What Do Sea Urchins Eat And How They Eat It? Fun Facts For Kids

Tanya Parkhi
Mar 24, 2023 By Tanya Parkhi
Originally Published on Nov 22, 2021
Edited by Monisha Kochhar
Fact-checked by Amatullah Gulbargawala
Purple sea urchin in a sunny day.
Age: 3-18
Read time: 7.1 Min

Sea urchins are spiny, usually spherical creatures belonging to the class Echinoidea.

Also known as sea biscuits, sea hedgehogs, and sand dollars - sea urchins are undoubtedly an important part of the marine ecosystem. All over the world, they are found in waters, ranging from rocky shores to within the deepest depths of the ocean.

As sea urchins do not have much range of locomotion, nor a complex feeding system, it is quite difficult for them to feed on fish, crabs, and other small creatures of the ocean.

Which is the reason why they have adapted to a mostly herbivorous diet, and feed on organic plant matter like algae, kelp, and seaweed. They have a special, beak-like mouth, which they use to scrape algae off of rocks and take it into their bodies.

The spines of their bodies help them to move across the ocean floor, as well as helps to protect them from predators like larger fish, lobsters, sea otters, and the like.

Though the sea urchins' diet is mostly herbivorous, it is not exempt from occasionally feeding on the bodies of dead fish and other creatures which sink down to the ocean depths. There are a few carnivorous sea urchin species as well, which can move swiftly across the ocean floor and prey on smaller, slow-moving marine animal species.

To learn more about the sensational sea urchin's feeding habits, read on!

If you enjoy this article, check out our pages on what parrots eat and what prairie dogs eat.

Sea Urchin Feeding Habits

Sea urchins are inherently herbivorous in nature and mostly eat algae. They are also present in kelp forests, where they are considered ecologically significant.

Sea urchins eat kelp, barnacles, and seaweed, feather star, and other crinoids. However, they are opportunistic eaters and can be found feeding on anything and everything edible which floats by or sinks to the bottom of the ocean floor.

Sea urchins can be found mostly feeding at night, however, it seems that they are constantly consuming something or the other. If they run out of algae to chomp on, they may destroy decorative plants, rocks, and coral present in the surroundings by scraping them in an attempt to find food.

The largest species of sea urchin - the red sea urchin - can be found from the coastlines of Alaska all the way to Baja California, Mexico. They are quite stationary and depend on the heavy water currents to bring them algae and seaweed, which they feed on.

Another well-known sea urchin species is the purple sea urchin. They are present along the western coastline of the Americas and are quite abundant in nature. Natural predators such as wolf eels in the Northern Hemisphere and sea otters in British Columbia help to keep populations of sea urchins in check.

The overpopulation of sea urchin species can be quite problematic, as too much sea urchin feces may not be able to break down in time, polluting the coral reefs. Though sea urchin waste is normally considered beneficial to the ecosystem as its breakdown causes important nutrients to cycle back into the environment, an excess of it is definitely problematic.

The presence of predators like sea birds, foxes, and sea otters helps curb this problem.

What is Aristotle's lantern?

Sea urchins feed with the help of a beak-like structure which is known as 'Aristotle's lantern'. The beak is made out of five hard plates and opens and closes in a star-shaped pattern.

This beak-like structure is located on the underside of the urchin's body. They use this beak-shaped mouth to scrape the algae off of rocks and coral. The constant scraping motion can cause the teeth to wear down, prompting the growth of new ones.

A fleshy, protruding tongue is located inside the mouth. Some sea urchins have venomous jaws called pedicellariae, which they may use to clamp onto any intruders which may step on them.

Stepping on a venomous sea urchin can cause searing pain which may last for many hours. Treating a sea urchin bite as soon as possible is essential, as it can prove to be deadly if it enters the bloodstream.

Their spines may also break off if stepped on and cause painful wounds. Extracting these spines is a difficult task, as any pieces left behind can infect the bloodstream. Sea urchins can also inject poison into their predators through hollow spines.

Sea urchin closeup.

Sea Urchin Feeding in Aquariums

Sea urchins mostly feed on algae, which make them great additions to any aquarium or tank. Being algae-eaters, they help to keep the tank naturally clean and prevent the build-up of too much algae which may prove harmful to the other inhabitants. They also eat pieces of marine plants, plankton, and kelp.

