Story Behind the Science
Sea stars are unlike many other animals that live in the ocean, one reason being that they are technically not fish. This is why the correct and updated reference for this animal would be sea star rather than starfish. Sea stars are closely related to sea urchins, sea cucumbers and sand dollars, all different kinds of echinoderms. Echinoderms are marine invertebrates whose name means “spiny skin.” These animals also have radial symmetry and unique water vascular systems, which assist in transportation, eating and breathing. Their tough skin helps protect echinoderms from predators.
The most common sea stars have five arms, though some species can have up to 40 arms. They are popularly known for their shape, as well as their ability to re-grow their limbs and bodies when broken off. The reason for this is that most of the sea star’s essential organs are located in its arms.
Sea stars, echinoderms and all of marine biodiversity are threatened mainly by humans. Pollution, caused by urbanization and other industrial human processes, can be easily washed into the ocean, especially when houses, buildings and roads are built near the shore. The resulting pollution can potentially kill echinoderms.
Humans don’t usually eat many echinoderms, but some sea urchins and sea cucumbers are sometimes consumed or even used as fishing bait. When a large quantity of these animals is taken out of a single area, overharvesting can be the devastating result. This directly damages the marine ecosystem that those echinoderms were harvested from.
Another threat to echinoderms is bottom trawling, a destructive fishing technique that results in habitat modification. Bottom trawling is a way for fisherman to collect large amounts of fish, using large nets that are dragged through the open ocean or along the seafloor. This method of commercial fishing causes overharvesting of echinoderms and marine animals, as well as the destruction of delicate communities of sponges, coral and other marine biodiversity.
Did You Know?
- The sea star is nicknamed the “crown of thorns” because of the spines covering the surface of its body and the shorter spines found on its underside.Learn More »
- Echinoderms do not have a head.Learn More »
- Sea stars can weigh up to 11 lbs and live an average of up to 35 years in the wild.Learn More »
- Sea stars have eyespots at the end of each leg that are used to detect light.Learn More »
- Sea lilies look like plants but are actually an animal from the echinoderm group.Learn More »
|Compare & Contrast||Sea Stars||Echinoderms|
|How many species?||2,000||20,000 total (7,000 still exist and 13,000 are known in fossil records)|
|Where do they occur?||Sea stars can be found in all of the world’s oceans, anywhere from tropical environments to the cold ocean floor.||Aside from a few species of echinoderms that live in brackish water, most live throughout the world’s oceans, but none live in freshwater.|
|What do they eat?||They eat coral and a variety of shellfish (like oysters, clams and snails), sea anemone, sponges, small animals and sometimes other sea stars.||These marine animals have varied diets. Some consume floating particles, plankton or algae, in addition to shellfish and other small animals.|
|Do they have predators?||Some animals that prey on sea stars include king crabs, gulls and sea otters.||Otters, flounders and haddock are a few of the echinoderm’s main predators.|