How Do Sponges Reproduce? – (Reproduction In Sponges)

Sponges are classified under Phylum Porifera of the Animal Kingdom. They have simple multicellular body organization without any tissues or organs.

Majority of the sponges are marine in nature and, only a few are freshwater species.

They show a very fast rate of reproduction and regeneration into new individuals.

Sponges can reproduce both asexually and sexually. In asexual reproduction, they reproduce by forming buds, gemmules, reduction bodies, by regenerating themselves, and by fission. In sexual reproduction, they reproduce by the cross-fertilization of male sperms and female ova.

All sponges show both asexual and sexual reproduction during their lifetime but, both type of reproduction cannot take place at the same time.

All sponges are hermaphrodite (having both male and female sex organs in one body) but, cross-fertilization is the only rule of sexual reproduction.

ASEXUAL REPRODUCTION

By Regeneration

Regeneration is a type of asexual reproduction in which the sponges are capable of regrowing their complete body parts. Regeneration occurs via. mitosis.

This power of regeneration also helps the sponges to repair the damage caused in various harsh environment.

All sponges possess a remarkable ability to regenerate their lost parts and grow into new adults. In fact, they show the high capability to regenerate themselves.

If an individual is cut into pieces, each piece, however small but carrying the amoebocytes and choanocytes, will grow into a complete and full-size organism, if the conditions are favourable.

It is also to be noted that if all the small cut particles of a particular sponge species are kept in a nutritious aqueous medium, they all can come and accumulate in one place thus forming a new sponge body over a period of time. That’s also a way to unite the cells and then regenerate in sponges.

The elements like calcium and magnesium along with some other unknown aggregation factors are actually needed for proper regeneration to take place.

By Budding

In this type of asexual reproduction, the sponge first develops into a new partially developed individual while being attached to the parent sponge’s body. This newly formed sponge later gets detached from the parent and becomes a new adult.

In budding, numerous amoebocytes (also called archaeocytes) collect at the surface of the body of the sponge. This results in a small outgrowth on the pinacoderm (dermal epithelium).

These pinacoderms that receive the amoebocytes shows a bulged out appearance from the body surface of the sponge. This bulged out appearance is actually the bud.

The bud that is formed usually grows outward of the parent’s body to produce a small individual, which either remains attached with the parent individual or gets detached and attached to a nearby rock substratum.

This newly formed sponge later shows branching to grow into an independent colony.

By Fission

In this type of asexual reproduction, the parent sponge reproduces and multiplies itself by fission that is by throwing off the body parts which later grows into a new individual.

This process starts when the sponge gets hypertrophied that is when they excessively become enlarged as a result of increased size in the body parts.

With the interval of time, that hypertrophied body part starts slowly developing a line of fission (weakness). Later along this weak line, splitting occurs and the body part is separated and thrown off from the main parental body.

This separated body part later develops into a new sponge. They can also follow multiples fission techniques at the same time.

In multiple fission, sponges can break into several pieces along several lines of weakness and these later develop into various separate individuals.

By Formation of Reduction Bodies

This type of reproduction occurs only during the harsh and unfavourable environmental conditions when the sponge gets in a very poor health state.

During such harsh conditions, the ailing sponge usually disintegrates itself and gets collapsed into small rounded balls, called reduction bodies.

Each of these reduction bodies consists of an internal mass of amoebocytes, covered externally by a pinacoderm and spicules. These bodies just remain freely floating in the ocean bed or stay near the coral reefs.

The formation of the reduction body entails the withdrawal of the sponge from its skeleton (spicules) to form a spherical structure that is devoid of a canal system but, composed of an inner cell mass of amoebocytes, which is surrounded by a simple squamous pinacocyte epithelium.

If adverse conditions persist, the reduction body will undergo progressive degeneration leading to eventual death.

And, if the favourable conditions return, these reduction bodies can grow into complete new sponges.

By Formation of Gemmules

It is a type of asexual reproduction that follows a bit complex mechanism than that which was followed during the formation of reduction bodies. Here inspite of the reduction bodies, gemmules are formed.

