| Sponge Fossil range: Ediacaran–Recent |
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The sponges or poriferans (from Latin porus "pore" and ferre "to bear") are animals of the polyphyletic phylum Porifera (pronounced /pɒˈrɪfərə/). They are primitive, sessile, mostly marine, water dwelling filter feeders that pump water through their bodies to filter out particles of food matter. Sponges represent the simplest of animals.These are non-motile animals attached to some solid support.They are covered with a hard outer layer or skeleton. With no true tissues (parazoa), they lack muscles, nerves, and internal organs. Their similarity to colonial choanoflagellates shows the probable evolutionary jump from unicellular to multicellular organisms. However, recent genomic studies suggest they are not the most ancient lineage of animals, but may instead be secondarily simplified. There are over 5,000 modern species of sponges known, and they can be found attached to surfaces anywhere from the intertidal zone to as deep as 8,500 m (29,000 feet) or further. Though the fossil record of sponges dates back to the Neoproterozoic Era, new species are still commonly discovered. Their body form is supported by a skeleton of spicules.
Contents
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Distinguishing features
| Sponges[1][2] | Cnidarians and ctenophores[3] | |
|---|---|---|
| Nervous system | No | Yes, simple |
| Cells in each layer bound together | No | Yes: inter-cell connections; basal lamina |
| Number of cells in middle "jelly" layer | Many | Few |
| Cells in outer layers can move inwards and change functions | Yes | No |
Sponges consitute the phylum Porifera, and have been defined as sessile metazoans (multi-celled animals) that have water intake and outlet openings connected by chambers lined with choanocytes, cells with whip-like flagella. However a few carnivorous sponges have lost these water flow systems and the choanocytes.[4] All known living sponges can remold their bodies, as most types of their cells can move within their bodies and a few can change from one type to another.[1][4]
Like cnidarians (jellyfish, etc.) and ctenophores (comb jellies), and unlike all other known metazoans, sponges' bodies consist of a non-living jelly-like mass sandwiched between two main layers of cells.[2][3] Cnidarians and ctenophores have simple nervous systems, and their cell layers are bound by internal connections and by being mounted on a basal lamina (thin fibrous mat, also known as "basement membrane").[3] Sponges have no nervous systems, their middle jelly-like layers have large and varied populations of cells, and some types of cell in their outer layers may move into the middle layer and change their functions.[1]
Description
Sponges consitute the phylum Porifera, and have been defined as sessile metazoans (multi-celled animals) that have water intake and outlet openings connected by chambers lined with choanocytes, cells with whip-like flagella. However a few carnivorous sponges have lost these water flow systems and the choanocytes.[4] All known living sponges can remold their bodies, as most types of their cells can move within their bodies and a few can change from one type to another.[1][4]
Basic structure and cell types
A sponge's body is held in shape by the mesohyl, a jelly-like substance made mainly of collagen and reinforced by a dense network of fibers also made of collagen. The inner surface is covered with choanocytes, cells with cylindrical or conical collars surrounding one flagellum per choanocyte. The wave-like motion of the whip-like flagella drives water through the sponge's body. All sponges have ostia, channels leading to the interior through the mesohyl, and in most sponges these are controlled by tube-like porocytes that form closable inlet valves. Pinacocytes, plate-like cells, form a single-layered skin over all other parts of the mesohyl that are not covered by choanocytes, and the external pinacocytes also digest food particles that are too large to enter the ostia,[2][1] while those at the base of the animal are responsbile for anchoring it.[2]
Other types of cell live and move within the mesohyl:[1][2]
- Lophocytes are ameba-like cells that move slowly through the mesohyl and secrete collagen fibres.
- Collencytes are another type of collagen-producing cell.
- Rhabdiferous cells secrete polysaccharides that also form part of the mesohyl.
- Oocytes and spermatocytes are reproductive cells.
- Sclerocytes secrete the mineralized spicules ("little spines") that form the skeletons of many sponges and in some species provide some defense against predators.
- In addition to or instead of sclerocytes, demosponges have spongocytes that secrete a form of collagen that polymerizes into spongin, a thick fibrous material that stiffens the mesohyl.
- Myocytes ("muscle cells") conduct signals and cause parts of the animal to contract.
- "Grey cells" act as sponges' equivalent of an immune system.
