While starfish live underwater, they're not actually fish at all. In fact, marine scientists have replaced the beloved starfish's common name with the name "sea star" because, well, the starfish is not a fish.
So, what exactly are they? "They're a type of invertebrate, meaning they don't have a backbone," explains Kim Stone, the Georgia Aquarium's curator of fish and invertebrates, in an email interview. "Their body consists of a central disc with arms that radiate out, and on the underside, there are hundreds to thousands of small suction cups called 'tube feet' that help the sea star move around, stick to different surfaces and eat."
There are some other big differences that set starfish apart from fish, according to the National Ocean Service. These cool creatures don't have gills, scales or fins; live only in saltwater; and use sea water instead of blood to pump nutrients through their bodies by means of a "water vascular system."
Eager to find out more about these beautiful marine animals found in a variety of colors, shapes and sizes? Here are 11 more fun facts you might not know about these echinoderms.
1. They First Existed Millions of Years Ago
"Sea stars belong to a group of marine invertebrates called echinoderms, which first appeared more than 500 million years ago," says Stone. "The ancestors of modern-day sea stars appeared more than 450 million years ago during the Ordovician Period." Echinoderms include five classes of marine life: sea stars; brittle and basket stars; sea urchins and sand dollars; sea cucumbers; and sea lilies and feather stars.
2. There Are Around 2,000 Different Species
"They can be found in a variety of habitats from shallow sandy bottoms and cold rocky environments to the bottom of the sea floor," says Stone. Some kinds of starfish are even found on sandy sea beds 29,500 feet (9,000 meters) deep! One of the most unique, adds Stone: the crown-of-thorns sea star, a tropical species found in the Pacific and Indo-Pacific named for the long spines that cover its body. Made up of plates of calcium carbonate with tiny spines on their surface, a sea star's spines are used for protection from predators, which include birds, fish and sea otters.
3. A Sea Star's Mouth Is on its Underside
Upon capturing food (typically a bivalve such as a clam or mussel) with its tube feet, the sea star wraps its arms around the animal's shell and pulls it open just slightly. Then the sea star pushes its stomach through its own mouth and into its prey's shell. It then digests the animal and slides its stomach back into its own body. This unique feeding mechanism allows the sea star to eat larger prey than it would otherwise be able to fit into its tiny mouth. Among the other sea star menu favorites: mollusks like clams, oysters and snails.
4. Some Species May Have as Many as 40 Arms
Although starfish have five-point radial symmetry, that doesn't mean all of them have five arms. "Species with 10, 20, or even 40 arms exist," says Stone, adding that if one of these arms is lost, a sea star has the amazingly ability to regenerate it. Sea stars accomplish this by housing most or all of their vital organs in their arms, which means some species can even regenerate an entirely new sea star from just one arm and a portion of the star's central disc.
The ability to regenerate lost arms is especially useful if a sea star is injured by a predator. It can lose an arm, escape and grow a new arm later. This won't happen too quickly, though; it takes about a year for an arm to grow back. Some require the central body to be intact to regenerate, but a few species can grow an entirely new sea star just from a portion of a severed limb.
5. They Have Eyes
The eyes are there, just not in the place you would expect. Sea stars have an eye spot at the end of each arm. That means a five-armed sea star has five eyes, while the 40-armed sun star has 40 eyes. Each sea star eye is very simple and looks like a red spot. It doesn't see much detail, but it can sense light and dark. And that's just enough for the environments in which these animals live.
6. Some Starfish May Live for More Than Three Decades
The average life span of a starfish is an impressive 35 years. Usually, large starfish species tend to live longer than their smaller counterparts.
7. It Is Not OK to Take Them Out of the Water
"Like many aquatic animals, sea stars get their oxygen from the water," says Stone. "They also have a unique circulatory system that pumps water through their body instead of blood, so taking them out of water for extended periods of time can stress the animal and cause them harm." Unlike fish that have gills and mammals that have lungs, she adds, sea stars "breathe" by absorbing oxygen from the water through different parts of their body, such as their skin and tube feet.
8. They Can Reproduce Sexually or Asexually
"Sea stars can reproduce sexually through spawning or asexually by dividing their central disc," says Stone. Male and female sea stars are even difficult to tell apart because they look identical. They reproduce sexually by releasing sperm and eggs (called gametes) into the water. The sperm fertilizes the gametes and produces swimming larvae, which eventually settle on the ocean floor, growing into adult sea stars. Sea stars also can reproduce asexually through regeneration, which is what happens when the animals lose an arm.
9. They Are Omnivores
Starfish eat a wide variety of plant and animal life. What they eat can depend on the species, according to Stone. "Many species are scavengers and carnivores that eat gastropods, bivalves, barnacles, marine worms and other invertebrates," she says. "Some species are suspension feeders that capture plankton and organic material from the water."
10. They Don't Swim
Sea stars use hundreds of small suction cups called tube feet on the underside of their bodies to move from one area to another. The tube feet are filled with seawater, which the sea star brings in through the madreporite (a sort of trap door) on its top side. According to the National Ocean Service, adult sunflower sea stars can move at the astonishing speed of 3 feet (around 1 meter) per minute using 15,000 tube feet. Tube feet also help sea stars hold their prey. If you get a chance, visit a tide pool or aquarium, and take a moment to watch a sea star moving around — you'll find it's truly a most amazing sight.
11. They Don't Hurt Humans, But We Are Dangerous to Them
Because they're literally shaped like stars, humans have the tendency to keep starfish as souvenirs or even hold them out of the water just for photos. Forcing starfish out of the water, or throwing them back in, is a big no-no. Just like sea cucumbers and corals, starfish are born with intricate and fragile arms and tiny body structures. Despite their regeneration capabilities, even the slightest poke may hurt or damage them, most especially when people carelessly throw them out of the water. Aside from that, human hands are naturally dangerous to all sea creatures due to billions of bacteria that exist on them and contact can lead to a possible slow death for these creatures.