What do sea urchins eat? – Technology Org

Sea urchins, those small creatures resembling balls full of sharp spikes sometimes are called porcupines of the sea. Contrary to some beliefs, sea urchins have a mouth and will eat nearly anything they find – as long as the food matches their size.

A sea urchin.

A sea urchin. Image credit: Pxhere, CC0 Public Domain

This article explains what is the favorite food of a sea urchin, and how these animals eat. You will also find more interesting information about the origin of sea urchins, their biology, and even the origin of the name of their species. Let’s start with a brief introduction to this interesting creature.

What is a sea urchin?

Sea urchins are little animals that live on the seabed of every ocean. They slowly move on the ocean floor with their tube feet, or propel themselves with their spines. They indeed remind porcupines or hedgehogs, because these long spikes are a weapon to deter predators who would otherwise gladly feast on sea urchins.

Despite looking very similar, there are about 950 species of sea urchins on our planet, living on the seabed of every ocean, inhabiting every depth zone down to 5,000 meters (16,000 ft). Their spherical hard shells full of spikes are a common feature. The shells range in diameter from 3 to 10 cm (1 to 4 inches).

This is the body (also called test) of the Echinus esculentus, a regular sea urchin, without its needles.

This is the body (also called test) of the Echinus esculentus, a regular sea urchin, without its needles. Image credit: Esko Jäntti via Wikimedia, Public Domain

Ten fused plates encircle the body of a sea urchin and for their outer skeleton. This creature can extend its tube feet through holes between those sections. Nearly everyone gets this false first impression thinking that these sea hedgehogs can not move at all. But this animal can move using its feet, by changing the amount of water inside. Most of the time they use their feet to hang on the bottom of the ocean while feeding, but if needed, they can reach relatively high walking speeds.

What is the origin of sea urchins?

The fossil record shows that the Echinoids – the general class of animals that includes sea urchins – first appeared on our planet in the Ordovician period, about 450 million years ago, or 220 million years before the first dinosaurs.

Modern sea urchins are the only representatives of this class of animals. Their closest relatives are the sea cucumbers (Holothuroidea). More distant relatives are sea stars, sand dollars, and sea lilies.

Lantern of Aristotle, the entire chewing organ of a sea urchin, viewed in lateral section.

Lantern of Aristotle, the entire chewing organ of a sea urchin, viewed in lateral section. Image credit: Philippe Bourjon via Wikimedia, CC BY-SA 4.0

Do they really have a mouth?

Sea urchins have a mouth on the bottom of their shells. It faces the ground and is made up of five calcium carbonate teeth with a fleshy, tongue-like structure within. The mouth can grasp and tear, allowing urchins to eat a wide variety of food.

What do sea urchins eat?

The diet of sea urchins consists of both plants and animals. They most often eat algae and kelp which form their primary diet, but sometimes also prey on slow-moving animals, such as jellyfish, mussels, or sponges. Sea urchins can even eat deceased fish.

Some species are popular in aquariums, because they offer a natural way to control the growth of algae. And, of course, these spikey balls captivate the attention of any observer.

Teeth of a sea urchin.

Teeth of a sea urchin. Image credit: Bundesstefan via Wikimedia, CC BY-SA 3.0

Why sea urchins are called like that?

Their name comes from the old French word irichon/yrichonwhich was a variant of herichun, meaning a hedgehog. Later this word transitioned into English and was used to name sea urchins – because to people they looked like small hedgehogs.

Sea urchin spikes are sharp but cannot protect against all predators

Despite being protected by their spiny shells, sea urchins often become a victim of predators. Animals such as wolf eels, sea otters, lobsters, starfish, crabs, marine mammals, and birds have sea urchins in their diet.

They have specially adapted teeth, pincers, or claws that allow them to overcome the protective features of sea urchins. In fact, having predators is essential for the health of an ecosystem. Otherwise, urchins devastate their environments, like algae and kelp forests.

Moreover, humans also eat these little animals. They are considered a delicacy in some countries, especially Japan. There, they are often eaten raw as sashimi or in sushi. In particular, the gonad (or genital gland) of sea urchins is edible. You can also find dishes from sea urchins in Mediterranean cuisines or California.

What happens when a sea urchin dies?

Adult sea urchins reach their maximum size, usually in about five years. They can live up to twenty years.

When a sea urchin dies, its body loses all spines – they simply fall off, leaving tiny bumps at the spots where they were once connected. When a sea urchin is alive, it can rotate its spines around these bumps using its skin and muscle that can be pulled to move them.

Sea urchins are very sensitive to changes in water quality. Even under slight contamination, they are usually the first to show signs of stress. When weakened or ill, they lay their spines down, until they finally fall off.

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