to larger, more familiar aquatic organisms such as fish and whales.

Though many planktic (or planktonic—see section on Terminology) species are microscopic in size, plankton includes organisms covering a wide range of sizes, including large organisms such as jellyfish.

The name plankton is derived from the Greek adjective πλαγκτός - planktos, meaning "errant", and by extension "wanderer" or "drifter". Plankton typically flow with ocean currents. While some forms are capable of independent movement and can swim hundreds of meters vertically in a single day (a behavior called diel vertical migration), their horizontal position is primarily determined by the surrounding currents. This is in contrast to nekton organisms that can swim against the ambient flow and control their position (e.g. squid, fish, and marine mammals).

Within the plankton, holoplankton spend their entire life cycle as plankton (e.g. most algae, copepods, salps, and some jellyfish). By contrast, meroplankton are only planktic for part of their lives (usually the larval stage), and then graduate to either a nektic or benthic (sea floor) existence. Examples of meroplankton include the larvae of sea urchins, starfish, crustaceans, marine worms, and most fish.

The characters in SpongeBob SquarePants were created by artist, animator and former marine biologist Stephen Hillenburg. In addition to the series' main cast, various celebrities have voiced roles in SpongeBob SquarePants. Notably, Ernest Borgnine and Tim Conway voice the roles of recurring characters Mermaid Man and Barnacle Boy respectively, while others have taken a cameo part.

Stephen Hillenburg conceived the characters for SpongeBob SquarePants in 1984, while he was teaching and studying marine biology at what is now the Orange County Ocean Institute. During this period, Hillenburg became fascinated in animation, and wrote a comic book entitled The Intertidal Zone starring various anthropomorphic forms of sea life, many of which would evolve into SpongeBob SquarePants characters, including "Bob the Sponge", who was the co-host of the comic and resembled an actual sea sponge as opposed to SpongeBob. In 1987, Hillenburg left the institute to pursue his dream of becoming an animator.

Lil' O (born Ore Magnus Lawson on November 22, 1977) is a southern rapper, born in Lagos, Nigeria and raised in Southwest Houston, Texas. He is an original member of DJ Screw's Screwed Up Click. He was also known as O or Da Fat Rat Wit Da Cheeze. His name references his height, at 5 feet tall.

His father was a doctor and Lil’ O and his four brothers and one sister had a middle class upbringing. His mother died from ovarian cancer when Lil’ O was 13 and suddenly his father’s strict Nigerian upbringing began to chase him to rebellion.

Soon after his mother’s death, Lil’ O began hanging in the streets with friends who were in the drug game, and he became addicted to the fast money that drug dealing brought in. “I got street fame, women, and a lot of money quickly and easily,” he says. Of course, he eventually got collared. After his arrest, his father kicked him out of the house and Lil’ O fell deeper into the lure of the drug game. While out on bond, he continued to sell drugs and was caught again. This time he attempted to swallow the evidence before the police could arrest him. He swallowed an ounce-and-a-half of crack cocaine and his lungs collapsed. He was put on life support and his father was told to prepare for his death. “It’s really a miracle that I woke up at all,” says Lil’ O. “I was breathing through tubes on my deathbed and I looked down and saw that my ankles were cuffed to the bed. I realized, if I recovered I was going to jail, and if I didn’t I was going to die.”

Sheldon James Plankton or simply Plankton, is the main antagonist on the American television cartoon series SpongeBob SquarePants. He is voiced by Mr. Lawrence. The character is also in video games and other media based on the series. Additionally, he has been featured on countless toys, games, dolls, and other popular merchandise items.

He operates a struggling restaurant called the Chum Bucket.

Mr. Krabs is Plankton's arch-nemesis.

He operates a struggling restaurant in the series called the Chum Bucket, located directly across the street from the Krusty Krab. His primary goal in the series is to steal the Krabby Patty secret formula and put Mr. Krabs out of business. Plankton succeeds at stealing the recipe in the movie, but his goal of world domination is thwarted by SpongeBob. In "Friend or Foe," it was revealed that Mr. Krabs and Plankton were once best friends, but a dispute over the Krabby Patty recipe ended their friendship.

Plankton has a large extended family, containing more than 10,000 members of different forms of plankton, most of whom are supposedly from Texas and a character that people might consider hillbillies. It is from them that the audience learns his full name, Sheldon J. Plankton. He is also married to his super computer wife, Karen.