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What are the differences between the Common Sea Urchin, Green Sea Urchin, and Purple Sea Urchin?

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What are the similarities and differences between the Common Sea Urchin (Echinus esculentus), the Green Sea Urchin (Psammechinus miliaris or Strongylocentrotus droebachiensis), and the Purple Sea Urchin (Paracentrotus lividus or Strongylocentrotus purpuratus)? 

The Common Sea Urchin, Green Sea Urchin, and Purple Sea Urchin are all spiny marine (saltwater) echinoderms in the Parechinidae family. Echinoderm means hedgehog skin.

The Common Sea Urchin, Green Sea Urchin, and Purple Sea Urchin are all spherical with short spines. They all have a hard shell called a test.

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Green Sea Urchin

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The Green Sea Urchin (Psammechinus miliaris) is a marine (saltwater) echinoderm in the Parechinidae family. It is also known as the Shore Sea Urchin.

The Green Sea Urchin is spherical-shaped (ball-shaped). Its rounded “body” is called a test. The test is covered with short, thick spines of about the same length. It is purple-brown in shallow water and greenish in deeper water. The spines are a paler colour with purple tips. It has tubed feet in groups of five or six in a small arc shape. It has a small mouth. It does not have eyes. It is sensitive to touch, light, and chemicals due to the numerous sensitive cells in its spines, tube feet, and around its mouth.

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Black Longspine Sea Urchin

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The Black Longspine Sea Urchin (Diadema setosum) is a marine (saltwater) invertebrate echinoderm in the Diadematidae family. It is related to the Starfish (Sea star). 

The Black Longspine Sea Urchin has extremely long, narrow, hollow spines that are mildly venomous. It has a hard, spherical (ball-shaped) shell, called a test. On its spherical body are five white dots. It does not have eyes. It is sensitive to touch, light, and chemicals, due to the numerous sense cells around its mouth.

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Spineless Sea Urchin

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The Spineless Sea Urchin (Abatus cordatus) is a marine (saltwater) invertebrate echinoderm. It is also known as the Kangaroo Sea Urchin because the female keeps her eggs in a pocket or pouch. It is related to starfish (sea stars). 

The Spineless Sea Urchin has a hard, spherical (ball-shaped) shell, called a test, with no backbone and no spines sticking out of its body (like the Spiny Sea Urchin). Its mouth, with a small jaw, is in the centre of the urchin on its underside. It does not have eyes. It is sensitive to touch, light, and chemicals, due to the numerous sense cells around its mouth.

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Common Sea Urchin

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The Common Sea Urchin (Echinus esculentus) is a spiny marine (saltwater) echinoderm in the Echinidae family. It is an animal that lives on the bottom of the ocean. It is related to the Starfish (Sea Star). It is also called a Sea Hedgehog.

The Common Sea Urchin has a hard shell, called a test. It is spherical (round like a globe), shiny and spiny. Young urchins have bilateral symmetry, and as they become adults, they have fivefold symmetry (similar to starfish with five appendages). Its mouth, with a small jaw, is in the centre of the urchin on its underside.

It does not have eyes. It is sensitive to touch, light, and chemicals, due to the numerous sensitive cells in its spines, tube feet, and around its mouth.

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