- Archaeocytes (or amoebocytes) are ameba-like cells that are totipotent, in other words each is capable of transformation into any other type of cell. They also have important roles in feeding and in clearing debris that block the ostia.
Glass sponges' syncytia
Glass sponges present a distinctive variation on this basic plan. Their spicules, which are made of silica, form a scaffolding-like framework between whose rods the living tissue is suspended like a cobweb that contains most of the cell types.[1] This tissue is a syncytium that in some ways behaves like many cells that share a single external membrane, and in others like a single cell with multiple nuclei. The mesohyl is absent or minimal. The syncytium's cytoplasm, the soupy fluid that fills the interiors of cells, is organised into "rivers" that transport nuclei, organelles ("organs" within cells) and other substances.[7]Instead of choanocytes they have further syncytia, known as choanosyncytia, which form bell-shaped chambers which water enters via perforations. The insides of these chambers are lined with "collar bodies", each consisting of a collar and flagellum but without a nucleus of its own. The motion of the flagella sucks water through passages in the "cobweb" and expels it via the open ends of the bell-shaped chambers.[1]
Some types of cells have a single nucleus and membrane each, but are connected to other single-nucleus cells and to the main syncytium by "bridges" made of cytoplasm. The sclerocytes that build spicules have multiple nuclei, and in glass sponge larvae they are connected to other tissues by cytoplasm bridges; such connections between sclerocytes have not so far been found in adults, but this may simply reflect the difficulty of investigating such small-scale features. The bridges are controlled by "plugged junctions" that apparently permit some substances to pass while blocking others. [7]
Water flow and body structures
Most sponges work rather like chimneys: they take in water at the bottom and eject it from the osculum ("little mouth") at the top. Since ambient currents are faster at the top, the suction effect that they produce does some of the work for free. Sponges can control the water flow by various combinations of wholly or partially closing the osculum and ostia (the intake pores) and varying the beat of the flagella, and may shut it down if there is a lot of sand or silt in the water.[1]
Although the layers of pinacocytes and choanocytes resemble the epithelia of more complex animals, they are not bound tightly by cell-to-cell connections or a basal lamina (thin fibrous sheet underneath). The flexibility of these layers and re-modeling of the mesohyl by lophocytes allow the animals to adjust their shapes throughout their lives to take maximum advantage of local water currents.[1]
The simplest body structure in sponges is a tube or vase shape known as "asconoid", but this severely limits the size of the animal. If it is simply scaled up, the ratio of its surface area to volume increases, because surface increases as the square of length or width while volume increases proportionally to the cube. The amount of tissue that needs food and oxygen is determined by the volume, but the pumping capacity that supplies food and oxygen depends on the area covered by choanocytes. Asconoid sponges seldom exceed 1 millimetre (0.039 in) in diameter.[1]
Some sponges overcome this limitation by adopting the "syconoid" structure, in which the body wall is pleated. The inner pockets of the pleats are lined with choanocytes, which connect to the outer pockets of the pleats by ostia. This increase in the number of choanocytes and hence in pumping capacity enables syconoid sponges to grow up to up to a few centimeters in diameter. The "leuconid" pattern boosts pumping capacity further by filling the interior almost completely with mesohyl that contains a network of chambers lined with choanocytes and connected to each other and to the water intakes and outlet by tubes. Leuconid sponges grow to over 1 metre (3.3 ft) in diameter, and the fact that growth in any direction increases the number of choanocyte chambers enables them to take a wider range of forms, for example "encrusting" sponges whose shapes follow those of the surfaces to which they attach. All freshwater and most shallow-water marine sponges have leuconid bodies. The networks of water passages in glass sponges are similar to the leuconid structure.[1]
In all three types of structure the cross-section area of the choanocyte-lined regions is much greater than that of the intake and outlet channels. This makes the flow slower near the choanocytes and thus makes it easier for them to trap food particles.[1] For example in Leuconia, a small leuconoid sponge about 10 centimetres (3.9 in) tall and 1 centimetre (0.39 in) in diameter, water enters each of more than 80,000 intake canals at 6 cm per minute. However, because Leuconia has more than 2 million flagellated chambers whose combined diameter is much greater than that of the canals, water flow through chambers slows to 3.6 cm per hour, making it easy for choanocytes to capture food. All the water is expelled through a single osculum at about 8.5 cm per second, fast enough to carry waste products some distance away.[9]
