Home Animals Sea Urchins: The Spiny and Spectacular Wonders of the Ocean Floor

Sea Urchins: The Spiny and Spectacular Wonders of the Ocean Floor


Have you ever heard of sea urchins? They are mysterious creatures that can live up to an astounding 200 years! Sea urchins are among the many unique organisms that inhabit the ocean floor. With their spiny exterior, it’s easy to mistake them for dangerous creatures. 

However, these little animals are far from dangerous and play an essential role in our ecosystem. Did you know there are numerous species of sea urchins, each with their distinct features? 

Although they can’t swim, their hard shells and spines protect them from predators. These remarkable creatures have much to teach us about life in the ocean. So, let’s dive deep and explore all there is to know about sea urchins.

Taxonomic Classification

Sea Urchins

Kingdom: Animalia

Phylum: Echinodermata

Subphylum: Echinozoa

Class: Echinoidea

(Sea urchins also have multiple subclasses and orders, but we’ll get into that another time.)

Common Species

There are a total of 950 species of sea urchins that are widely distributed across all ocean floors. However, among this vast number, only approximately 18 species are considered edible and consumed by humans.

Some well-known sea urchin species include:

  • Red sea urchin (genus Strongylocentrotus): The edible part is its gonads, commonly known as “uni.”
  • Pacific purple sea urchin (Paracentrotus lividus): An important ingredient in uni sushi.
  • Green sea urchin (genus Strongylocentrotus): The spines of this species are not venomous and are also edible.
  • European sea urchin (Echinus esculentus): Found in coastal areas of western Europe.
  • Diadema setosum: Possesses long, hollow, and mildly venomous spines.
  • Diadema antillarum (lime urchin): Characterized by long and hard black spines.
  • Tripneustes gratilla (collector urchin): Found in the red sea, Indo-Pacific, the Bahamas, and Hawaii.
  • Arbacia lixula (black sea urchin): A European sea urchin species.
  • Kina: An edible sea urchin species endemic to New Zealand.
  • Colobocentrotus atratus (shingle urchin or helmet urchin): Found on the shores of Hawaii.
  • Banded sea urchin (double-spined urchin): Possesses venomous spines.
  • Tripneustes depressus (sea egg or white sea urchin): Contains harmful toxins.
  • Slate pencil urchin: Used in aquariums to control algae numbers.
  • These edible sea urchin species are appreciated for their unique flavors and culinary uses in various regions around the world.

Conservation Status

Although sea urchins are not currently facing the risk of extinction, they are classified as “near threatened” sea creatures. This designation indicates that while their populations are not critically endangered at present, they are still at risk of becoming vulnerable in the future if appropriate conservation measures are not taken.

Other Names Of Sea Urchin

Indeed, the name “sea urchin” is derived from the Latin word “ericius,” meaning hedgehog, and the old French term “herichun.” This name was given to them due to their resemblance to hedgehogs in appearance.

Scientifically, sea urchins are classified under the class name “Echinoidea,” which is a part of the larger phylum Echinodermata, which also includes creatures like sea stars and sea cucumbers.

Apart from “sea urchin,” they are also known by several other common terms, including:

  1. Sea biscuits: Referring to some flattened and disk-shaped sea urchins with a biscuit-like appearance.
  2. Sand dollars: Specifically used for certain types of flat, round sea urchins found in sandy marine environments.
  3. Sea hedgehogs: Another reference to their physical resemblance to terrestrial hedgehogs.

These names are used interchangeably to describe different species of sea urchins based on their unique characteristics and features.


Sea urchins have a rich history dating back approximately 450 million years to the Ordovician period. They are part of the class Echinoidea, and their closest echinoderm relatives are sea cucumbers, as both belong to the deuterostomes clade, which also includes chordates. Fossilized sea urchins have been utilized as protective amulets. 


These unique marine creatures have fascinating lifespans, ranging from 15 to 200 years, making them one of the organisms with remarkably long life expectancies. However, the life duration can vary from species to species; for instance, purple sea urchins have a life expectancy of about 20 years.


The body of a sea urchin is spherical and rigid, featuring locomotive spines over its shell called tests. The anus is located at the top, while the face is situated at the base, making the upper body surface aboral and the lower body surface oral. Some species also possess statocysts in their spheridia, which assist in gravitational orientation.

Senses and Nervous System

Sea urchins lack a distinct brain but have a neural center in the form of a large ring surrounding the mouth. Despite this, they are highly sensitive to chemicals, light, and touch.


These creatures exhibit various appearances and come in different colors, such as green, black, white, orange, purple, yellow, pink, blue, brown, and gray. Sensitive cells are located in their tube feet, spines, around the mouth, and in the pedicellaria.

In terms of weight and size, sea urchins typically weigh around one pound, but this can vary depending on the species. Their size ranges from one inch to 14 inches (3cm to 36 cm).

Weight and Size

Sea urchins move with the help of hundreds of small adhesive and transparent structures called tube feet.


