Reproduction is key to life on earth.
Reproduction ensures genes are passed on to the next generation. This prevents entire species become extinct.
Sponges are no exception. They reproduce to ensure the continuity of the species.
However, sponge reproduction is unlike typical animal reproduction.
There are two ways in which sponges reproduce: sexual reproduction and asexual reproduction.
How Do Sponges Reproduce Sexually?
Sexual reproduction is perhaps one of the most widely recognised forms of reproduction.
But what really does it mean?
Sexual reproduction is a complex process whereby two gametes (sex cells) fuse to form a zygote. A zygote is a eukaryotic cell that is formed by the fertilisation of two gametes. This newly fertilised cell contains the genetic material from both gametes (organismalbio.edu).
So, that was a crash course into sexual reproduction. But where do sponges fit in to this?
In mobile species, fertilisation typically occurs through sexual intercourse. Adult sponges, however, are sessile. This means they cannot move.
Now, how can a sponge expect to get any action of they’re fixed to one spot?
Despite lacking a brain, sponges are clever, and have found an ingenious to reproduce sexually, whilst avoiding internal fertilisation.
A sponge will release millions of sperm cells into the water current. Here, sperm will drift until they make contact with another sponge of the same species. The receptor sponge releases an influx of choanocytes, or collar cells; a versatile, flagellated cell with a multitude of uses (zoology.edu).
Typically, collar cells aid in water circulation and nutrient absorption. During reproduction, however, collar cells transform into carrier cells. As the name suggests, the carrier cells simply carry the male sperm to the female sex cell: the egg.
Speaking of versatility, these same collar cells form a chamber using their tail-like flagella in which the egg is enclosed.
Following fertilisation of the egg, which occurs within the mesenchyme, a sponge larva will develop.
Now things get interesting.
Up until now, sponges have been referred to as sessile and immobile. And whilst this is true of adult sponges, the larval form breaks boundaries. For a period of time, ranging from hours to days, millions of sponge larval flood the water column, creating a haze of ciliated zooplankton (berkeley.edu).
Sponges are viviparous. This means, for the most part, they give birth to live young. Sponge larva are released through the excurrent (outgoing) channels and through the osculum; the main opening of the sponge whereby water is expelled (queenslandmuseum.gov).
Eventually, the larva settles on a suitable substrate, such as a rocky outcrop, and the magic begins.
Following attachment, sponge larva metamorphose into young sponges. This process involves drastic morphological and physiological changes, especially on a cellular level. Depending on sponge species, cells transform to form entire structures, such as the mesohyl layer.
Collar cells regain their function of creating water currents and capturing food and the cycles begins again.
Sexual reproduction of sponges is determined by the temperature of the surrounding water and other external abiotic factors. Conditions need to be optimal for mass spawning to occur, oftentimes in the spring of temperate regions (britannica.com).
However, in regions where seasons are undefined, spawning and fertilisation can occur year round.
When conditions are not favourable, a sponge may use asexual reproduction as a form of creating offspring.
How Do Sponges Reproduce Asexually?
Unlike sexual reproduction, asexual reproduction does not involve the fusion of gametes. Rather, sponges can self-fertilise.
Sponges are not alone when it comes to asexual reproduction, with many species of bacteria, plants and fungi adopting this method to pass on their genetic material. Even some animals, such as lizards, can achieve a certain type of asexual reproduction.
In sponges, asexual reproduction occurs in three distinct ways:
- Gemmulation
- Fragmentation
- Budding
Let’s take a closer look.
Gemmulation
One of the most common means of asexual reproduction within freshwater sponges, as well as some marine species, is the formation of gemmules.
The process begins with the aggregation of archaeocyte cells. These are specialised cells, not dissimilar to stem cells in mammals. These cells contain high cytoplasmic content, allowing them to morph according to their function.
When archaeocyte cells become saturated with food reserves, granules become isolated at the surface of the sponge. A protective covering encases the granules, now called gemmules, and are ejected from the mature sponge.
Free-floating gemmules are adept at attaching to benthic substrate and generating a new sponge.
Some species use this process as a normal method of reproduction. Most species of sponge, however, will resort to gemmulation in times of unfavourable environmental conditions, such as extreme temperatures. This is because gemmules are environmentally-resistant structures (zoology.edu).
Fragmentation
A biologically simple means of asexual reproduction, fragmentation is where an organism splits into, you guessed it, fragments.
If a fragment of the parent sponge lands on suitable substratum, it is capable of growing independently into a new organism. Fragments are genetically identical to their parents.
But how is this possible?
Sponges are incredibly simple multicellular animals. They lack organs and tissues, and as such, their cells have no specialised functions (nationallibraryofmedicine.gov) when compared to complex multicellular animals. This means sponges can break into several pieces along lines of weakness, at no detriment to the parent sponge.
Budding
Similarly to the process of gemmulation, budding involves the creation of a new organism via outgrowths, or buds. This process occurs due to cell division at a particular site.
The newly created offspring will be an exact replica, or clone, of it’s parent.
The bud appears as a protruding appendage, or outgrowth, from the parent. After time, the bud will either detach from the parent, or remain as an outgrowth, and form a new colony.
The key difference between budding and gemmulation is that a bud develops on the external surface of a parent. Gemmulation, on the other hand, involves the formation of gemmules from within the internal body of a parent (ncbi.gov).
Negatives of Asexual Reproduction
As we’ve seen, asexual reproduction eliminates the need of organisms finding mates. This conserves vital energy reserves in individuals.
However, when species use asexual reproduction as a form of reproduction, genetic diversity is not produced in the offspring.
When individuals of the same species all share the same gene pool, there is a higher chance that entire populations can become extinct. When a group of individuals, sharing the same genetic makeup, are faced with harsh environmental conditions (climate extremes, diseases or infections), there is an inability to adapt and overcome (ncbi.gov).
Conclusion
Sponges are capable of reproducing both sexually, using gametes, and asexually, using gemmulation and fragmentation.
Sexual reproduction increases genetic diversity within populations, but is energetically costly to individuals.
Asexual reproduction eliminates the need to finding a mate and ensures offspring are created. However, these offspring have limited genetic diversity. Consequently, they are at risk of extinction if environmental factors are not favourable.
