Marine invertebrates
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Starfish and sea urchins. (2023). In Q-files Encyclopedia, Life, Marine invertebrates. Retrieved from
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"Starfish and sea urchins." Life, Marine invertebrates, Q-files Encyclopedia, 8 Nov. 2023.
https://www.q-files.com/life/marine-invertebrates/starfish-and-sea-urchins.
Accessed 19 Mar. 2024.
Starfish and sea urchins 2023. Life, Marine invertebrates. Retrieved 19 March 2024, from
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Life, Marine invertebrates, s.v. "Starfish and sea urchins," accessed March 19, 2024.
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Starfish and sea urchins
Starfish, or sea stars, have sucker-lined arms and mouths on the undersides of their bodies. Most starfish have five arms, but some species have more. The arms are not like our own arms, but more like extensions of the head. Starfish use their arms to prise open shellfish. Once they are open, the starfish pushes its stomach through its mouth into the shell to feed on the soft, fleshy parts inside. If a starfish is attacked, it can lose an arm in order to make its escape. This will then grow back. Starfish belong to a group called echinoderms. The echinoderm group also includes brittle stars, crinoids, sea urchins and sea cucumbers.
Echinoderms
The echinoderms live only in marine habitats. They all have tough coverings on their upper sides, made of chalky calcium carbonate plates. The spines on the surface help protect the creatures from predators. Most echinoderms also have the ability to regrow body tissue, including organs and limbs.
Tube feet
Like all echinoderms, starfish and sea urchins have "tube feet". These are small, tube-shaped projections along their arms that allow them to move around. They also use their tube feet for breathing and to help them feed, by passing food to the mouth in the centre of their bodies.
Crown-of-thorns
The crown-of-thorns is a large, brightly coloured starfish. It has at least 12 arms covered in long, venomous spines. It is named after the crown of thorny twigs placed on Christ's head before he was crucified. The crown-of-thorns preys on coral polyps, usually emerging at night to feed on the living tissue from inside the skeletons. When large numbers of these starfish collect on the reef, they can cause significant damage in short periods.
Sea urchins
Sea urchins are small and round with spiny shells for protection. They feed on seaweeds and other plants that grow on the seabed. A sea urchin's mouth is on the underside of its body, while its anus is on the top side of its body. Sea urchins eat by climbing on top of their meals. They have branched tentacles, their tube feet, protruding through holes in their shells. These enable them to move across the ocean floor.
Sea cucumbers
Sea cucumbers are soft-bodied invertebrates that live on the ocean floor. They filter the sandy seabed for dead plant and animal material. Sea cucumbers have five rows of tube feet and a mouth surrounded by tentacles. They use their feet to move across the ocean floor, burrow into the ooze or pass food to their mouths.
Consultant: Chris Jarvis