Visually arresting, hazardous to swimmers, and—to some cultures—delicious, sea urchins are also revealing new information to the scientists who study them.

Here are 16 unbelievable facts about urchins that you probably didn't know.

1.
Sea urchins belong to the class Echinoidea. They are known for their spiny, spherical bodies, making them easily recognizable underwater.

2.
There are more than 950 sea urchin species.
These spiky invertebrates are incredibly diverse, boasting over 950 known species of sea urchins. Smaller species are usually found in shallow waters, while larger sea urchins prefer deeper ocean parts.
Furthermore, sea urchin species come in different colors. The most common colors are yellow, red, black, brown, and purple.

3.
Sea urchins live in all five oceans.
Sea urchins are one of the few animals that thrive in all oceans. They live in the warm waters of the tropics to the icy waters of the Arctic. They also thrive in shallow areas with crashing waves to the deep sea, reaching depths of over 5000 meters.
Additionally, they can inhabit various habitats, including rocky shores, coral reefs, seagrass beds, sandy or muddy bottoms, kelp forests, or a rocky sea bed.

4.
Sea urchins don't have brains.
Sea urchins, unlike animals with more complex nervous systems, lack a centralized brain or ganglia. Their nerve cells spread throughout their body, allowing for localized processing of sensory information and coordination of basic behaviors.

5.
Sea urchins reproduce sexually through a behavior called broadcast spawning.
Several females and males release their eggs and sperm into the water column, where fertilization takes place. Sea urchins can also reproduce asexually by cloning themselves during the larval stage.

6.
The red sea urchin is the largest of the species.
In general sea urchins grow to be around 1.18 inches (3 cm) to 3.93 inches (10 cm).
The red sea urchin is the biggest of the species, and it will enter adulthood when it reaches 1.96 inches (5 cm) in diameter.
It will then continue to grow until it reaches around 3.93 inches (10 cm), whereas most other sea urchins will stop growing around 1.96 inches (5 cm).

7.
Urchins have zero bones in their body.
But they have a hard shell, which is called a "test."
The hard shell made from calcium carbonate is similar to that of starfish and sand dollars and is their main form of protection.
The structure consists of small plate segments that enclose the urchin; it looks a bit like the segments of an orange.
As well as the "test," they have spines that help them protect themselves from predators.

8.
Sea urchin's spines are divided into two types: primary and secondary.
Primary spines are the longer and more prominent ones on a sea urchin's surface. They are typically thicker and provide structural support, stability, and protection to the sea urchin. Additionally, primary spines aid in locomotion, allowing sea urchins to move across surfaces or navigate through their habitat.
Secondary spines, called tube feet and pedicellariae, are smaller and shorter spines found between the primary spines.

9.
Sea urchins have just 5 teeth!
Sea urchins are omnivores with just 5 teeth, which are held together in the center of their bodies.
Urchins mostly feed on algae found on rocks or corals, as well as any debris or decomposing matter from fish or sea creatures.

10.
Urchins have hundreds of tiny feet.
At first glance, watching a sea urchin looks as though they're just bouncing along with the water, or using their spines to move.
In fact, Sea urchins have 5 paired rows of tubular tiny feet called pedicellariae.
These tiny rows are located between the spines.
Each foot is a small sucker-like shape which allows the urchin to grip surfaces in order to pull itself along.
The suckers are also useful to help catch food and attach themselves to the ocean floor.

11.
Toxopneustes pileolus, known to most as the flower urchin, is the deadliest of all.
When a human or animal steps on, touches, or brushes past the flower urchin, its "flower-shaped" spines will break or pierce the skin of the predator.
Its venom contains two poisonous elements: Contractin A, which can cause red blood cells to clot and can cause muscle spasms. The other element is Peditoxin, which can cause anaphylactic shock, convulsions, and death.

12.
The oldest living species found was the red sea urchin, which was dated to be around 200 years old.
In captivity, their life expectancy is reduced to around only 30 years, while in the wild, it is usually over 30 years.

13.
The clownfish is one of the only species that purposefully try to build immunity to the flower urchin's venom.
Clownfish will dance around the flower urchin, lightly brushing itself against the spines.
This means they are exposed to a small amount of venom but not enough to kill them.
After repeating this a few times, they become immune to the venom.
You might wonder why they choose to do this and the reason is so they can eat the parasites that live on the flower urchin.
This relationship is beneficial to both creatures as the clownfish gets food, and the flower urchin has its parasites removed.

14.
Sea urchins have a number of natural predators, including fish, crabs, sea otters, lobsters, triggerfish, wolf eels, wrasse, birds, and starfish. These predators play an important role in controlling sea urchin populations and maintaining the balance of the ecosystem.

15.
In 2006 British marine biologist Dr. Simon Coppard came across an urchin for sale on eBay that he had never seen before.
It was given the scientific name Coelopleurus Exquisitus, also known as The Exquisite Urchin.

16.
People around the world eat sea urchins.
Sea urchin gonads, or Uni in Japan, is a premium seafood with a salty taste and creamy texture.
Traditional Japanese restaurants serve them raw. However, they can also flavor sauce, soup, and pasta. Due to their high demand and the low supply of quality gonads, Uni is very expensive.
A single 200-gram box of premium gonads costs $100 in the U.S. However, in Japan, it can sell for up to 500 dollars!

Sources
Study / BBC / Marine Bio / Navajo Code Talkers / A-Z Animals / Wikipedia / Trvst