Hexactinellid sponges are sponges with a skeleton made of four- and/or six-pointed siliceousspicules, often referred to as glass sponges. They are usually classified along with other sponges in the phylumPorifera, but some researchers consider them sufficiently distinct to deserve their own phylum, Symplasma. Some experts believe glass sponges are the longest-lived animals on earth;[2] these scientists tentatively estimate a maximum age of up to 15,000 years.
Glass sponges are relatively uncommon and are mostly found at depths from 450 to 900 metres (1,480 to 2,950ft) below sea level. Although the species Oopsacas minuta has been found in shallow water, others have been found much deeper. They are found in all oceans of the world, although they are particularly common in Antarctic and Northern Pacific waters.[3]
They are more-or-less cup-shaped animals, ranging from 10 to 30 centimetres (3.9 to 11.8in) in height, with sturdy skeletons made of glass-like silicaspicules, fused to form a lattice.[4][5] In some glass sponges such as members of the genus Euplectela, these structures are aided by a protein called glassin. It helps accelerate the production of silicas from the silicic acid absorbed from the surrounding seawater.[6] The body is relatively symmetrical, with a large central cavity that, in many species, opens to the outside through a sieve formed from the skeleton. Some species of glass sponges are capable of fusing together to create reefs or bioherms. They are generally pale in colour, ranging from white to orange.[3]
Much of the body is composed of syncitial tissue, extensive regions of multinucleatecytoplasm. The epidermal cells characteristic of other sponges are absent, being replaced by a syncitial net of amoebocytes, through which the spicules penetrate. Unlike other sponges, they do not possess the ability to contract.[3]
Their body comprises three parts: the inner and outer peripheral trabecular networks, and the choanosome, which is used for feeding purposes. The choanosome acts as the mouth for the sponge while the inner and outer canals that meet at the choanosome are passages for the food, creating a consumption path for the sponge.[7]
All hexactinellids have the potential to grow to different sizes, but the average maximum growth is estimated to be around 32 centimeters long. Some grow past that length and continue to extend their length up to 1 meter long. The estimated life expectancy for hexactinellids that grow around 1 meter is approximately 200 years (Plyes).
Glass sponges possess a unique system for rapidly conducting electrical impulses across their bodies, making it possible for them to respond quickly to external stimuli.[8] In the case Rhabdocalyptus dawsoni, the sponge uses electrical neuron signaling to detect outside stimuli, such as sediments, and then send a signal through its body system to alert the organism to no longer be actively feeding. Another glass sponge species in the same experiment of R. dawsoni, showed that the electrical conduction system for this class of sponges all has its own threshold of how much outside stimuli, sediments, etc, it can endure before it will stop its feeding process.[9] Species like "Venus' flower basket" have a tuft of fibers that extends outward like an inverted crown at the base of their skeleton. These fibers are 50 to 175 millimetres (2.0 to 6.9in) long and about the thickness of a human hair.
Glass sponges are different from other sponges in various other ways. For example, most of the cytoplasm is not divided into separate cells by membranes but forms a syncytium or continuous mass of cytoplasm with many nuclei (e.g., Reiswig and Mackie, 1983). The remaining cells are connected to the syncytium of cytoplasmic bridges. This physiology is what allows for a greater flow of ions and electrical signals to move throughout the organism, with around 75% of the sponge tissue being fused in this way.[6] Another way is their role in the nutrient cycles of deep-sea environments. One species for example, Vazella pourtalesii, has an abundance of symbiotic microbes which aid in the nitrification and denitrification of the communities in which they are present. These interactions help the sponges survive in the low-oxygen conditions of the depths.[10]
These creatures are long-lived, but the exact age is hard to measure; one study based on modelling gave an estimated age of a specimen of Scolymastra joubini as 23,000 years (with a range from 13,000 to 40,000 years). However, due to changes in sea levels since the Last Glacial Maximum, its maximum age is thought to be no more than 15,000 years,[11] hence its listing of c. 15,000 years in the AnAge Database.[12] The shallow-water occurrence of hexactinellids is rare worldwide. In the Antarctic, two species occur as shallow as 33 meters under the ice. In the Mediterranean, one species occurs as shallow as 18 metres (59ft) in a cave with deep water upwelling (Boury-Esnault & Vacelet (1994))
Pattersonia ulrichi Rauff, 1894; an Ordovician hexactinellid sponge from near Cincinnati, Ohio.
