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 in the episode "Nature Pants" and make many appearances in the series onward. One has a major role in "Eek, an Urchin!"
The sea urchins have different appearances shown in different episodes. In most episodes, they appear as round, purplish-black animals with black spikes.
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 SpongeBob, Squidward, Mr. Krabs, and Plankton'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.
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.
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.
Unlike sea urchins in real life, they climb and bite fish and do not have spines to hurt them.
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.
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.