Legend of the Starfish
Once upon a time, a man witnessed a storm off the coast that resulted in thousands of starfish being washed ashore. The man would have liked to help, but there were simply too many. He was staring at the vast tragedy before him when something caught his eye. A small boy was walking along the beach, tossing the creatures back into the ocean.
The man approached the boy, and asked him what he was doing.
"Saving the starfish," replied the boy.
"You can only save a few, you know. There are thousands. That little bit can't make a difference."
In response, the boy bent over and picked up another starfish. He examined it briefly, then threw it into the sea. He then turned to the man. "It made a difference to that one."
What are they?
Starfish aren't really fish; they lack fins, scales, and gills. That's why scientists prefer to call them by one of their other nicknames: sea stars. They are members of the class asteroidea, giving rise to another name for them: asteroids.
They are also echinoderms, members of the phylum echinodermata. Adults of that phylum are characterized by their radial symmetry; other members include sea cucumbers, sand dollars, and sea urchins. Starfish typically have five arms, though some species have more, and one can have up to 40!
Where are they?
Starfish live in saltwater, and only in saltwater; they have a water vascular system that pumps surrounding seawater instead of blood through their bodies to carry nutrients. They can be found in icy polar seas, warm mediterranean currents, and all temperatures in between. Some favor ocean depths, though most occupy shallow waters.
They occupy kelp forests, sandy beaches, tide pools, sea grass beds, rocky and sandy shores, even up to 400 meters from water.
Their bodies are really weird.
Starfish, or sea stars, have body parts in some unexpected places, and no bones or brains! The mouth of a starfish is in the center of it's body, on the underside; it's rectum is on the top. The creatures have two stomachs. One stays in their body, and the other one comes out of their mouth to engulf prey, digesting what it can before bringing the now-pulp back into the body for further digestion.
Their eyes are at the ends of their arms, though they likely can't do much beyond differentiate between light and dark. The skin of a sea star has chemoreceptors that allow it to smell it's prey (often clams), and even allow it to determine what direction the smell is coming from.
The creature crawls along the ocean floor using it's hundreds of tube feet, which it also uses to pry open clam shells. This works because their 'feet' are tubes that it extends, fills with water, places in the right spot, then creates suction by emptying the water again, so anchoring each leg in place. It uses this same suction power to continuously tug on clam shells until the clam's muscle wears out from exhaustion and the clam is no longer able to hold the shell shut. Then the starfish can get to the meat inside.
The more common means of reproduction...
All species can regenerate new arms should they lose one, and some species can regenerate a whole new starfish, so long as they have an arm and a piece of the central disk. Not all of the species are able to reproduce from pieces, but all starfish can reproduce sexually. In most cases, this involves a male and female.
When a male and female are involved, usually the female starfish releases eggs into the water, and male starfish release sperm nearby. Those meet and form swimming larva, which join the ocean's other zooplankton (small and weak or immature forms of animals that float in the water, often snacks for other ocean creatures). Some of those survive and settle to the bottom, where they grow into the creatures we recognize.
The trigger for this release is a pheromone released by the first starfish to spawn, which triggers spawns in nearby starfish. The females of some species also use pheromones to attract males to the area. Certain species form a pair, where a male will join arms with a female and they spawn together.
Sexes are not always obvious and not always constant.
The gonads of a sea star are on the inside; the sex can often only be told when it spawns.
Some species have simultaneous hermaphrodites (creatures with both male and female parts), and some are what's called sequential hermaphrodites, which change sexes during their lifespan.
Of those, some start as males then change into females as they get older; some species' females can split in half if they are large enough. The pieces become male, then become females again once they are large again.
Not all species treat their eggs the same way
A few species stick their eggs under rocks. The eggs can also be brooded, which means kept on the body of the parent longer than required to transport it elsewhere. Sometimes they are sat on, sometimes they are kept internally. Of the brooders, some are fed yolk. Those tend to keep their young internal until they are fully developed starfish. The young of some brooders eat their fellow embryos. Brooding is more common in environments hostile to larva (cold or very deep water).
Their role in the ecosystem
Sea stars are what's known as a keystone species. A keystone species is one on which other entire ecosystems depend, and one whose removal would change the ecosystem drastically. The reason these animals are so important is because they often prey on animals that have no other natural predators. The starfish help other life forms flourish, as without them, animals that are their natural prey often drive out other species.
Most are carnivorous, their diet consisting of clams, oysters, molluscs, and small fish. Some species supplement their diets with algae and whatever they can scavenge.
Don't touch them!!!
A few species have poisonous spines. While those don't kill people, they can make humans somewhat sick.
Most species aren't poisonous, however it's still not a good idea to pick them up out of the ocean. They need to stay in the water because they can't breathe outside of it. Humans can transfer bacteria that makes them horribly sick, and sometimes kills them. The starfish can also be damaged if not handled gently.
Strange Wonders Trading Cards
While looking for starfish photos, I ran across Weird and Wild Card Sets. I don't know if people play with them or just collect them, but the cards are gorgeous and cover all kinds of living things. There is an entire community dedicated to these card sets.
https://weirdnwildcreatures.fandom.com/wiki/Category:Strange_Wonders