Scientific Classification

KINGDOM: Animalia

PHYLUM: Echinodermata

SUBPHYLUM: Echinozoa

CLASS: Echinoidea

Conservation Status

small burgundy sea urchin
Sea urchin (Uni)
An isolated blue sea urchin while climbing on a rock
blue and red colored sea urchin

Many sea urchins can vary widely with some having long spikes and others with a round hard shell composed of hard plates.

All sea urchins, regardless of the species, can be divided into five equal parts. They also have hundreds of little “tube feet” that end in small suction cups to help them move around on rocks or the ocean floor since they lack the muscle that would allow them to move their arms. A sea urchin’s tube feet are controlled through pumping water in and out of them, which is why it’s important for them to remain in the water.

The spikes found on most sea urchins are not only used for food collection, but also for defense against predators. In fact, some sea urchins, such as the flower urchin, have venomous spikes.
There are hundreds of species of sea urchins found all over the world. They inhabit tropical and polar climates, to rocky shores all the way to the hadal zone depths, which is the deepest region within oceanic trenches.
Sea urchins are omnivores as they eat sea weed, algae, plankton, and floating rotten organic matter. They have a specialized mouth called “Aristotle’s lantern” that consists of five strong plates. Each plate has a tooth that points towards the center of the mouth and specialize in gripping, scraping, and tearing. Their mouths are so good at grabbing and grinding, that scientists are trying to find real-world applications for their structure.
Similar to other sea creatures, sea urchins will release eggs and sperm into the water. In most cases, the females’ eggs will float freely in the sea, but some sea urchin species carry them with their spines for extra protection. Once the egg is fertilized, the embryo can start swimming freely in as quickly as 12 hours. Then, the embryo will undergo 10 cycles of cell division before it matures.

Crabs have even been seen carrying sea urchins for protection! Some find it difficult to view these little balls of spikes as being a living organism, but most urchins will have a prompt, defensive response to touch; converging its spikes towards the touch point.