Reefs
The sponges form reefs (called sponge reefs) off the coast of British Columbia, southeast Alaska and Washington state,[13] which are studied in the Sponge Reef Project. In the case of Sarostegia oculata, this species almost always hosts symbiotic zoanthids, which cause the hexactinellid sponge to imitate the appearance and structure of coral reefs.[14] Only 33 species of this sponge have ever been reported in the South Atlantic until 2017 when the Shinkai 6500 submersible went on an expedition through the Rio Grande Rise.[14] Reefs discovered in Hecate Strait, British Columbia, have grown to up to 7 kilometres long and 20 metres high. Prior to these discoveries, sponge reefs were thought to have died out in the Jurassic period.[15][16]
Reports of glass sponges have also been recorded on the HMCS Saskatchewan and HMCS Cape Breton wrecks off the coast of Vancouver Island. Species of zoantharin that rely on hexactinellid have also been found off the coast of the Japanese island of Minami-Torishima. Unidentified species of zoanthids have also been found in Australian waters, if these are identified as the same as the ones found in Minami-Torishima, this could potentially be proof of hexactinellids existing in all of the Pacific Ocean.[17]
Conservation
Most hexactinellids live in deep waters that are not impacted by human activities. However, there are glass sponge reefs off the coast of British Columbia. The Canadian government designated 2140km² of the Hecate strait and Queen Charlotte sound as a marine protected area. This area contains four glass sponge reefs. The new regulations prohibit bottom contact fishing within 200 meters of the sponge reefs.[18] Although human activities only affect a small portion of glass sponges, they are still subject to the threat of climate changes. Experiments using the species Aphrocallistes vastus have shown that increases in temperature and acidification can lead to weakened skeletal strength and stiffness.[19] In 1995, an Antarctic ice shelf collapsed due to climate change. Since then, studies of the area have shown that hexactinellid reefs have been increasing in size despite the changes in climate.[20]
Classification
The earliest known hexactinellids are from the earliest Cambrian or late Neoproterozoic eras. They are fairly common relative to demosponges as fossils, but this is thought to be, at least in part, because their spicules are sturdier than spongin and fossilize better. Like almost all sponges, the hexactinellids draw water in through a series of small pores by the whip-like beating of a series of hairs or flagella in chambers which in this group line the sponge wall.
The class is divided into two subclasses and several orders:[21]
Sponges, the members of the phylum Porifera, are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate through them, consisting of jelly-like mesohyl sandwiched between two thin layers of cells.
The Venus' flower basket is a glass sponge in the phylum Porifera. It is a marine sponge found in the deep waters of the Pacific ocean, usually at depths below 500 m (1,600 ft). Like other sponges, they feed by filtering sea water to capture plankton and marine snow. Similar to other glass sponges, they build their skeletons out of silica, which forms a unique lattice structure of spicules. The sponges are usually between 10 cm (3.9 in) and 30 cm (12 in) tall, and their bodies act as refuge for their mutualist shrimp partners. This body structure is of great interest in materials science as the optical and mechanical properties are in some ways superior to man-made materials. Little is known regarding their reproduction habits, however fluid dynamics of their body structure likely influence reproduction and it is hypothesized that they may be hermaphroditic.
Demosponges (Demospongiae) are the most diverse class in the phylum Porifera. They include greater than 90% of all species of sponges with nearly 8,800 species worldwide. They are sponges with a soft body that covers a hard, often massive skeleton made of calcium carbonate, either aragonite or calcite. They are predominantly leuconoid in structure. Their "skeletons" are made of spicules consisting of fibers of the protein spongin, the mineral silica, or both. Where spicules of silica are present, they have a different shape from those in the otherwise similar glass sponges. Some species, in particular from the Antarctic, obtain the silica for spicule building from the ingestion of siliceous diatoms.
The siliceous sponges form a major group of the phylum Porifera, consisting of classes Demospongiae and Hexactinellida. They are characterized by spicules made out of silicon dioxide, unlike calcareous sponges.
Hexasterophora are a subclass of glass sponges in the class Hexactinellida. Most living hexasterophorans can be divided into three orders: Lyssacinosida, Lychniscosida, and Sceptrulophora. Like other glass sponges, hexasterophorans have skeletons composed of overlapping six-rayed spicules. In addition, they can be characterized by the presence of hexasters, a type of microsclere with six rays unfurling into multi-branched structures.
Sponge reefs are reefs produced by sea sponges. All modern sponge reefs are formed by hexactinellid sponges, which have an endoskeleton made of silica spicules and are often referred to as "glass sponges", while historically the non-spiculed, calcite-skeletoned archaeocyathid and stromatoporoid sponges were the primariy reef-builders.
