Sea urchin

Sea urchins, also known as simply urchins and sometimes called sea hedgehogs, are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum.

They first appear at the beginning of the episode "Help Wanted", where some of them lie on the ocean floor as the French narrator introduces Bikini Bottom. They make their first centric appearance in the episode "Nature Pants" and make many appearances in the series onward. One has a major role in "Eek, an Urchin!"

Description
The sea urchins have different appearances shown in different episodes. In most episodes, they appear as round, purplish-black animals with black spikes.

In "Squidward's School for Grown-Ups," a brown and hairy sea urchin was seen sleeping on 's face, and later woke up due to Patrick's tears. It is, besides Sir Urchin, the only sea urchin that could actually talk.

Intelligence
As seen in the episode "Eek, an Urchin!," the sea urchins are shown to have a crafty side to them. They manage to move around and dodge all of, , , and 's attempts at catching them.

They also have mouths and seem to talk in their own language.

The sea urchins even have superhuman strength as only one urchin managed to drag SpongeBob across the floor while being trapped in a bucket and even pick up Mr. Krabs by the end of the broom, spin him around, then slam him numerous of times.

Role in series
Sea urchins appear in many episodes, including "Nature Pants," "Prehibernation Week," "Your Shoe's Untied," and "No Hat for Pat."

The sea urchins make their largest appearance in the episode "Eek, an Urchin!" It starts out as one urchin causing trouble at the Krusty Krab. The situation is so bad that even Plankton decides to help: by using a robot and even dressing as a female urchin. It is eventually discovered that there are many more sea urchins hiding under the stove. SpongeBob and Mr. Krabs eventually get them out but SpongeBob ends up putting them back into the Krusty Krab again after Mr. Krabs tells him to drop them somewhere far away.

A sea urchin also appears in "Plankton's Pet" where it, along with a guard worm and a sea monkey, are accidentally released from their cages at the animal shelter and attempt to attack SpongeBob and Plankton. However, the sea urchin and the other animals are scared back into their cages by Spot. Like with the one from "Squidward's School for Grown-Ups," this sea urchin has a mouth and two eyes.

Lots of sea urchins appear in "Plankton Retires," where they attack Plankton.

A sea urchin appears in "The Incredible Shrinking Sponge," where it is found in Shubie's Krabby Patty when she complains about it to Squidward. He then tries to squish the sea urchin with the cash register, before SpongeBob takes it and puts it under the grill where it will be safe.

Later, when SpongeBob shrinks, he sees the sea urchin trapped in a mousetrap and uses his teeth to free the sea urchin. However, the sea urchin tries to eat him, until he and the sea urchin fall into a drain.

Incidental D15, part of the Bikini Bottom Hospital audience in "Code Yellow," appears to be a sea urchin wearing a head mirror.

Trivia

 * Unlike sea urchins in real life, they climb and bite fish and do not have spines to hurt them.
 * In "Pineapple RV," Gary is seen chewing on a sea urchin chew toy.
 * They do not appear in seasons 3, 4, and 5.
 * The spikes can easily snrap off as seen in "Eek, an Urchin!."
 * A sea urchin appears on SpongeBob's bumper sticker in "Pizza Delivery."
 * Unlike in other episodes, the sea urchins that appear in the episode "Eek, an Urchin!" are blue instead of black. They are also bigger and rounder.
 * Both the brown sea urchin from "Squidward's School For Grown-Ups" and all of the blue urchins from and "Eek, an Urchin!" are male since the brown sea urchin has a male voice and calls SpongeBob - who was dressed as a female Viking - a "cutie," and the blue sea urchins all fall in love with Plankton dressed as a female urchin.
 * Both of these types of urchins are classified as male due to them thinking a male character is really female.
 * They seem to be the underwater equivalent of a spider/cockroach.
 * As seen in the episode "Growth Spout," sea urchins can actually be eaten, as one of the foods Mr. Krabs takes from Squidward's fridge is pickled urchins.