Sea urchin

Sea urchins, or simply urchins, are small sea creatures that belong to the group of echinoderms. They live on the seabed in all oceans, from shallow waters to deep seas. These creatures have a round body covered with sharp spines and a hard shell, usually from 3 to 10 centimeters across. They move slowly using tiny tube feet and sometimes push themselves along with their spines.

Sea urchins mainly eat algae but will also eat slow-moving animals like crinoids and sponges. Their predators include sharks, sea otters, starfish, wolf eels, triggerfish, and humans. Without enough predators, sea urchins can damage ocean environments by eating too much algae.

Like all echinoderms, adult sea urchins have a special five-sided symmetry. Their young larvae have a mirror-like symmetry. Sea urchins have existed for a very long time, with fossils dating back to the Ordovician period, about 450 million years ago. Scientists have studied sea urchins for over a century because their embryos are easy to observe. Some sea urchin species are popular in aquariums for helping control algae.

Diversity

See also: List of echinodermata orders

Sea urchins belong to the phylum Echinodermata. This group also includes starfish, sea cucumbers, sand dollars, brittle stars, and crinoids. Like these animals, sea urchins have a special balance called five-fold symmetry. They move using many tiny, clear, sticky "tube feet" tube feet. This balance is hard to see when the sea urchin is alive but becomes clear when its hard shell, called a "test," is dried test.

The name "sea urchin" usually means "regular echinoids," which are round. There are different groups of these, split into two subclasses: Euechinoidea, known as "modern" sea urchins, and Cidaroidea, called "slate-pencil urchins." These have thick, blunt spines with algae and sponges on them. Inside the Euechinoidea group, there is also a special group called Irregularia. This group includes flattened shapes like sand dollars, sea biscuits, and heart urchins.

Together with sea cucumbers (Holothuroidea), sea urchins form a group called the subphylum Echinozoa. These animals are known for their round shape. Even though some sea cucumbers and irregular sea urchins have many different shapes, many still keep this round form.

_[Paracentrotus lividus](/wiki/Paracentrotus_lividus)_, a regular sea urchin ([Euechinoidea](/wiki/Euechinoidea), infraclass [Carinacea](/wiki/Carinacea))

A [sand dollar](/wiki/Sand_dollar), an irregular sea urchin ([Irregularia](/wiki/Irregularia))

_[Phyllacanthus imperialis](/wiki/Phyllacanthus_imperialis)_, a cidaroid sea urchin ([Cidaroidea](/wiki/Cidaroidea))

Description

Urchins are usually between 3 to 10 cm (1 to 4 in) in size, but some very large ones can be up to 36 cm (14 in). They have a round body covered with moving spines, which is why they are called Echinoidea (meaning "spine" in Greek). The word "urchin" is an old name for a hedgehog, because sea urchins look similar to them.

Like other sea creatures called echinoderms, young sea urchins start with a mirror-image body shape, but as they grow, they develop a five-part symmetry. This means their bodies have five equal sections coming from the center. In many sea urchins, the mouth is on the bottom and the top part is called the anus. Some sea urchins, like sand dollars, have an oval shape and can dig through sand because of their flat underside.

Systems

Musculoskeletal

Further information: Test (biology) and Tube feet

Sea urchins have a hard shell called a test, made of fused plates of calcium carbonate. This shell is covered by a thin layer of skin and muscle. Even though it surrounds most of the urchin, it is called an endoskeleton because it has skin and muscle on top. The test has five grooves and five wider areas. Inside the test, there are tiny holes where tube feet can extend.

The test is covered with small bumps called tubercles, and spines attach to these bumps. Spines help protect the urchin and let it move. They come in different shapes and sizes. Sea urchins can move by walking with their tube feet, which work like the tube feet of starfish. These tube feet stick out through small holes in the test and are moved by a water system. This system uses water pressure to push water in and out of the tube feet, helping the urchin move. Some sea urchins can even flip themselves back over if they get upside down.

Feeding and digestion

The mouth of a sea urchin is in the middle of its underside. It is surrounded by soft lips and small bony pieces. The mouth has five strong, arrow-shaped plates with teeth that help the urchin grab and scrape food. This setup is so good at grinding that scientists have studied it for use in machines.

On the top of the urchin, there is a membrane called the periproct around the anus. It has hard plates, including five special ones that have openings called gonopores.

Most sea urchins have a special set of teeth and a tongue-like structure inside their mouth, called Aristotle’s lantern. This name comes from an ancient description by Aristotle. The mouth leads to a tube that goes to the stomach, which is divided into parts. Food moves through the stomach and then to the intestine, which has a small tube called a caecum that helps with digestion.