The cloud sponge(Aphrocallistes vastus) is a species of sea sponge in the class Hexactinellida. It is a deep-water reef-forming animal. The species was first described by F.E. Schulze in 1886.
Spicules are structural elements found in most sponges. The meshing of many spicules serves as the sponge's skeleton and thus it provides structural support and potentially defense against predators.
Euplectella is a genus of glass sponges which includes the well-known Venus' Flower Basket. Glass sponges have a skeleton made up of silica spicules that can form geometric patterns. These animals are most commonly found on muddy sea bottoms in the Western Pacific and Indian Oceans. They are sessile organisms and do not move once attached to a rock. They can be found at depths between 100 m and 1000 m but are most commonly found at depths greater than 500 m.
Reticulosa is an extinct order of sea sponges in the class Hexactinellida and the subclass Amphidiscophora. Reticulosans were diverse in shape and size, similar to their modern relatives, the amphidiscosidans. Some were smooth and attached to a surface at a flat point, others were polyhedral or ornamented with nodes, many were covered in bristles, and a few were even suspended above the seabed by a rope-like anchor of braided glass spicules.
Amphidiscosida is an order of hexactinellids. The Amphidiscosida are commonly regarded as the only living sponges in the subclass Amphidiscophora.
Silicateins are enzymes which catalyse the formation of biosilica from monomeric silicon compounds extracted from the natural environment. Environmental silicates are absorbed by specific biota, including diatoms, radiolaria, silicoflagellates, and siliceous sponges; silicateins have so far only been found in sponges. Silicateins are homologous to the cysteine protease cathepsin.
Sceptrulophora is an order of hexactinellid sponges. They are characterized by sceptrules, a type of microsclere with a single straight rod terminating at a bundle of spines or knobs. An anchor- or nail-shaped sceptrule is called a clavule. A fork-shaped sceptrule, ending at a few large tines, is called a scopule. A broom-shaped sceptrule, ending at many small bristles, is called a sarule.
Claviscopulia is a genus of glass sponge in the family Farreidae.
Bolosoma is a genus of pedunculated siliceous sponges belonging to the family Euplectellidae. This genus lives in deep-sea environments and provides a habitat for a plethora of other benthic species, giving Bolosoma an incredibly important ecological role in the ecosystems it is a part of.
Lychniscosida is an order of sponges belonging to the class Hexactinellida and subclass Hexasterophora. They are dictyonal sponges characterized by the presence of additional struts at the nodes of the skeleton. These struts create octahedral frames, known as lychniscs ("lanterns").
Inflatella belli, or the gooseberry sponge, is a species of demosponge that appears to be restricted to the southern hemisphere, where it is widespread. It occurs from the coast of Namibia and the Indo-pacific, down to the Subantarctic and Antarctic regions.
Rosella is a genus of glass sponges in the family Rossellidae. It is found in the Antarctic and sub-Antarctic regions.
Rossella antarctica is a relatively small species of glass sponge. It is widely distributed in the southern hemisphere, particularly in the Antarctic and sub-Antarctic regions.
Oopsacas minuta is a glass sponge that is a member of the Hexactinellida. Oopsacas minuta is found in submarine caves in the Mediterranean. It is reproductive year-round. This species is a part of a class that are usually bathyal and abyssal. Meaning they grow at a depth over 200 meters. At this depth the temperature is low and constant, so silica metabolism is optimized. However, this species has been observed in shallow water. O. minuta have only been observed by exploring caves that trap cold water. The shape of the sponge is elongated, cylindrical and a little flared. It is between a few millimeters and 3.5 centimeters. O. minuta are white are held up with a siliceous skeleton. The spicules of the skeleton intersect in an intricate network. These spindles partially block the top of the sponge. There are no obvious oscules. The sponge is anchored or suspended from the cave by silica fibers. This class of sponge is different from the three other classes of Porifera. It differs in tissue organization, ecology, development and physiology. O. minuta belongs to the order Lyssacinosida. Lyssacinosida are characterized by the parenchymal spicules mostly being unconnected; this is unlike other sponges in the subclass where the spicules form a connected skeleton. The genome of O. minuta are one of the smallest of all the animal genomes that have been sequenced so far. Its genome contains 24 noncoding genes and 14 protein-encoding genes. The spindles of O. minuta have three axes and six points. This species does not have pinacocytes, which are the cells that form the outer layer in other sponges. Instead of true choanocytes it has frill structures that bud from the syncytium.
This page is based on this Wikipedia article Text is available under the CC BY-SA 4.0 license; additional terms may apply. Images, videos and audio are available under their respective licenses.
