Starfish look like something from a faraway planet — they're colorful and strange, with no discernible faces and five or more arms. Yet starfish are perfectly adapted to living in the ocean, where their bright colors help them blend into tide pools and coral reefs, and their many flexible limbs help them hunt for food. Their appearance is only one of the fascinating things about this unique marine animal.
Starfish are more closely related to sand dollars than to fish. They're part of a group of animals called echinoderms, ocean-dwelling invertebrates with bodies divided into five equal segments. Sea urchins, sea cucumbers, and brittle stars are also echinoderms. These types of animals are common in coastal waters, where they play important roles in the food chain.
There are over 1500 species of starfish, and they live everywhere, from tidal pools along the Hawaiian shoreline to the harsh seafloor of the Arctic. One deep-sea starfish species, Novodinia Americana, makes it's home almost a mile beneath the ocean's surface, where the waters are cold and dark. This doesn't bother the starfish, which communicates and sees using bioluminescence.
Starfish don't have their own blood like humans do. Instead, they have a water vascular system that takes in seawater from outside and uses it to pump nutrients through their bodies. They get oxygen via osmosis and don't need to breathe. Blood isn't the only thing starfish can live without; they also have no brains. A nerve ring around the center of their bodies helps coordinate their decentralized net of a nervous system.
Each limb of a starfish has a separate eye, which resembles a little red spot. These eyes aren't quite as complex as ours; they only allow the starfish to sense shades of light and dark. It's just enough to allow starfish to see the bodies of their prey and the shadows of predators overhead.
Starfish have had to adapt to survive many ocean environments as well as the turtles, sharks, and manta rays who all see them as a tasty meal. Some starfish have brightly colored bodies or sharp spines to scare away predators. Their skin is tough and bony. If one of their arms is bitten off, a starfish can grow a new one in about a year. Starfish will sometimes tear off an arm to get away from danger.
Baby starfish come in two ways. Female starfish lay eggs that the males then fertilize and create tiny larval starfish. Some species can also reproduce through regeneration. When they drop an arm, assuming it's not eaten by a predator, that arm can grow into an entirely new starfish that's genetically identical to the parent.
Many starfish are small enough to fit in a person's hand, but a few species grow bigger. Sunflower starfish can grow to nearly 5 ft tall and 3 feet across. They also have more limbs than average; sunflower starfish can have anywhere from 16 to 24 arms.
Starfish eat mollusks, sea snails, coral, algae, and even other starfish, and they have two stomachs. One of them, the cardiac stomach, gets vomited out where it can pierce or surround potential prey. The cardiac stomach secretes powerful enzymes that digest the other animal outside the starfish's body. When the prey is broken down, the starfish pulls it and the cardiac stomach back inside their body, where their second pyloric stomach digests the remains.
About 500 million years ago, the earth had an explosion of life. The fossil record from this time features armored slugs, trilobites, ancient ancestors of shrimps, and the first recorded starfish. They have been in our oceans ever since, living alongside the Dunkleosteus, who came 150 million years later, the T-Rex, 400 million years later, and the Megalodon, who roamed seas where starfish had already been living for about 477 million years.
Many species of starfish are endangered because of habitat destruction and a catastrophic pandemic of sea star wasting syndrome. Starfish are, however, essential to the ecosystem. They're the main predators of sea urchins that feed on kelp. A healthy starfish population means healthy kelp forests, and kelp absorbs carbon dioxide more effectively than trees. Scientists are breeding starfish in labs in hopes of repopulating the oceans and reducing carbon in the atmosphere.
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