Sea Urchin

A sea urchin is a marine invertebrate characterized by its spines. They are found in all oceans of the world, from shallow coral reefs to deep-sea environments.

Sea urchins are small, spiky creatures found in various habitats, from shallow coral reefs to deep-sea trenches, and they play important roles in the ecosystems in which they live. Approximately 950 species of these creatures live on the seabed of every ocean and inhabit almost every depth zone.

Which phylum is sea urchins part of? These spiny, globular animals are part of the phylum Echinodermata, which includes starfish and sand dollars. They are best known for their round, spiny, spherical shells called ‘tests.’ These sea animals have spines, which they use to protect themselves from predators and cling to rocks and other surfaces as they crawl along the ocean floor. This article looks at some key features and characteristics of sea urchins that will enrich the reader’s understanding, such as appearance, diet, reproduction, and more, in more detail. Keep reading for all these.


Sea urchins look like porcupines, and that’s why some people call them sea porcupines. They are small and measure between 3 and 10 cm (1 to 4 inches), weigh an average of one pound, and are covered by sharp spines like porcupines, as we’ve mentioned above.

These spines vary in length and color depending on the species, but they are typically short and black or brown. Some species have longer, more colorful spines.

Purple sea urchin
The purple sea urchin

Credit: Charles Hollahan / Santa Barbara Marine Biologicals

Some mature sea urchins have spherical bodies with five equally sized parts. Several sea urchins are oval with distinct rear and front ends, which gives them a degree of bilateral symmetry. Due to this ‘irregular’ body form, these sea urchins can easily burrow through soft materials like sand.


A sea urchin also has a small mouth on the underside of their body and five rows of tube feet for movement and feeding. The head lies towards one end of the oral surface in irregular urchins or the center in regular urchins. Lips of softer tissue, with embedded bony pieces, surround the mouth.

The white hole in the middle of the sea urchin is its mouth
The white hole in the middle of the sea urchin is its mouth

 Most sea urchins are omnivorous, meaning feeding on various plants and animals. They are known to graze on algae, which they scrape off rocks and other surfaces using their small, tooth-like structures called ‘Aristotle’s lantern.’ Some species also feed on small invertebrates, such as worms and snails, and even dead plant and animal matter.


Sea urchins are found in many marine environments, from shallow coral reefs to deep-sea trenches. However, they are commonly found in rocky areas, where they can find shelter and food, and in sandy or muddy areas. Some species are found in the intertidal zone, where they are exposed to the air during low tide, while others are found at deeper depths.

In short, sea urchins can inhabit any depth zone from the intertidal seashore to over 5,000 meters. A few species, such as Cidaris abyssicola, can live at depths of several kilometers.


Sea urchins are dioecious. In simple terms, they have separate female and male sexes even though no distinguishing features are visible externally.

So, sea urchins reproduce sexually, with males and females releasing their gametes (sperm and eggs) into the water. When the gametes meet, fertilization occurs, and the resulting zygote becomes a larva, which settles on the ocean floor and develops into an adult urchin.

Please note that the female’s eggs often float freely in the sea. However, some species hold them with their spines to increase their security.

Some species of sea urchins also can regenerate lost or damaged body parts, including their spines.


Sea urchins face several threats, including habitat destruction, pollution, and overfishing. In some areas, urchins are harvested for their edible roe or eggs. It’s often served raw, with rice, or tucked into dumplings in Japanese restaurants. In Mediterranean cuisine, you’ll mostly find it blended into sauces, tossed with pasta, or spread on crostini, and this alone is enough to deplete their populations over a short period.

Some species of sea urchins are also vulnerable to many diseases, which can affect their survival. Climate change is also a concern, as rising ocean temperatures and acidification can negatively impact urchin populations in days.

The sharp spines protect the urchin from predators, increasing their safety. However, human beings have invented ways to catch and kill these animals. Some health conditions like the bald sea urchin disease can also cause loss of the spines and lead to an increase in exposure to the threats.

Sea urchin sharp spikes
Sea urchin sharp spikes

Sea urchin species, such as Cidaris abyssicola, that live at depths of several kilometers face fewer threats than their counterparts that struggle to deal with overfishing and other similar threats.

Facts About Sea Urchins

  1. There are over 950 species of sea urchins.
  2. Sea urchins are related to starfish and sand dollars.
  3. Sea urchins have a hard, calcified skeleton made of plates called ossicles.
  4. Sea urchins are found in every ocean in the world.
  5. Sea urchins can move slowly using their tube feet but are mostly stationary creatures.
  6. Sea urchins can regenerate lost spines and tube feet.
  7. Sea urchins are an important part of marine ecosystems, as they help control algae growth and provide food for other animals.

In conclusion, sea urchins are fascinating creatures that are an important part of the marine environment. They play many important roles in their habitats and are integral to the ocean’s food chain. Their unique anatomy and adaptations make them well-suited to life in the ocean, and they have a long history of being valued as a source of nutritious human food.


What are sea urchins?

Sea urchins are small, spiny, globular animals that live in the sea. They are found in all oceans, from the tropics to the poles. Sea urchins are related to sand dollars, starfish, and sea cucumbers. They are important members of the marine ecosystem and are often used as a food source by humans.

Where do sea urchins live?

Sea urchins are found in the world’s oceans, including the Arctic, Atlantic, Indian, and Pacific Oceans. They are distributed across various latitudes and can be found in temperate and tropical waters. Some species of sea urchins are found in specific areas, while others have a more widespread distribution.

Are sea urchins poisonous to humans?

Most sea urchins are not poisonous to humans, but their spines are often sharp and can cause injury if handled carelessly. Sea urchins are found in all oceans, and their spines are used for protection against predators. So, if you are handling sea urchins, it is important to be careful and avoid getting poked by the spines. If they harm you, clean and treat the wound with an antiseptic to prevent infections.

Why do people eat sea urchins?

Sea urchins are eaten for their unique flavor and texture. Sea urchins are also considered a delicacy in many cultures because of their rarity and perceived high price.
In addition to their culinary uses, sea urchins are also eaten for their nutritional value. They are a good source of protein, omega-3 fatty acids, and various vitamins and minerals. Sea urchins are also low in calories and contain no cholesterol.

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