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.
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 Patrick's face, and later woke up due to Patrick's tears. It is, by far, the only sea urchin that could actually talk.
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.
Only in the episode "Squidward's School for Grown-Ups," there was a sea urchin that could actually talk. It also had eyes (which even the sea urchins from "Eek, an Urchin!" don't have).
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.
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 (with SpongeBob hanging onto the drain's cover).
Unlike sea urchins in real life, they climb and bite fish and don't 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.
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 was pickled urchins.