It should be noted that these 3 body grades are useful only in describing morphology, and not in classifying sponge species, although the asconoid and syconoid construction is present in Calcarea only[10].[citation needed]
Syconoids do not usually form highly branched colonies as asconoids do. During their development, syconoid sponges pass through an asconoid stage.[citation needed]
Skeleton
In zoology a skeleton is any fairly rigid structure of an animal, irrespective of whether it has joints and irrespective of whether it is biomineralized. The mesohyl functions as an endoskeleton in most sponges, and is the only skeleton in soft sponges that encrust hard surfaces such as rocks. More commonly the mesohyl is stiffened by mineral spicules, by spongin fibers or both. Spicules may be made of silica or calcium carbonate, and vary in shape from simple rods to three-dimensional "stars" with up to six rays. Spicules are produced by sclerocyte cells,[1] and may be separate, connnected by joints, or fused.[4]
Some sponges also secrete exoskeletons that lie completely outside their organic components. For example sclerosponges ("hard sponges") have massive calcium carbonate exoskeletons over which the organic matter forms a thin layer with choanocyte chambers in pits in the mineral. These exoskeletons are secreted by the pinacocytes that form the animals' skins.[1]
Movement
Although sponges are fundamentally sessile animals, some marine and freshwater species can move across the bottom at speeds of 1–4 millimetres (0.039–0.16 in) per day, as a result of ameba-like movements of pinacocytes and other cells. A few species can contract their whole bodies, and many can close their oscula and ostia.[1]
Respiration, feeding and excretion
Sponges do not have distinct circulatory, respiratory, digestive, and excretory systems – instead the water flow system supports all these functions. They filter food particles out of the water flowing through them. Particles larger than 50 micrometres cannot enter the ostia and pinacocytes consume them by phagocytosis (engulfing and internal digestion). Particles from 0.5 to 50 micrometres (2.0×10−5 to 0.00197 in) are trapped in the ostia, which taper from the outer to inner ends. These particles are consumed by pinacocytes or by archaeocytes which partially extrude themselves through the walls of the ostia. Bacteria-sized particles, below 0.5 micrometres, pass through the ostia and are caught and consumed by choanocytes.[1] Since the smallest particles are by far the commonest, choanocytes typically capture 80% of a sponge's food supply.[11] Archaeocytes transport food packaged in vesicles from cells that directly digest food to those that do not. At least one species of sponge has internal fibers that function as tracks for use by nutrient-carrying archaeocytes,[1] and these tracks also move inert objects.[2]
It used to be claimed that glass sponges could live on nutrients dissolved in sea water and were very averse to silt. However a study in 2007 found no evidence of this and concluded that they extract bacteria and other micro-organisms from water very efficiently (about 79%) and process suspended sediment grains to extract such prey.[12] Collar bodies digest food and distribute it wrapped in vesicles that are transported by dynein "motor" molecules along bundles of microtubules that run throughout the syncytium.[1]
Sponges' cells absorb oxygen by diffusion from the water flow system, into which carbon dioxide and other soluble waste products such as ammonia also diffuse. Archeocytes remove mineral particles that threaten to block the ostia, transport them through the mesohyl and generally dump them into the outgoing water current, although some species incorporate them into their skeletons.[1]
Carnivorous sponges
A few species live in waters where the supply of food particles is very poor, and prey on crustaceans and other small animals. Most belong to the family Cladorhizidae, but a few members of the Guitarridae and Esperiopsidae are also carnivores.[13] In most cases little is known about how they actually capture prey, although some species are thought to use either sticky threads or hooked spicules.[14][13] Most carnivorous sponges live in deep waters, up to 8,840 metres (5.49 mi),[15] and the development of deep-ocean exploration techniques is expected to lead to the discovery of several more.[1][13] However one species has been found in Mediterranean caves at depths of 17–23 metres (56–75 ft), alongside the more usual filter feeding sponges. The cave-dwelling predators capture crustaceans under 1 millimetre (0.039 in) long by entangling them with fine threads, digest them by enveloping them with further threads over the course of a few days, and then return to their normal shape; there is no evidence that they use venom.[15]
Most known carnivorous sponges have completely lost the water flow system and choanocytes. However the genus Chondrocladia uses a highly modified water flow system to inflate balloon-like structures that are used for capturing prey.[13][16]
Endosymbionts
