The poriferous they are the simplest multicellular animals and belong to the Phylum Porifera, commonly known as sponges. These animals are totally aquatic, approximately 15,000 species of sponges inhabit the seas and only about 150 are found in fresh waters..
Sponges are extremely variable in size: they can measure from a few millimeters to more than two meters in diameter. They are very colorful organisms, since they have multiple pigments in the cells of the dermis..
Regarding their diet, they are capable of taking food particles suspended in water, since they are sessile organisms and they are not able to actively seek their food. However, there is a family of carnivorous sponges that breaks the filter feeding pattern..
Sponge skeletons can be rigid and / or fibrous. The fibrous parts of the skeleton are made up of collagen fibers, such as spongin, embedded in the cell matrix. In contrast, the rigid portion is composed of calcareous or silica-like structures called spicules..
Sponges play an important role in biogeochemical cycles, such as the nitrogen cycle. Likewise, they can form symbiotic associations with other organisms, from microscopic to fish, polychaetes, among others. Currently the Phylum Porifera is divided into four classes: Calcarea, Hexactinellida, Demospongiae and Homoscleromorpha.
The organisms belonging to the Phylum Porifera are characterized by being multicellular, diblastic and acellomed animals composed of different cell types.
Morphologically, a series of pores, channels and chambers are organized that allow the transit of water within the animal, and in this way they obtain food and oxygen..
Unlike other animals, sponges - in their adult state - are completely sessile and are anchored to a substrate, such as corals, rocks or other surfaces..
The shape of the sponge is quite variable, it can present radial symmetry or not show any symmetry. They can grow in a wide range of shapes, from erect to branched or lobed sponges, and generally live in colonies..
Sponges do not have real organs or tissues; therefore, the digestion of food particles occurs intracellularly and the processes of respiration and excretion by diffusion. They have a nervous system considered diffuse, although the presence of a nervous system in porifers is a controversial issue..
Sponges boast an incredible cell regeneration process. In fact, if a sponge is cut into pieces, each fragment can develop a new sponge by a process called somatic embryogenesis..
Historically sponges were classified as marine plants. However, in the mid-1765 researchers noted its undoubted animal nature.
Sponges are distributed worldwide and can inhabit a wide range of aquatic environments, from calm and shallow waters to the polar regions..
The body plan of sponges is extremely simple: an outer cellular layer called pinacoderm that separates the inner region called mesoglea or mesohilo, a gelatinous region composed of collagen. The inner surfaces are surrounded by choanocytes, cylinder-shaped cells with a flagellum..
The regions that are not lined with choanocytes, are lined with another cell type called pinacocytes.
Sponges have three types of designs that differ in the location of choanocytes, a class of flagellated cells that create a current that facilitates the flow of water and nutrients. The following types can be distinguished:
Asconoid sponges are small, primitive, simple forms punctured by incurring pores that open into a cavity called a spongocele. The spongocele opens to the outside through the osculum.
The asconoid type of sponge constitutes an inefficient primitive morphology, since the volume of water that houses the spongocele is high and its expulsion to the outside is difficult..
Syconic sponges have horizontal folds in the body wall, which is complex and thick. The water enters through the incurring channels through the dermal pores, the ostioli and to the radiated channels - covered by choanocytes - through the prosopilos, which are fine orifices.
Leuconoid sponges exhibit a greater degree of complexity thanks to the presence of folds in the flagellate canals to form chambers, which greatly increase the surface area for obtaining nutrients..
The Phylum Porifera is divided into three classes of sponges: class Calcarea, class Hexactinellida, and class Demospongiae. We will describe each class in detail below:
Porifers of the Calcarea class have needle-shaped spicules or with three or four rays, composed of calcium carbonate. Species of this class are small and rarely exceed 10 centimeters.
However, in some estuaries it has been found that the sponge Sycon ciliatum it can reach up to 50 centimeters. Similarly, the species Leucetta avocado Y Pericharax heteroraphis inhabit coral reefs in the Pacific and reach 20 centimeters.
They are usually considered shallow water species, although there is evidence that they can inhabit abyssal areas, between 4,000 and 6,000 meters deep..
All species are marine and present the three types of channel systems: asconoid, syconoid and leuconoid. About 300 species are known, some examples are: Leucosolenia complicata, Sycon gelatinosum, Grantia compress Y Clathrina.
The sponges belonging to this group are called vitreous sponges, since the spicules usually group together to form a network and are composed of silicon and have six rays (triaxonic).
All species are marine, predominate in Antarctica and inhabit deep water. The flagellate chambers are of the syconoid and leuconoid type. About 500 species are known, among these Hexactinella, Farrea, Euplectella, Aphrocallistes, among other.
They possess silica spicules that are not triaxonic, but can be monoaxonic, tetraxonic, or polyaxonic. In addition, they can present only spongy or both..
In this class are the famous "bath" sponges, belonging to the Spongiidae family, which have abundant spongy.
