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While most animals are born as either male or female, the line separating genders gets a little blurry when it comes to starfish. These slow-moving creatures do have distinct genders and can mate with other individuals, but they can also reproduce asexually and some species can even switch their sex.
What's the Difference?
It can be difficult to distinguish male starfish from females based on appearance alone. In some species, like the common sea star (Archaster typicus), males are noticeably smaller than females. In others, there are virtually no visible traits that set them apart. There are some physiological differences, but you may need a microscope to see them. Males produce sperm and females create eggs, so the gonad structures of the two genders are distinct. The reproductive organs are often small or housed on the inside of the animal's body, so it's difficult to tell them apart even if you flip the starfish over to look at its underside.
Gender Isn't Always Forever
There are hundreds of different species of starfish crawling around the ocean floor, so there's plenty of variety among them. Many species are born as either male or female and retain their assigned gender until death. The cushion star (Asterina gibbosa) does things a little differently though. All of their babies are born as males. Once the males mature to a certain point, they stop producing sperm and begin creating eggs. This type of age-related gender change is called sequential hermaphroditism.
It Takes Two
Even though starfish reproduce sexually, the males and females don't usually get physical during the event. Animals of both genders release gametes into the water when they detect a potential mate nearby. It's common for several males to breed with one or more females at a time, as the sperm and eggs float freely in the nearby water. There are a few exceptions to this rule. In some species, males crawl on top of the female in a basic mating ritual that prompts her to release eggs. While females often leave their fertilized eggs to fend for themselves, the mothers of some species carry the fertilized eggs on their body and guard the larvae as they develop.
Divide and Conquer
Luckily for the lone wolf starfish out there, mating isn't the only reproductive trick that these hardy animals have up their sleeve. When starfish are dismembered by a predator, strong water currents or other physical damage, they can regenerate their lost tissue. As the main starfish body regenerates its lost limb, the missing leg can also grow a whole new body and take on a life of its own. Tiny pieces usually don't possess the biological resources to do this. Dividing isn't the primary mode of reproduction for starfish, but it does allow individuals to procreate in the absence of the opposite gender.
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