Marine Invertebrates

Sea sponge

Porifers (sea sponges) are invertebrate animals known as sponges. Its main characteristic is that its body is completely full of pores through which water circulates, which is why they are called «poriferous».
Until 1765 it was thought that sponges were plants, until it was discovered that they possess internal currents and that they perform intracellular digestion.

Another fact that had gone unnoticed until recently is that
sponges can move, but they move so slowly (about 4 mm a day) that the fact had gone completely unnoticed.


There are about 9,000 different species of sponges around the world, of which only 150 species live in fresh water, the rest of the sponges live in the sea.

Its most important characteristic, which gives its name to this group of animals (poriferous), is that their bodies are covered by a succession of pores and channels, through which water passes, and that they serve to obtain their food and oxygen needed to live.

Morphology. Characteristics of sea sponges

Perhaps morphologically the most outstanding thing about sponges is that they do not have body symmetry. But they don’t need it either.

Visually they
are sack-shaped and, although the shape can vary between different sea sponges, they all have a similar body structure.

Some species are capable of adapting their shape to the environment, changing their appearance depending on the inclination of the substrate, or if they collide with a rock.

are made up of two layers of cells, separated from each other by an interior space.

The outer layer of the sea sponge is called pinacoderm, and it is made up of cells very similar to epithelial (skin) cells, called pinacocytes.

The inner layer is called the coanoderm, and it is made up of cells called choanocytes.

These cells are provided with a flagellum and microvilli on their entire surface, to promote the circulation of water and in turn, take advantage of its nutrients.

They have an upper opening called the osculum or atrial cavity,  responsible for the circulation of the porifers. Through the atrial cavity the sponge pumps water throughout the body, making it pass through the walls filled with pores of different sizes.

They have no mouth or digestive system.
It obtains the food it needs by filtering the water, thanks to cells that only sponges have, the choanocytes.

They also don’t have nerve cells.
Interestingly, they
are the only animalsthey lack a nervous system.

Although they are animals, porifers do not have differentiated tissues or organs.
Instead they have cells called totipotent, which mutate to adapt at all times to the cellular needs of the animal. Thus, if the animal loses mass for any reason, it regenerates again.

They have no natural predators.
Its spicule skeleton and its toxicity keep the rest of the marine animals at bay.

The spicules are calcareous or siliceous skeletal units, which are part of the skeleton of porifers.
Calcareous spicules . They are typical of calcareous sponges, they are formed by crystallized calcium carbonate.
Siliceous spicules. They originate from crystalline accumulations of hydrated silica. They are only present in the skeleton of sponges, no other animal uses silica in their skeleton.

Types of sponges

Sponges are classified into four different classes:
Calcareous sponges

They are characterized by having
calcareous spicules and lacking spongin fibers.

They have
three different types of morphology: ascon, sicon and leucon , they are small in size and usually pale in color.

90% of sponges belong to this type of sponge, and
its main characteristic is that its skeleton is made up of skeletal units of silica , sponge fibers or a mixture of the two options.

Its coloration is very striking: reddish, green or bluish tones.

Its shape is varied, which allows it to adapt very well to the environment, colonizing all types of surfaces and cracks.

Vitreous sponges (Hexactinellida)

They are
poriferous with a skeleton formed by six- ray siliceous spicules , known as hexactins, which gives the group its name. In addition to the siliceous spicules, they can also have spongy fibers.

They are usually glass-shaped and have a preference for deep waters, between 200m and 2,000m.


This is a group of sponges that is now extinct.

Distribution and habitat

Poriferous animals are animals with a brutal adaptability, they are capable of living in highly polluted waters, under conditions in which others could not survive.

However, what nature has not been able to do, human beings are achieving with climate change, which kills thousands of sponges every year.

Even so,
we can find sea sponges throughout all the oceans and seas of the world.

The most prominent seas, for the amount of sponges that can be found, are the Mediterranean in its easternmost area, in the Gulf of Mexico, the Caribbean and in the seas surrounding Japan.

Sponges can live at many levels of depth, although they prefer environments where sunlight reaches weakly.

Diet: How do sponges feed?

Sponges get the food they need by filtering the water.

Through filtration they obtain plankton, bacteria and small organic particles, which are dissolved in sea water.

can establish relationships with other marine animals, such as fish and other invertebrates , as well as bacteria and other single-celled organisms, providing access to organic matter that also serves as food.

Reproduction of sponges

Sea sponges can reproduce in two different ways: sexual and asexual.

Sexual reproduction

Sexual reproduction is perhaps the most complicated. Sponges lack sexual organs, as well as any other specialized organs, so fertilization actually occurs externally.

The sea sponge are mostly hermaphrodites, although it differs from one species to another.

For sexual reproduction,
cross-fertilization is needed.

Both sperm and ovules develop from choanocytes, which are expelled by sponges and it is outside, where the union between cells occurs.

Once the ovum is fertilized,
the sponges go through different larval stages, until they develop into adult individuals.

Asexual reproduction

Remember that before I mentioned the cells called totipotent, which take on all kinds of functions …
Sea sponges produce buds. They are bumps that grow until they detach from the original sponge, and thanks to their totipotent cells, these new individuals are completely autonomous.

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