There are about 1,500 species of starfish in the world, some of them living at depths of 20,000 feet below the surface of the ocean. They have five arms (usually, but sometimes they have six or seven) lined with tubes, which help them attach to rocks and move around. Amazingly, starfish can regenerate lost limbs and can willingly shed an arm to escape a predator, much like the gecko sheds its tail.
How starfish reproduce is thoroughly fascinating and also confusing in the sheer variety of ways new starfish can be either born or formed. That’s right, not all starfish are born, some are formed in a different way. But we’ll get to that.
Of course, some species of starfish have distinct males and females who reproduce in the way we’re used to thinking about reproduction. We can’t tell which starfish is male and which is female by looking at their external appearance, but they’re separate and the females lay the eggs from which the baby starfish hatch.
But there are other species of starfish that want nothing to do with that. Some produce both the egg and sperm, and sometimes in the same organ, and so they self-fertilize. Those species could go on even if two starfish never meet one another. This ability is called being a simultaneous hermaphrodite, and it’s a trait shared with a few other animals, such as sea slugs and earthworms.
Some starfish are sequential hermaphrodites. These starfish species begin life as a male uniformly. There are no female baby starfish of these species. As they get older, though, every one of them turns into a female starfish. And this phenomenon actually isn’t unheard of in other species. Parrotfish almost always change from female to male throughout their lifetime, and the largest clownfish in a group of clownfish will turn into a female as they get older.
But now we’ve come to the point where I explain what I meant when I said some starfish are formed. These species of starfish also begin life as male, without exception. And then, when they get older, they also turn into females without exception. However, these species of starfish reproduce neither sexually nor in a self-fertilizing way. An adult female will, when ready to reproduce, rip herself in two and the two halves of herself will regenerate into two male starfish, who in turn will grow up to be females and tear themselves in two to make two new baby starfish.
That’s right, the reproduction is entirely asexual in these species and seemingly bizarre, but that’s just life if you’re one of those starfish.
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