Sea urchins are constantly eating, and are pretty sure to clear your tank of any algae growing in no time. This can make them susceptible to eating beautiful decorative corals and live rock, as they look for algae to scrape off.

If there is not enough algae matter available in your tank, you can provide your sea urchin with pieces of seaweed, kelp, and algae wafers. They are omnivores and can be fed mussels and other bits of meat and seafood, such as invertebrates or cut-up shrimp, as well as fish flakes and pellet food.

Sea urchins excrete waste through their anus which is located on the top of their bodies. Any tank holding sea urchins must be cleaned thoroughly at least once a month.

Transfer the urchins as well as any other inhabitants to other containers, and scrub the tank well with soap and warm water. Before moving your pets back, rinse the tank well with seawater, and then proceed to fill it up.

Carnivorous Sea Urchins

Though most sea urchins are inherently herbivorous or omnivorous in nature, there does exist an order of urchins called Cidaroida, which consists of active predators.

These primitive sea urchins move across the ocean floor using their tube feet and pursue slower-moving sea creatures like tiny starfish, feather stars, and sea cucumbers, engulfing them in their spines. They use their feet and spiny spikes to latch onto their prey.

As these feet are present all over the body, the entire body is considered as the oral surface of the sea urchin except for the upper part which contains the anus.

The sharp spines of sea urchins are very useful, as they act as a defense against predators and also help them to move across the ocean floor and sense other creatures and obstacles in their vicinity.

Some sea urchin species hold their eggs among their spines rather than letting them float free, which helps protect them from fish and other predators that may consume them.

They are fertilized by sperm released in the water and develop into embryos which then float off into the ocean.

Sea urchins are bottom feeders and do not actively hunt marine creatures such as fish, crustaceans, and sea snakes as these are either stronger than them or are present in the middle or upper layers of the water, making them inaccessible to them.

They do however feed on any decaying fish or any other animal matter which floats down to the bottom of the ocean.

Though sea urchins do not have eyes, it is theorized that their entire body functions as a compound eye- being sensitive to light.

Sea urchins are also common prey for many marine species on the ocean floor such as crabs, larger starfish, and lobsters. Crabs are also known to hold sea urchins in their large claws and move them around like weapons!

They are also preyed upon by birds, foxes, cod, and sea otters. They use their spiny exoskeleton to protect themselves from these animals, and some species have poisonous spines to stun their predators.

Sea urchins, in many parts of the world, are considered a delicacy as well, and their gonads are typically eaten drizzled with olive oil and lemon as a special treat.

They are rare as they are considered difficult to harvest, and most species can only be recovered from the ocean depths by specialized divers. They must also be eaten fresh, and these factors can cause the price to shoot quite high.

Sea urchin sushi called uni in Japan is one of the most popular ways to eat this sea animal.

Here at Kidadl, we have carefully created lots of interesting family-friendly facts for everyone to enjoy! If you liked our suggestions for what sea urchins eat then why not take a look at what seals eat or green sea urchin facts.

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Written by Tanya Parkhi

Bachelor of Arts specializing in Economics

Tanya Parkhi picture

Tanya ParkhiBachelor of Arts specializing in Economics

Tanya is a skilled content creator with a passion for writing and a love for exploring new cultures. With a degree in Economics from Fergusson College, Pune, India, Tanya worked on her writing skills by contributing to various editorials and publications. She has experience writing blogs, articles, and essays, covering a range of topics. Tanya's writing reflects her interest in travel and exploring local traditions. Her articles showcase her ability to engage readers and keep them interested.

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Fact-checked by Amatullah Gulbargawala

Bachelor of Arts specializing in English, Bachelor of Education specializing in the Language Arts

Amatullah Gulbargawala picture

Amatullah GulbargawalaBachelor of Arts specializing in English, Bachelor of Education specializing in the Language Arts

Amatullah is a passionate student pursuing a Bachelor of Arts and Bachelor of Education from Ashoka College of Education. With a keen interest in literature, she has excelled in elocution competitions and is an accomplished writer. She has completed courses like "History of English Language and Literature", "Introduction to Western Political Thought and Theory", and "Development of Soft Skills and Personality". In her free time, Amatullah enjoys reading books and writing poetry.

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