Freshwater sponges often host green algae as endosymbionts within archaeocytes and other cells, and benefit from nutrients produced by the algae. Many marine species host other photosynthesizing organisms, most commonly cyanobacteria but in some cases dinoflagellates. Symbiotic cyanobacteria may form a third of the total mass of living tissue in some sponges, and some sponges gain 48% to 80% of their energy supply from these micro-organisms.[1] Sponges that host photosynthesizing organisms are commonest in waters with realtively poor supplies of food particles, and often have leafy shapes that maximize the amount of sunlight they collect.[2]
A recently-discovered carnivorous sponge that lives near hydrothermal vents hosts methane-eating bacteria, and digests some of them.[2]
"Immune" system
Sponges do not have the complex immune systems of most other animals. However they reject grafts from other species but accept them from their own species. In a few marine species grey cells play the leading role in rejection of foreign material. They produce a chemical that stops movement of other cells in the affected area, thus preventing the intruder from using the sponge's internal tranport systems. If the intrusion persists, the grey cells concentrate in the area and release toxins that kill all cells in the area. The "immune" system can stay in this activated state for up to three weeks.[2]
Reproduction
Asexual
Sponges have three asexual methods of reproduction: after fragmentation; by budding; and by producing gemmules. Fragments of sponges may be detached by currents or waves, and perhaps by predators. They use the mobility of their pinacocytes and choanocytes and reshaping of the mesohyl to re-attach themselves to a suitable surface and then rebuild themselves as small but functional sponges over the course of several days. The same capabilities enable sponges that have been squeezed though a fine cloth to regenerate.[1] A sponge fragment can only regenerate if it contains both collencytes to produce mesohyl and archeocytes to produce all the other cell types.[11] A very few species reproduce by budding.[1]
Gemmules are "survival pods" which a few marine sponges and many freshwater species produce by the thousands when dying and which some, mainly freshwater species, regularly produce in autumn. Spongocytes make gemmules by wrapping shells of spongin, often reinforced with spicules, round clusters of archeocytes that are full of nutrients.[1] Freshwater gemmules may also include phytosynthesizing symbionts.[17] The gemmules then become dormant, and in this state can survive cold, drying out, lack of oxygen and extreme variations in salinity.[1] Freshwater gemmules often do not revive until the temperature drops, stays cold for a few months and then reaches a near-"normal" level.[17] When a gemmule germinates, the archeocytes round the outside of the cluster transform into pinacocytes, a membrane over a pore in the shell bursts, the cluster of cells slowly emerges, and most of the remaining archeocytes transform into other cell types needed to make a functioning sponge. Gemmules from the same species but different individuals can join forces to form one sponge.[1] Some gemmules are retained within the parent sponge, and in spring it can be difficult to tell whether an old sponge has revived or been "recolonized" by its own gemmules.[17]
Sexual
Most sponges are hermaphrodites (function as both sexes simultaneously), although sponges have no gonads (reproductive organs). Sperm are produced by choanocytes or entire choanocyte chambers that sink into the mesohyl and form spermatic cysts while eggs are formed by transformation of archeocytes, or of choanocytes in some species. Each egg generally acquires a yolk by consuming "nurse cells". During spawing, sperm burst out of their cysts and are expelled via the osculum. If they contact another sponge of the same species, the water flow carries them to choanocytes that engulf them but, instead of digesting them, metamorphose to an ameboid form and carry the sperm through the mesohyl to eggs, which in most cases engulf the carrier and its cargo.[1]
A few species release fertilized eggs into the water, but most retain the eggs until they hatch. There are four types of larvae, but all are balls of cells with an outer layer of cells whose flagellae or cilia enable the larvae to move. After swimming for a few days the larvae sink and crawl until they find a place to settle. Most of the cells transform into archeocytes and then into the types appropriate for their locations in a miniature adult sponge.[1]
Glass sponge embryos start by dividing into separate cells, but once 32 cells have formed they rapidly transform into larvae that externally are ovoid with a band of cilia round the middle that they use for movement, but internally have the typical glass sponge structure of spicules with a cobweb-like main syncitium draped around and between them and choanosyncytia with multiple collar bodies in the center. The larvae then leave their parents' bodies.[18]
Life cycle