These marine creatures are distributed throughout the world’s oceans, from the deepest sea trenches to shorelines in tropical, temperate, and arctic regions. They prefer habitats such as coral reefs and rocky ocean floors, as they are not capable of swimming. Some species, like shingles, inhabit shallow waters near ocean beaches.


These creatures can be found in various locations across the world’s oceans. They inhabit the deepest sea trenches as well as shorelines, thriving in tropical, temperate, and arctic regions alike.


These species prefer to inhabit coral reefs and rocky ocean floors as their preferred environments, where they spend the majority of their time. Their inability to swim leads them to thrive in these stationary habitats. Certain species, such as shingles, reside in the shallow waters close to ocean beaches.


The population of sea urchins varies greatly among the more than 950 species. For instance, the population of purple sea urchins can be vast, with around 350 million found on a single Oregon reef. However, certain populations may face threats due to various factors, including predation, diet, and changes in sea temperature, leading to an imbalance in the ecosystem and reducing the numbers of some species.


Sea urchins are omnivores and feed on marine vegetation such as kelp and algae, as well as other sessile ocean creatures like sea sponges and coral.


They have several predators, including sea otters, lobsters, crabs, birds, and fish. Sea otters play a crucial role in maintaining the population of sea urchins by consuming them, thus protecting kelp forests from overconsumption.


Sea urchins also face certain diseases, such as bald sea urchin diseases caused by pathogens like Aeromonas salmonicida and Listonella anguillarum.

Diseases In Sea Urchins

These creatures have various self-defence mechanisms, including spines and venomous spines that protect them from predators. Pedicellaria also helps defend them from ectoparasites, and they possess a hemal system that protects them from endoparasites.

Self-Defense Mechanisms

Sea urchins possess protective spines that act as a defence mechanism against predators. Among these species, some have venomous spines, which not only safeguard the sea urchins but also provide refuge to shrimps, crabs, and other marine creatures seeking protection from predators.

Additionally, sea urchins benefit from a defensive feature called Pedicellaria, which helps shield them from ectoparasites. Furthermore, their hemal system ensures protection against endoparasites, enhancing their overall safety and well-being.

Reproduction Process of Sea Urchins

Sea urchins are dioecious, meaning they have separate male and female sexes. They reproduce once a year using external fertilization, releasing eggs and sperm into the water. The fertilized eggs develop into embryos, and only a small number of them eventually grow into adult sea urchins. The time to reach maturity varies among species, with the purple sea urchin taking about three to five years to mature.

Humans and Sea Urchins

#1 Scientific Uses:

Sea urchins have significant scientific uses and are employed as model organisms in developmental biology since the 1800s. Researchers find them valuable due to their long lifespan, high fecundity (reproductive capacity), and easy availability. They are particularly useful for studying cis-regulatory elements, which are essential for gene expression regulation. Moreover, scientists also use sea urchins to assess factors like ocean acidification, ecological impacts, temperature changes, and the health and population dynamics of sea urchin populations.

#2 Source of Food:

Sea urchins are a popular source of food in various parts of the world, particularly in Japan. Both male and female sea urchins have edible gonads called “corals” or “sea urchin roe.” Japan is the largest consumer of sea urchins globally, consuming over 50,000 tons annually, which accounts for more than 80% of the total global production. Additionally, sea urchins are utilized in various culinary forms for their taste and texture.

#3 Folk Tradition:

In certain regions, sea urchin fossils were once associated with folk traditions. For instance, in southern England and Denmark, people believed that these fossils were apotropaic symbols, capable of repelling harm caused by witchcraft or lightning. They were considered to be thunderbolts with protective properties.

Special Characteristics of Sea Urchins

Sea urchins indeed possess several unique characteristics that set them apart as fascinating ocean creatures. Here are some of these distinctive features:

  1. Spines and Hair-like Structures: While most sea urchins are known for their spines, not all species have them. Some sea urchin shells are instead covered with hair-like structures, giving them a different appearance and texture.
  2. Absence of Separate Eyes: Unlike many other marine creatures, sea urchins do not have separate eyes. They lack complex visual organs and primarily rely on their highly sensitive sensory cells to detect changes in light, chemicals, and touch.
  3. Five Sets of Teeth: Sea urchins have a unique feeding structure known as Aristotle’s lantern, which consists of five sets of teeth. These teeth continuously replace the worn-out material through abrasion, and their self-sharpening mechanism helps them maintain their efficiency in feeding on algae and other marine vegetation.

These characteristics, along with their intricate anatomy and ecological significance, make sea urchins intriguing and essential members of marine ecosystems.

