Hexactinellids are sessile sponges that form the class Hexactinellida, of the phylum Porífera. They are characterized by having a symmetrical body, with a skeleton composed of triaxonic spicules. These are generally fused, giving a characteristic rigidity to said clade.
Another relevant aspect is that the cytoplasm forms a soft tissue, where there is no barrier that divides it and the nuclei are scattered.
Hexactinellids lack speciesized nerve structures. However, they can transmit electrical impulses through your body, through soft tissue. This allows them to react quickly to any external stimulus.
Regarding food, glass sponges, as species of this class are known, filter the water that enters the body. Thus, they consume detritus material and bacteria, among others, that are found in the environment that surrounds them.
The food particles are absorbed as the water moves through the channels that make up the body.
Taxonomy and classification
Distribution and habitat
Examples of species
Bird’s Nest Sponge (Pheronema carpenteri)
Cloud Sponge (Aphrocallistes vastus)
The basket of venus flowers (Euplectella aspergillum)
Taxonomy and classification
– Animal Kingdom.
– Subkingdom Radiata.
– Filum Porífera.
Orders: Aulocalycoida, Lychniscosida, Hexactinosida, Lyssacinosida.
Molecular phylogenetic investigations support the monophyly of the Hexactinellida class and of the two subclasses into which it is divided. As well as in the vast majority of the families and genders that make it up.
The two subclasses into which this clade is divided are Amphidiscophora and Hexasterophora.
The body of these sponges generally remains anchored to somewhat soft substrates, in deep water. It does this through a basal plume or through a grouping of spicules. These structures are differentiated megascleras and are not fused. In addition, they have amphidisk microsclerae.
This subclass is divided into a single order, Amphidiscosida, and three families: Hyalonematidae, Pheronematidae, and Monorhaphididae.
Members of this group possess hexaster microsclera. In addition, they show a great diversity of forms, in terms of spicules and skeletons. Taxonomically it is made up of four orders: Lyssacinosida, Aulocalycoida, Hexactinosida and Lychniscosida,
Lyssacinosida has three families, where most of the representatives are characterized because their spicules are not fused. The remaining orders possess fused skeletons.
The body is recognized by its relative radial symmetry, and can be cylindrical or in the shape of a glass, tube or cup. In the center they have a cavernous cavity, which in most species has an exit to the outside, through a kind of sieve that forms the skeleton.
The height could be between 10 and 30 centimeters, with a coloration that ranges from whitish tones to orange.
All glass sponges are upright and have specialized structures at their bases to quickly attach to the ocean floor.
Contrary to the rest of the sponges, the cytoplasm is not divided into individual cells, with a nucleus each. Rather, it forms a kind of soft tissue, known as the trabecular reticulum.
In this, the multinucleated cytoplasm moves freely, because they do not have any membrane as a barrier. This network is attached to the skeleton by fine strands and extends from the dermal layer to the ear layer, the innermost of the two.
Between the syncytial and cellular components there is a thin layer of collagen, called mesolium. The researchers suggest that, because they are so thin, the cells are unable to migrate inwards, as happens in the rest of the sponges.
However, the exchange of nutrients could occur in the microtubule networks that exist within the polynucleated tissue.
Hexactinellids have specialized cells, which can be linked to each other and to the trabecular reticulum, by a multilaminar structure of the cell membrane. However, it is not an extension of this.
In particular, the cells of the epidermis that characterize the other sponges are absent. Instead they have a syncytial network of amoebocytes, which is traversed by the spicules.
In the inner part of the syncytia there are cells known as collar bodies. These have a similar structure to choanocytes, but without nuclei. In addition, they have flagella, which help water circulate through the sponge.
Likewise, they have functional units comparable to archaeocytes, present in other sponges, but unlike these, they have very limited mobility. Because Hexactinellids do not have myocytes, they do not have the ability to contract.
Glass sponges have a skeleton formed by siliceous spicules, usually composed of 3 perpendicular rays, which originate six points.
The spicules are generally fused. This gives the hexactinellids a stiffness rare in other sponge clades. Species of this class often have finger-like protrusions on the body walls. In each projection they have a kiss.
However, in each subclass there are species whose spicules are attached only by living tissue.
The species have peculiarities regarding the skeleton. For example, the Monorhaphis chuni has a long spicule, which allows it to anchor its body to the seabed.
Distribution and habitat
Hexactinellids are widely distributed in marine waters worldwide, being very common in the North Pacific and Antarctica. They generally live between 200 and 6000 meters deep.
However, they could live in shallower areas, such as on the coasts of British Columbia, New Zealand or the Mediterranean underwater caves. On the Canadian coast, they usually form reefs, in waters of 180 to 250 meters. These can rise up to 18 meters above the sea floor and extend up to 7 kilometers long.
Likewise, glass sponges are currently abundant at various levels in polar waters. Thus, they are part of the benthic life of the cold Antarctic waters. There they can be important elements in the biodiversity of the slopes and continental shelf of Antarctica.
One of the characteristics of the habitat is the water temperature, which can range between 2 and 11 ° C. In addition, it is important that there is a high level of dissolved silica and a low intensity of sunlight.
Although some species require a firm substrate to fixate, others grow on dead sponge skeletons or on soft substrates.
Examples of species
Bird’s Nest Sponge ( Pheronema carpenteri )
This species belongs to the order Anfidiscosida. Its size could reach 25 centimeters high and 20 wide. The body walls are cavernous, tapering at the top into a serrated opening.
As for the silica spines, they are sharp and thin. They are projected on the lower part of the body, so they serve as an anchor in the sea mud. They are distributed in the northeastern Atlantic, spanning from Iceland to the northern region of Africa, including the Mediterranean Sea.
Cloud Sponge ( Aphrocallistes vastus )
Its habitat is located north of the Pacific Ocean, including Japan, the Aleutian Islands, and Siberia. He also lives on the west coast of North America. In these regions you can build slow-growing reefs.
The species is part of the Aphrocallistidae family and is characterized by having a cone shape, with external projections, similar to the fingers. Its body can measure up to 1 meter and is made up of a siliceous skeleton, which gives the sponge rigidity.
The basket of venus flowers ( Euplectella aspergillum )
This representative of the order Lyssacinosida has a tubular body, with thin walls 50 millimeters wide and 240 millimeters long. The spicules fuse to form a rigid network.
To attach themselves to the ocean floor, they use fine vitreous strands, 5 to 20 centimeters long. They are located in the Pacific Ocean, from the Philippines to the eastern area of Africa. In these regions they tend to live in muddy and soft bottoms.
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