How Do Sponges Reproduce?

The surprising ways this simple aquatic animal produces offspring

Sponges may seem a bit boring, since they stay rooted to one spot on the ocean floor, but they’re actually quite amazing. Believe it or not, they are one of the oldest animals on Earth and have managed to survive for 600 million years! How? Well, they are experts at reproduction and have managed to keep their animal group alive by producing offspring. But how do sponges actually reproduce?

The sponge species known as a giant barrel sponge, Xestospongia
The sponge species known as a giant barrel sponge, Xestospongia testudinaria, Credit: Wikimedia/Albert Kok

Table of Contents

Sponges and their reproduction: what’s the big deal?

First off, let’s introduce the main character of this article—the sponge! People often think that sponges (phylum Porifera) are plants, but they’re actually animals! They are the simplest animal group on our planet since they have no nerves, no digestive or circulatory systems, and no internal organs whatsoever. Instead, sponges are just a bunch of cells with a squishy body wall made of spongin and a skeleton made of hard, spiny spicules. Sponges come in all sorts of shapes, colors, and sizes, and there are approximately 8,550 different sponge species alive today. Sponges don’t move much but have still managed to spread throughout the world in both marine and freshwater habitats.

 

Now, think back to biology class. Remember, one of the most basic laws in biology is that animals must be able to produce fertile offspring to allow their species to continue living. Sponges don’t really seem like much, do they? So what is it about their reproduction that has allowed sponges to exist for millions of years? Their secret: being able to reproduce in more than one way. This simple, squishy, stationary animal can reproduce both sexually and asexually, and in multiple different ways. This ability has given them an evolutionary advantage. Pretty cool for a “boring” animal, right?

A blue sponge species of the genus Haliclona
A blue sponge species of the genus Haliclona, Credit: Wikimedia/Peter Southwood

How do sponges reproduce sexually?

Sexual reproduction requires two parents and occurs when an egg of a female is fertilized by a sperm of a male. This produces an offspring that has a mixture of genes from both parents. It might be strange to think that sponges are able to reproduce sexually. You might be thinking: do sponges even have different sexes? The answer might surprise you.

 

Most sponges are monoecious, or hermaphrodites, which means that each individual is both sexes simultaneously! Monoecious sponges produce both eggs and sperm at the same time. The production of eggs and sperm is influenced by environmental factors such as water temperature or food availability. Some sponges may become sequential hermaphrodites, which means that they will first produce the eggs and then the sperm to avoid self-fertilization. Other sponges are dioecious, meaning that the female and male reproductive organs are in separate individuals. This boosts the genetic diversity in the species, since the genes come from two different parents.

 

Fertilization can happen in two different ways. A sponge will release its sperm into the water , which then travel until the sperm enter a sponge with eggs. After fertilization is completed, sponge larvae develop and are released into the water. The second way that fertilization can happen is when sponges release both eggs and sperm into the water in a process known as broadcast spawning. Sponges send out huge numbers of sperm and eggs, and just hope for the best that they meet and develop into larvae! Most sponge larvae float around for several days searching for a suitable place to attach themselves to. Once found, the larvae settle and begin their growth into an adult sponge. This is seen in the monoecious “glass sponge” (Hexactinella). This sponge is found in deep sea habitats and releases live larvae after producing both sperm and eggs throughout the year.

A species of glass sponge known as the Venus flower basket glass sponge
A species of glass sponge known as the Venus flower basket glass sponge, Credit: Wikimedia/NOAA Okeanos Explorer Program, Gulf of Mexico 2012 Expedition

How do sponges reproduce asexually?

Asexual reproduction involves just one individual, and the offspring that are produced are exact copies of the parent. In other words, asexual reproduction creates clones! Sponges have developed not one, not two, but three different ways to reproduce asexually. These ways are fragmentation, budding, and gemmulation.

 

Fragmentation is the most common way sponges reproduce. It actually happens accidentally! Fragmentation is when a piece of a sponge breaks off, settles onto the seafloor, and then grows into a new sponge. How cool is that!? Imagine if new humans were created every time we cut our fingernails! Strong waves from storms or messy sponge-eaters such as fish or turtles can cause fragmentation. The broken-off pieces can travel over very long distances, which is probably a big reason why sponges are found all over the world! Fragmentation is seen in the yellow tube sponge—one of the most primitive animal groups on our planet. Yellow tube sponges are attached to reefs but still manage to produce offspring through this method of reproduction.

Small fish with a yellow tube sponge
Small fish with a yellow tube sponge, Credit: Wikimedia/Paolo Gamba

Budding occurs when buds grow out from the body of an adult sponge, a bit like buds on a tree. But instead of just opening to reveal leaves or flowers, these buds develop into new sponges. These new sponges can either detach and become independent or they can cling on to their parent. Budding is very common in the sponge group Tethya. Buds emerge from the adult sponge throughout the year, and take between 7–15 days to detach and become new individuals.

A Tethya species called a “puffball sponge
A Tethya species called a “puffball sponge.” Credit: Wikimedia/Ed Bierman

Gemmulation is less common and is mainly utilized by sponges that live in freshwater habitats. An adult sponge produces gemmules, which are like small packets of cells that each have nutrient storage and a protective outer layer. This outer layer acts like armor and helps the gemmules survive in harsh environmental conditions. Decreases in water temperature or periods of extreme dryness trigger gemmule production. Scientists believe that sponges produce gemmules to make sure that their species will survive the harsh conditions. Adult sponges may die and decay during the unfavorable conditions, but they will leave their gemmules behind. The gemmules will then attach themselves to the seabed and start their development into new sponge individuals during more favorable conditions.

Sponge gemmules
Sponge gemmules, Credit: Wikimedia/The Other 95%

Conclusion

There is clearly more to sponges than meets the eye. They are survivors and have come up with lots of different ways to reproduce and continue their existence here on Earth. So, don’t judge a book by its cover! Next time you see a sponge, think about where it might have come from and the wacky ways it might produce its own offspring.

Glossary

Broadcast spawning: The release of huge quantities of eggs and sperm into the water

 

Larvae: Immature offspring (like babies!) that go through developmental processes before becoming adults

 

Spongin: A flexible material that makes up the body wall of sponges

 

Spicules: Small, hard elements that support sponges’ bodies

 

Fertile offspring: Offspring of parents that can reproduce in turn

 

Fertilize: When a sperm fuses with an egg which starts the development of a new individual

 

Evolutionary advantage: Characteristics that allows an organism to survive and reproduce better than others

 

Primitive: Very basic, at an early stage of development

 

Self-fertilization: Fertilization between sex cells that came from just one individual

 

Genetic diversity: A range of different traits in one species. It allows species to adapt better to changing environments

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