The starfish, despite its name, is not a fish. In fact, marine scientists are trying to replace its name with sea star, which it is also commonly known as. They are echinoderms, spiny-skinned animals, and are related to sand dollars and sea urchins. More than 2,000 species exist and can be found living along beaches and up to a quarter mile offshore near and in oceans around the world.
Starfish can grow up to 10 inches in diameter, can weigh up to 11 pounds and come in a variety of colors. Most have five arms, while some can have as many as 40 (usually in multiples of five), connected to a small central disk. Under each arm are tube-like feet that help them crawl and eat, and at the tips of their arms are eyespots that can detect changes in light and dark. The central disk is flat, rough and usually has spines. The mouth is located on the underside and the anus is located on top. They have a nervous system that spreads through their arms, but they do not have a brain. Filtered sea water, instead of blood, flows through their vascular system, which is pumped into the body through a small structure called the madreporite located on top of the body.
Most starfish have a unique way of eating which allows them to eat prey larger than what their mouth would allow. They can push their stomach out of their mouth, digest their prey, then bring their stomach back into their body. Their diet consists of meat, including mussels, clams, oysters, small fish, small crustaceans, polychaete worms and other echinoderms. Some species will also eat plants. To consume an animal in a shell, they wrap their arms around the shell and use the suction from their feet to pry it open.
The female starfish normally produces a million eggs at one time, but can produce over 2 million. Fertilization takes place outside of the body. Eggs, and sperm from the male, are released into the water. Once fertilized, the eggs become free-swimming larvae that feed on phytoplankton - microscopic plants drifting in the water - for about three weeks before settling on the ocean’s surface to develop into an adult. Some species can reproduce asexually, meaning eggs and sperm are not needed. Instead, a detached arm (some requiring a piece of the central disk to be attached) can develop into a whole new starfish.
Here are some more interesting facts about starfish:
If an arm is lost, they can regrow it.
They move very slowly. It can take them about one minute to move one foot.
Their size depends on how much they eat, not how old they are.
Predators include crabs, fish, sharks, rays and birds when washed ashore.
Their ability to filter toxins and contaminants in the water is not that good, which can cause death when polluted water is pumped into the body.
In the wild, starfish can live up to 35 years.
National Geographic: https://animals.nationalgeographic.com/animals/invertebrates/starfish/
University of Michigan Museum of Zoology: https://animaldiversity.ummz.umich.edu/site/accounts/information/Asterias_rubens.html