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As the most simplistic forms of multicellular organisms, sponges differ from humans and complex animals in many ways, including their lack of internal organs and a nervous system. However, sponges are capable of sexual reproduction which involves the fertilization of eggs inside the sponge’s body. They never actually lay eggs. But the role of fertilized eggs is only part of the sponge's reproductive story since they can also reproduce asexually and because most sponges are hermaphroditic.
More than 5,000 species of sponges (known as Porifera scientifically) are known to exist. All of them attach to solid matter in order to grow and thrive. They are covered in ostia, which are pores that lead into their bodies. These pores play a critical role in their survival by allowing the entrance of food and water into their systems. Waste and carbon dioxide exit the sponge's body via the ostia, too. Sponges also have a unique feature called choanocytes or collar cells. The flagella on the collar cells move water through the sponge, while the sticky substance covering the cells catches nutrients as they float by in the water.
The majority of sponge species are both male and female. However, they do not play the same role at the same time. For example, a sponge may produce sperm during one spawning period, then switch to producing eggs for the next reproductive cycle. This ability to switch sex roles has helped sponges survive in a wide range of watery habitats.
Sponge Sexual Reproduction
Because sponges stay connected to a solid surface for their entire lives, they do not have physical contact with their mate. Instead, neighboring sponges release large amounts of sperm into the water. These sperm are carried in the water and into the ostia along with the nutrients and oxygen the sponge needs for survival. Once the sperm attaches to the collar cells, those cells change into archaeocytes, which transport the sperm to the eggs for fertilization. The fertilized eggs become larvae, which are eventually released by the female into the water. The larvae eventually find their own surfaces where they begin to grow into mature sponges.
Sponge Asexual Reproduction
Sponges can also reproduce asexually, meaning they can essentially make a clone of themselves. Some species will produce gemmules (protected packets of cells) that are released into the water. These gemmules can survive even if they are still inside the parent sponge when it dies. Additionally, if a piece of a sponge breaks off, that piece can become a separate sponge and the parent sponge can repair itself.
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