Is a starfish a fish? No, starfish are not fish. Fish are vertebrate animals that live in the sea, have gills to breathe through, have a two chambered heart, are cold blooded, and have a brain and central nervous system. Starfish live in the sea, but that is where the similarity stops.
These days, many people would prefer that starfish be called sea stars because that more closely resembles what they are. Their Latin name is Asteroidea, which means star appearance and was given to them in 1830 by A French zoologist called Henri Marie Ducrotay de Blainville. You can see the word “aster” in asteroid as well. Before that, they were called starfish in English. Every other language calls them sea stars, which probably came from Latin, through French, but in English people stuck with starfish. This is not too surprising because they called all the animals that lived only in the sea fish. Crabs, whales, dolphins, seahorses, and starfish, were all fish because they lived in the sea. The definition of what a fish actually is only came along later. The first written evidence of starfish comes from 1530 and it was spelled starrefyshe. The word was probably around for a long time before it was written down. The word sea star was written down in English about 30 years later, but it was probably just a translation from the French.
So, what is a starfish? It is an echinoderm and is closely related to the sea urchin and the sea cucumber. Echinoderms usually have radial symmetry, which means they are symmetrical around a central point. Our faces are symmetrical in that the left half and the right half are pretty similar. An echinoderm is symmetrical in that all of its points (usually five but not always) are identical. This can be seen with the average starfish because it has five points that are identical and they are arranged around the center of the starfish.
We are talking about starfish but, of course, there is more than one type of starfish. The one we are thinking of has five points, but there are 1,900 different types of starfish. The majority have five arms, but some have six, some have seven, and some have fifteen. However, no matter how many arms they have, there is always radial symmetry. Starfish have a number of unique features that make them very interesting.
So, fish have a backbone, a brain and nervous system, gills, along with a heart and blood. Starfish don’t have any of these. For a start, they don’t have a brain. They do have a nervous system, but they have no brain to control it all. In the center of their bodies there is a ring of nerves that pass signals from the nerves in each arm around the starfish. Whenever they move or whatever they do, they are reacting to a stimulus.
Starfish don’t have bones either. They are invertebrates and they are constructed of calcium carbonate, which is something found in eggshells and the shells of shellfish as well. It has a strong honeycomb structure, and it forms in plates that fit perfectly together and allow free movement. Starfish have a layer of muscle over this and a layer of skin on top.
Starfish don’t have blood or a heart. They have a water vascular system. Seawater is absorbed through a porous part of their body. This acts as a valve and the starfish can pump the seawater around its body by clenching and relaxing muscles. Starfish don’t have gills or lungs and the oxygen they need is carried through them in the water. Oxygen passes from the water into their bodies and carbon dioxide and other waste exits in the water. The water also pumps nutrients around their bodies. They can also use the water to move. Starfish have thousands of tube feet that they use to move on. They can fill these tube feet with water, making them stand out. On the end of each tube foot is a chemical sucker that attaches to whatever the starfish is moving over. By engorging and relaxing these tube feet, the starfish can move across a surface.
Many starfish also have the ability to regrow themselves. Some starfish can completely regrow from just one arm. Their mouth is in the center of the body, so they have to live on the nutrients they have stored until they can regrow the mouth. It can take many months. And this is what I learned today.
Photo by Myburgh Roux: https://www.pexels.com/photo/starfish-with-soft-papulas-on-arms-3466417/
Sources
https://en.wikipedia.org/wiki/Echinoderm
https://en.wikipedia.org/wiki/Symmetry_in_biology#Radial_symmetry
https://www.etymonline.com/word/starfish
https://en.wikipedia.org/wiki/Starfish
https://www.beachcombingmagazine.com/blogs/news/the-starfish-is-not-a-fish
https://www.shapeoflife.org/news/featured-creature/2019/03/19/radically-radial-sea-star
http://www.animalsworlds.com/characteristic-of-fish.html