Echinoderm, a member of a large group (phylum) of spiny-skinned marine animals, including starfish, sea urchins, and sea cucumbers. Echinoderms are found in all oceans at various depths. There are about 6,000 species.
All echinoderms have radial symmetry—that is, all body parts radiate from a central point. An echinoderm usually has five symmetrical sections around the central point. This body shape is clearly defined in the common starfish, which has five distinct appendages with the mouth in the center.
The phylum Echinodermata is divided into six classes: Asteroidea (starfish); Echinoidea (sea urchins, sand dollars, and allies); Ophiuroidea (brittle stars); Concentricycoidea (xyloplax); Crinoidea (feather stars and sea lilies); and Holothurioidea (sea cucumbers).
Many sea animals have rough or spiny skin. But sea stars and their relatives have some of the spiniest. That’s why these creatures are commonly called spiny-skinned sea animals. Scientists, however, call sea stars and their relatives by another name. They call these animals echinoderms (ih KY nuh durmz). That name also describes the skin of these animals. In Greek, echinoderm means “spiny skin.”
Echinoderms have much in common with one another. They all have skeletons made of bony plates. Some of the plates have spines that stick out to give these animals their spiny skin. Most have tiny feet that are hollow, like tubes. And all echinoderms have a network of tubes filled with seawater inside their bodies.
There are about 6,500 species, or kinds, of echinoderms living today. Sea stars, of course, are echinoderms. So are sea urchins and sand dollars.
Echinoderms live in every ocean of the world. Some species live in warm, tropical waters. Others live in the icy waters of the polar seas. No echinoderms live in fresh water.
Many echinoderms live in shallow water near shore. Scientists call this area the intertidal zone. Here, the level of the water rises at high tide and falls at low tide. The intertidal zone is a habitat for many kinds of sea stars.
Other echinoderms live in deeper waters. Some live where the water is hundreds of feet deep. No matter how deep the water is, these echinoderms spend most of their adult lives resting on the ocean floor. Some cling to rocks or coral reefs. Others prefer sandy or muddy places.
The sea lily looks like a flowering plant, but it is an animal that also belongs to the echinoderm group.
A sea lily may not look like a sea star or a sand dollar. But it does have a lot in common with these animals. The sea lily has five or more arms, just as a sea star does. A sea lily doesn’t have spines, but it does have hard plates in its skin. And a sea lily also has many tube feet.
Sea lilies live mostly on the ocean floors. There, sea lilies use their round center stalks to attach themselves to the ocean bottom. This keeps sea lilies firmly in place for their adult lives. The mouths of sea lilies are on top of their bodies, which are on top of the stalks.
Sea lilies have lived in the oceans for hundreds of millions of years. During that time they have changed very little. Scientists think they were in the oceans even before dinosaurs roamed the planet!
A feather star, of course! A feather star is a close relative of a sea lily. But unlike a sea lily, a feather star doesn’t have stalks. Instead it attaches itself to the bottom of the ocean with many short hooks. A feather star spends much of its time attached to the ocean floor, but it can also use its long arms to swim or to walk.
Both feather stars and sea lilies have tube feet that cover their long arms. The tube feet are coated with sticky mucus. The mucus helps sea stars capture food.
Feather stars and sea lilies eat tiny water organisms called plankton (PLANGK tuhn). When plankton bumps up against tube feet, it becomes stuck in the mucus. With a quick flick of its feet, the feather star tosses the plankton into a groove in the middle of its arm. From there, the groove carries the food straight to the mouth of the echinoderm.
The biggest danger to echinoderms is the same one that threathens other animals in the ocean—humans.
Echinoderms need clean water in which to live. When people build roads and buildings along the shore, pollutants can wash into the water. These can kill echinoderms. Pollution from other sources can also kill these animals.
People also eat a few types of echinoderms. Both sea urchins and sea cucumbers are collected for food. If people take too many out of one area, they can damage the population. This is called overharvesting.
Still, most types of echinoderms are thriving. And as people continue to explore the ocean’s depths, there is a good chance that more new species will be discovered. Who knows what fascinating echinoderm we’ll learn about next?