Most live in marine environments, although a family that lives in freshwater environments has been reported, such as Spongilia lacustris Y Ephidatia fluviatilis. They are of the leuconoid type.
In addition to bath sponges, other relevant genres belonging to this class can be mentioned, such as: Thenea, Cliona, Myenia, Poterion Y Callyspongia.
Within this class there is a very particular order, the Poecilosclerida, characterized by its peculiar carnivorous feeding habit..
Compared to their filter-filtering relatives, carnivorous sponges do not have an aquifer system (with the exception of genus Chondrocladia) with choanocytes, a diagnostic characteristic of poriferous.
Prey in this order includes small invertebrates, mostly crustaceans. There are some 119 carnivorous sponges within the Cladorhizidae family in eight genera, among these Cladorhiza, Asbestopluma Y Chondrocladia.
It is the smallest class of poriferous conformed by only 87 species belonging to the following genera: Oscarella, Pseudocorticium, Corticium, Placinolopha, Plakina, Plakinastrella Y Plakortis.
They are characterized by having flagellated pinacocytes; the skeleton is variable, may or may not have silica spicules, and have a basement membrane.
When the skeleton is present, it is composed of tetraxonic silicon spicules with four radii. Most of the species have cushion shapes and vary widely in their coloration, exhibiting shades of blue, purple, green, yellow, red, among others..
They inhabit dark or semi-dark ecosystems and can be located both in shallow water and at depths greater than 100 meters.
Previously it was considered a subclass belonging to Desmospongiae. Recently, studies based on molecular evidence have proposed the creation of this fourth class of sponges..
Sponges can experience both sexual and asexual reproduction. In the asexual, the sponge produces external buds that grow and, when they reach the appropriate size, they detach from the mother sponge and form a new, smaller individual. You can also remain as a member of the colony.
The asexual reproduction process can also occur by the formation of internal buds, called gemmules..
In an initial state, a type of cells called archeocytes clump together and are surrounded by a layer of spicules and spongins. These structures can escape from the parental body and form a new sponge.
Gemmules are produced when environmental conditions are unfavorable for the sponge and are also a way to colonize new habitats.
Gemmules can enter a dormant period during unfavorable periods (such as winter or low temperatures) and, when these end, they are reactivated and the formation of a new individual occurs; for this reason they are considered as an adaptation of sponges to survive adverse conditions.
Most sponges have male and female sex cells in the same individual. This dual condition is called "monoecious" or hermaphroditic..
Gametes (ovules and sperm) are generated from choanocytes or also from archeocytes, depending on the species. The sperm are released into the aquatic environment and enter the body of another sponge, where it enters the flagellated chamber and finds the ovum..
In most cases the parent sponge retains the zygote after fertilization and then a larva with cilia and is released. The larva is able to swim and is mobile, in contrast to the sessile adult. In other cases, the eggs and sperm are released into the water.
In some specific cases, the formation of a hollow blastula occurs, which experiences the opening of a "mouth" and the inversion of the blastula occurs; thus, the cells that were previously exposed to the blastocele face the outside.
Sponges do not have a digestive system or an excretory system. Instead, the water-transporting canal system fulfills these essential functions for the life of an organism..
Sponges feed mainly by taking particles suspended in the water that are pumped into the sponge..
The water enters through small pores located in an external cell bed. Inside the sponge, the food material is collected by the choanocytes, and thus suspension feeding is achieved..
Smaller particles can enter choanocytes through a phagocyte process. Two other cell types, pinacocytes and archeocytes, are also involved in particle uptake. On the other hand, respiration and excretion occur by simple diffusion processes..
Sponges lack nerve cells or "true neurons"; however, it has been shown that these animals can respond to external stimuli.
Sponges have contractile cells that respond to the environment through a type of slow conduction due to protoplasmic transmission..
In 2010, a group of researchers discovered that in the sponge genome Amphimedon queenslandica there are genes associated with neuronal cells similar to those found in cnidarians and in other animals.
These genes include those associated with rapid synaptic transmission, enzymes involved in the synthesis of neurotransmitters, among others..
When characterizing the cell types of the larvae of A. queenslandica, it has been possible to propose certain types of cells that are probably associated with sensory functions.
For example, photoreceptor cells that regulate phototaxis have been found in the posterior part of the larvae. In fact, the larva is able to select the substrate where the establishment of the adult will occur..
The Phylum Porifera is made up of the oldest existing metazoans on the planet. Sponges are a group that originated before the Cambrian. Probably a group of calcareous-like sponges occupied the Paleozoic seas; in the Devonian there was a rapid development of the group of vitreous sponges.
According to molecular studies, calcareous sponges belong to a separate clade from those sponges belonging to the Desmospongaie and Hexactenellida classes..
Molecular data suggest that the oldest group is Hexactinellida, while Calcarea is the closest to the Phylum of the metazoans..
With this evidence, two possibilities have been raised: calcareous sponges are the sister group of silica sponges, or calcareous sponges are more related to other metazoans than to silica sponges; in the latter case, the Phylum Porifera would be paraphyletic.