Circulation and respiration

Sea urchins have a system to move fluids through their body. This includes a network of vessels and a fluid that fills the body cavity. This fluid helps carry things around and remove waste.

Most sea urchins have small outer parts called gills around their mouths that help them breathe. These gills can take in oxygen when the urchin needs it. The tube feet can also help with breathing, especially in some types of urchins that do not have gills. Inside each tube foot, there is a wall that helps keep the water flowing in the right direction.

Nervous system and senses

Sea urchins do not have a brain, but they have a nerve ring around their mouth. From this ring, nerves go out to the tube feet, spines, and other parts. Sea urchins can feel touch, light, and chemicals. They do not have eyes, but some parts of their body may help them sense light. They usually try to hide from light and can sense which way is down with special structures called statocysts.

Life history

Sea urchins have male and female sexes, but you can't tell them apart by looking. Their organs for making new babies also store food. Most sea urchins have five of these organs, but some have fewer.

When they make new babies, the eggs and sperm float in the water and meet up. The egg becomes a tiny swimming ball that grows into a larva with arms. After several months, the larva settles down and changes into a baby sea urchin. Some sea urchins can live for over 100 years!

Ecology

Sea urchins live on the ocean floor, from shallow areas you can see at low tide to very deep waters. They are found all around the world, in both warm and cold seas. Many types live deep in the ocean, sometimes several kilometers below the surface.

Sea urchins mainly eat algae, so they are herbivores. But they sometimes eat other sea creatures too, making them omnivores. Their strong, sharp spines help protect them from animals that might want to eat them. Some sea urchins have spines that can cause a stinging feeling.

When there are too many sea urchins and not enough animals to eat them, they can damage their homes by eating too much kelp. This creates areas called "urchin barrens," where the kelp forests are gone, and the sea urchins dominate. The right number of sea urchins and their predators helps keep the ocean balanced and healthy.

Evolution

Fossil history

The earliest sea urchin fossils are from the Middle Ordovician period, about 465 million years ago. Their hard shells, made of calcite plates, are found in rocks from every time period since then. Sometimes we find spines, but often only the shell stays behind.

Most sea urchin fossils from the Paleozoic era are incomplete, with just spines and small shell pieces. In the late Paleozoic, sea urchins almost disappeared, with only six species surviving into the Permian period. Only two groups made it through this big change and into the Triassic period. One group led to today's pencil urchins, and the other to many modern sea urchins. Their numbers grew again by the late Triassic.

During the Jurassic and Cretaceous periods, sea urchins evolved into new groups. Some sea urchins, like Micraster from the Cretaceous, help scientists date rocks.

In the Paleogene and Neogene periods, sand dollars appeared. Their flat shells and tiny spines were perfect for living on sandy bottoms in shallow water.

• Archaeocidaris brownwoodensis, Cidaroida, Carboniferous, about 300 million years ago
• Miocidaris coaeva, Cidaroida, Middle Triassic, about 240 million years ago
• Clypeus plotti, Irregularia, Middle Jurassic, about 162 million years ago
• Echinocorys, Holasteroida, Upper Cretaceous, about 80 million years ago
• Echinolampas ovalis, Cassiduloida, Middle Eocene, about 40 million years ago
• Clypeaster portentosus, Clypeasteroida, Miocene, about 10 million years ago

Phylogeny

External

Sea urchins are deuterostome animals, like chordates. A study in 2014 looked at genes from all echinoderm classes.

Internal

A 2022 study shows a different family tree for sea urchins. Irregular sea urchins are closely related to Echinacea, including Salenioida, forming a group called Carinacea. Other groups like Aspidodiadematoida, Diadematoida, Echinothurioida, Micropygoida, and Pedinoida form another basic group called Aulodonta.

Relation to humans

Injuries

Sea urchins can hurt people by poking them with their sharp spines. These spines can leave small holes in the skin and might be painful. If you get poked, it’s important to remove the spine right away.

Science

Sea urchins are important for science because they help scientists study how animals grow and develop. Their clear eggs make it easy to see how they form and change. Scientists also use sea urchins to learn about how animals stay healthy and heal themselves.

As food

In many places around the world, people eat the parts inside sea urchins, called “roe” or “corals.” In Japan, this food is called uni and is often eaten raw in dishes like sushi. Japan eats a lot of sea urchin and imports it from many countries. Other cultures also enjoy eating sea urchin in different ways.

Aquaria

Some sea urchins are kept in aquariums because they can help control algae and make a nice addition to tanks with other sea creatures.

Folklore

Long ago, some people believed that sea urchin fossils could protect against harm. They thought these fossils could keep away bad luck or sickness.

Explanatory notes

There are no explanatory notes provided in the source content for this section.