Echinoderms
Echinodermata
Have exoskeletons
Echinoderms is the collective name for a large group of animals that live on the seabed. The echinoderms include starfish, sea urchins, brittle stars, crinoids, and sea cucumbers. In total, there are over 9,000 different species of echinoderms around the world. A large proportion live in coral reefs, but echinoderms can be found around Antarctica and at great depths. Some 80 species are found along the Swedish coast.
All echinoderms have an exoskeleton in the form of calcareous plates, which are usually covered with spines. Several of the species are venomous. They can vary in size, from a few millimetres up to 100 cm in diameter. Scientists have found fossils of extinct crinoids as large as 20 metres.
An Australian starfish with many colours.
Photo: John-Turnbull-CC-BY-SA
The icon star has a graphical pattern.
Photo: Blogie-Robillo-CC-BY-SA
Some starfish has round soft-looking bodies.
Photo: Edward-Callaghan-CC-BY
Most starfish has five arms. At the tip of each arm is a small eye spot, which can sense the difference between light and dark. Some species has up to 50 arms, many of them are part of the sun star group.
Photo: Lumpytrout-CC-BY-SA
Sea urchins have a round shape, and, more often than not, sharp spines.
Photo: Prilfish-CC-BY
Some sea urchins have thick and blunt spines.
Photo: Scott-Roy-Atwood-CC-BY-SA
The helmet urchin's spines are reshaped into a kind of scales.
Photo: Makuahine-CC-BY
The brittle star's body consists of a round disc with five arms attached to it. The disc can be up to 15 cm - and the arms up to 70 cm! The brittle star has no eyes, and no anus.
Photo: Philippe-Bourjon-CC-BY-SA
The basket stars are a family of brittle stars, with many curly branches on their arms.
Photo: Seascapeza-CC-BY-SA
Crinoids are believed to be the oldest echinoderms on Earth. The crinoid in the photo is a kind without a stalk. This group of crinoids is called feather stars.
Photo: Alexander-Vasenin-CC-BY-SA
Crinoids with stalks, growing upside down on a rock.
Photo: NobuTamura-CC-BY-SA
Sea cucumbers also belong to the echinoderms.
Photo: Diego-Delso-CC-BY-SA
A pineapple sea cucumber with robust spines.
Photo: Bernard-DUPONT-CC-BY-SA
Tube feet and light-sensitive organs
Echinoderms move slowly forward, using hundreds of so-called tube feet. The tube feet are special projections on the body of the echinoderm that it uses to move around. Echinoderms have neither eyes nor brain, but only a simple nervous system. However, some starfish have light-sensitive organs on their arms that can measure different levels of light.
Echinoderms reproduce by so-called external fertilization. This means that the male ejects sperm, and the female ejects eggs directly into the water. There, eggs and sperm meet before sinking to the bottom. At the bottom, they develop into full-grown organisms.
On the underside of a starfish are hundreds of small tube feet. They are used to move, and for the starfish to catch prey.
Photo: Stewart-Butterfield-CC-BY
A starfish which has used its tube feet to catch a fish.
Photo: Jinterwas-CC-BY
The starfish's mouth is located in the middle of its underside. It doesn't swollow its food in the same way you do, but turns its stomach inside out to eat the prey. The stomach secretes digestive fluids that break down the prey and allow the starfish absorb the nutrients.
Photo: Coughdrop12-CC-BY-SA
Sea urchins graze on algae and seaweed, either growing on something or floating in the water. The mouth is located on the sea urchin's underside, and is a complicated system of muscles and teeth. The teeth are incredibly strong, and can bite through steel.
Photo: Bundesstefan-CC-BY-SA
Researchers believe that the sea urchin's tube feet help the animal to see - or at least sense differences in light.
Photo: Jerry-Kirkhart-CC-BY
Sea cucumbers have tentacles around their mouths, that they use to put food into their mouths. They mostly eat dead and living organic matter from the sea floor.
Photo: Bernard-DUPONT-CC-BY-SA
Some sea cucumbers use their tentacles to filter plankton from the water.
Photo: Derek-Keats-CC-BY
A sea feather using its feather-like outgrowths to catch food.
Echinoderm larvae, like this sea urchin, live free swimming as plankton until they are big enough to settle to the bottom or a rock.
Photo: Bruno-C.-Vellutini-CC-BY-SA
A very young and newly settled sea urchin.
Photo: Philippe-Bourjon-CC-BY-SA
Capable of replacing lost body parts
Echinoderms are known as experts when it comes to regenerating lost body parts. If a starfish or brittle star loses an arm, a new one quickly grows back. But what’s even more spectacular is that sea cucumbers can vomit or excrete their own intestines, and then regrow them! The sea cucumber uses the ejection of its intestines as a means of defence. Regenerating lost body parts requires particular stem cells. These stem cells are of interest to scientists, given that echinoderms are distantly related to humans.
Fossils of crinoids, the oldest group of echinoderms.
Photo: James-St.-John-CC-BY
The sunflower sea star is one of the largest species of starfish on Earth. It can grow up to 1 meter in diameter.
Photo: Ed-Bierman-CC-BY
The morning sun star is a starfish that feeds on other starfish.
Brittle stars are, like other echinoderms, able to regrow lost body parts. The photo shows a brittle star with a couple of partly lost arms, on their way of regrowing.
Photo: Philippe-Bourjon-CC-BY-SA
A smaller brittle star also regrowing its lost arms.
Photo: Jay-Sturner-CC-BY
A sea cucumber which feels threatened can shoot its intestines out of its body - and regrowing them! Sea cucumbers breathe by taking in water through their anus, and passing it by its breathing organ.
Photo: Rpillon-CC-BY-SA
A common starfish regrowing two lost arms.
Photo: Dean-Franklin-CC-BY.