When the harsh conditions are reached the sponges start desiccating themselves. They due to food scarcity and other unfavourable conditions start to die and disintegrate themselves, thus leaving behind a large number of gemmules.

The internal buds of sponges that are are involved in asexual reproduction are called gemmules. It is an asexually reproduced mass of cells, that is capable of developing into a new adult sponge.

A full-grown gemmule is like a small-hard ball that consists of an internal mass of amoebocytes which remain surrounded by resistant chitinous covering.

The resistant chitinous covering consists of various spicules and is actually made up of an outer and inner membrane which is usually pierced at one end, by an outlet called micropyle.

When in favourable conditions that is during the time of abundance of water, the gemmules (internal buds) begin to hatch and the living content that is inside comes out through the micropylar end. These contents develop into all new sponges by gathering themselves together.

SEXUAL REPRODUCTION

Release of Sperms

Most sponges are monoecious (hermaphrodite), only a few are dioecious (unisexual) in nature. If monoecious, then the same organism can release both sperm and ovum but not at the same time.

Although most sponges are hermaphrodite but cross-fertilization is the rule because ova and sperms are produced at different times.

Gametes (sperm and ovum) contain half of the genetic information of each parent (half of their chromosomes), and their fusion during fertilization provides a complete set of chromosomes to the offspring.

The sperms and ova are derived from the amoebocytes or choanocytes depending on the species by gametogenesis.

When the sponges produce their sperms, its concentration in the body increases and thus get released into the aquatic environment through the oscula.

The sperms than flows freely in the surrounding water with the help of its tail. Its main motive is to find an egg (ovum) for fertilization.

The sperms are then delivered to the eggs by the amoebocytes.

Fertilization

When the sperms are released to the outside of the sponge body, it freely floats in the surrounding water.

That sperm through out-flowing water make their way into another sponge along with the ingressing water current via. the canal system.

Thus the sperm floating in the water reach the female sponge that has produced eggs at the time of reproduction

Choanocyte cells by their pumping action trap the sperm cells as they flow through the interior of the organism. It is done in the same way how food is obtained by the choanocytes.

The eggs remain inside the mesohyl chamber of the sponge. The mesohyl is actually the gelatinous matrix within a sponge that fills the space between the external pinacoderm and the internal choanoderm.

There the sperm nucleus then fuses with the nucleus of ovum, ensuring internal fertilization. Thus, fertilization (cross-fertilization) takes place in the mesohyl.

Larval Development

A few species release the fertilized eggs into the water soon after the fertilization process has occurred. But, most species retain the eggs until they hatch into larva.

So, the fertilized eggs are stored within the mesohyl, and that is where fertilization takes place to form a zygote.

Once the zygote develops into a larva, it is usually released back into the water.

In sponges larvae are of many types:

• Amphiblastula: It is a hollow, oval type larval stage that is seen in many calcareous sponges like Scypha. Anterior one-half of amphiblastula bears flagella, where the posterior half does not.
• Parenchymula: It is a solid, oval, or flattened larval stage. It is characteristic of only a few calcareous sponges like the deep-water Monaxonellid Sponge. Its entire outer surface bears flagella.
• Other larval types like Stomoblastula in syconoid sponges, Coeloblastula in asconoid and leuconoid sponges, Rhagon larva in Spongilla species are also seen.

Development Into Adult

Next, after the full development of the larvae, they become motile and free-swimming.

They soon with the help of their external flagella escape the mesohyl chamber of the mother sponge. They then swim for a few hours to many days in the water.

As the larvae continue to develop in the water, they become more similar to adult sponges and finally lose their ability to locomote from one place to another.

They do finally settle down, become attached to some solid surface, perform metamorphosis, and grow into an adult sponge.

During the metamorphosis of the larva into the adult stage, gastrulation takes place by delamination of the amoebocytes which are located on one end of the blastocoel.

The amoebocytes gradually fill the blastocoel completely and the gastrula becomes solid.

This solid gastrula having a larva swims about for some time and then settles on substratum to form Olynthus stage.

Soon, the inner archaeocytes migrate to form pinacocytes on the surface and the outer flagellated cells migrate towards inside to form choanocytes lining the spongocoel.