11 Facts About Sea Urchins That Will Blow Your Mind

Sea urchins are indeed fascinating and unique ocean residents, with a multitude of amazing facts that highlight their intelligence and distinct characteristics:

  • Five Symmetrical Sections: Unlike mammals with bilateral symmetry (two symmetrical sections), sea urchins display pentaradial symmetry, featuring five symmetrical sections in their body design.
  • Strong Teeth: Sea urchins possess remarkably strong teeth within their feeding structure, Aristotle’s lantern, allowing them to efficiently feed on algae and other marine vegetation.
  • Carrier Crab Defense: The carrier crab cleverly uses sea urchins as a self-defense mechanism against predators by carrying them on its back, providing protection from potential threats.
  • Aristotle’s Lantern: The mouth structure of sea urchins is unique and intricately designed, earning it the nickname “Aristotle’s lantern.”
  • Compound Eye Sensitivity: While sea urchins don’t have separate eyes, expert zoologists suggest that their entire body may function as a compound eye, making them sensitive to light. This adaptation leads them to live on the seafloor, avoiding direct exposure to light.
  • Bumpy Outer Surface: Dry sea urchins have a distinctive appearance, covered with numerous bumps resembling beads.
  • Versatile Temperature Range: Various sea urchin species thrive in a wide range of water temperatures, from cold to warm, demonstrating their adaptability to diverse environments.
  • Test instead of Bony Skeleton: Sea urchins lack a hard bony skeleton like some other marine creatures, but their outer shell, known as a test, is composed of tough calcium carbonate, providing protection and support.
  • Largest Red Sea Urchin: Among the 950 species of sea urchins, the red sea urchin stands out as the largest and most significant in size.
  • Simple Reproductive and Nervous System: Sea urchins possess relatively simple reproductive and nervous systems, yet they exhibit remarkable adaptability and survival skills.
  • Dangerous Flower Urchin: The flower urchin is known to be the most dangerous among all sea urchin species, possibly due to its venomous spines or other unique defensive mechanisms.

These fascinating facts highlight the complexity and intelligence of sea urchins, making them intriguing and essential components of marine ecosystems worldwide.

Frequently Asked Questions

Is sea urchin safe to eat?

Both male and female sea urchins have delectable gonads that are often served raw in sushi or used to flavor pasta dishes. The texture of the gonads is often compared to that of custard, and the flavor is described as both sweet and savory. 

Are sea urchins poisonous to touch?

They possess two dangerous venomous organs within their bodies that could cause some serious harm. The pedicellaria and spines are both capable of inflicting pain and inflammation upon contact with human skin. 

What kills sea urchins?

Autoclaved seawater, which has been heated to high temperatures to sterilize it, can have devastating effects on the sea urchin population. The water becomes lethal, and the urchins are unable to survive in the environment. This can lead to drastic reductions in the population of sea urchins in affected habitats.

Can you survive a sea urchin sting?

While they may not be fatal in most cases, the pain and potential symptoms can be significant. It’s always better to err on the side of caution and seek treatment for any symptoms you may experience. Whether it’s pain, swelling, or redness, it’s best to get help and take care of yourself. 

Do sea urchins cut you?

While they may not be fatal in most cases, the pain and potential symptoms can be significant. It’s always better to err on the side of caution and seek treatment for any symptoms you may experience. Whether it’s pain, swelling, or redness, it’s best to get help and take care of yourself. 

What is the deadliest sea urchin?

Although its colorful appearance may entice some, this particular species has a secret weapon that can cause severe pain, paralysis, and respiratory issues. The toxins found in their pedicellariae and spines can quickly turn a fun day at the beach into a dangerous situation. 

Final Words

It’s not every day that we come across a marine creature as striking as the sea urchin. With over 950 species worldwide, these spiny organisms boast a range of special qualities that set them apart from the rest of the marine world. Although only 18 of these species are deemed edible, sea urchins are particularly famous for their succulent gonads and the intricate shells they carry, lined with venomous spines, providing them with enough protection against predators. 

While they can primarily be found on the ocean floor, some species also occur on the shores. As for their population, sea urchins are currently listed as near threatened due to the imbalance in the ecosystem. There’s no denying that these adorable creatures are a true marvel, worth marvelling at whenever the opportunity arises.


Author Profile
Zahra Makda
Wildlife Enthusiast | Explorer at Animals Research

Growing up enjoying the beauty of my village, a good passion for nature developed in me from childhood. Following my passion for the natural world, I have chosen zoology for my graduation, during my undergraduate degree, I participated in many nature trails, bird watching, rescues, training for wildlife conservation, workshop, and seminars on biodiversity. I have a keen interest in invertebrate biology, herpetology, and ornithology. Primary interests include studies on taxonomy, ecology, habitat and behavior.

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Growing up enjoying the beauty of my village, a good passion for nature developed in me from childhood. Following my passion for the natural world, I have chosen zoology for my graduation, during my undergraduate degree, I participated in many nature trails, bird watching, rescues, training for wildlife conservation, workshop, and seminars on biodiversity. I have a keen interest in invertebrate biology, herpetology, and ornithology. Primary interests include studies on taxonomy, ecology, habitat and behavior.


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