Sponges in temperate regions live for at most a few years, but some tropical species and perhaps some deep-ocean ones may live for 200 years or more. Some calcified demosponges grow by only 0.2 millimetres (0.0079 in) per year and, if that rate is constant, specimens 1 metre (3.3 ft) wide must be about 5,000 years old. Some sponges start sexual reproduction when only a few weeks old, while others wait until they are several years old.[1]
Coordination of activities
Adult sponges lack neurons or any other kind of nervous tissue. However most species have the ability to perform movements that are co-ordinated all over their bodies, mainly contractions of the pinacocytes, squeezing the water channels and thus expelling excess sediment and other substances that may cause blockages. Some species can contract the osculum independently of the rest of the body. Sponges may also contract in order to reduce the area that is vulnerable to attack by predators. In cases where two sponges are fused, for example if there is a large but still unseparated bud, these contraction waves slowly become co-ordinated in both of the "Siamese twins". The co-ordinating mechanism is unknown, but may involve chemicals similar to neurotransmitters.[19] However glass sponges rapidly transmit electrical impulses through all parts of the syncytium, and use this to halt the motion of their flagella if the incoming water contains toxins or excessive sediment.[1] Myocytes are thought to be responsible for closing the osculum and for transmitting signals between different parts of the body.[2]
Sponges contain genes very similar to those that contain the "recipe" for the post-synaptic density, an important signal-receiving structure in the neurons of all other animals. However in sponges these genes are only activated in "flask cells" that appear only in larvae and may provide some sensory capability while the larvae are swimming. This raises questions about whether flask cells represent the predecessors of true neurons or are evidence that sponges' ancestors had true neurons but lost them as they adapted to a sessile lifestyle.[20]
Ecology
Habitats
Sponges are worldwide in their distribution, from the polar regions to the tropics,[11] but the great majority of individuals and species live in warmer waters.[citation needed] They live in quiet clear waters, because sediment stirred up by waves or currents would block their pores, making it difficult for them to feed and breathe.[citation needed] The greatest numbers of sponges are usually found on firm surfaces such as rocks, but some sponges can attach themselves to soft sediment by means of a root-like base.[citation needed]
The different classes of sponge live in different ranges of habitat:
| Water type[2] | Depth[2] | Type of surface[2] | Body form[2] | |
|---|---|---|---|---|
| Calcarea | Marine | less than 100 metres (330 ft) | Hard | Asconoid, syconoid or leuconoid |
| Glass sponges | Marine | Deep | Soft or firm sediment | Leuconoid |
| Demosponges | Marine, brackish; and about 150 freshwater species[1] | Inter-tidal to abyssal;[2] a carnivorous demosponge has been found at 8,840 metres (5.49 mi)[15] | Any | Leuconoid |
As primary producers
Sponges with photosynthesizing endosymbionts produce up to three times more oxygen than they consume, as well as more organic matter than they consume. Such contributions to their habits' resources are significant along Australia 's Great Barrier Reef but relatively minor in the Caribbean.[11]
Defenses
Many sponges shed spicules, forming a dense carpet several meters deep that keeps away echinoderms which would otherwise prey on the sponges.[11] They also produce toxins that prevent other sessile organisms such as bryozoans or sea squirts from growong on or near them, making sponges very effective competitors for living space.
A few species, such as the Caribbean fire sponge Tedania ignis, cause a severe rash in humans who handle them.[1] Turtles and some fish feed mainly on sponges. It is often said that sponges produce chemical defenses against such predators.[1]However an experiment showed that there is no relationship between the toxicity of chemicals producd by sponges and how they taste to fish, which would diminish the usefulness of chemical defenses as deterrents. Predation by fish may even help to spread sponges by detaching fragments.[2]
Bioerosion
The Caribbean chicken-liver sponge Chondrilla nucula secretes toxins that kill coral polyps, allowing the sponges to grow over the coral skeletons.[1] Others, especially in the family Clionidae, use corrosive substances secreted by their archeocytes to tunnel into rocks, corals and the shells of dead molluscs.[1] Sponges may remove up to 1 metre (3.3 ft) per year from reefs, creating visible notches just below low-tide level.[11]
Diseases
Caribbean sponges of the genus Aplysina suffer from Aplysina red band syndrome. This causes Aplysina to develop one or more rust-colored bands, sometimes with adjacent bands of necrotic tissue (dead). These lesions may completely encircle branches of the sponge. The disease appears to be contagious (spread by physical contact). The rust-colored bands are caused by a cyanobacterium, but it is unknown whether this organism actually causes the disease.[21]
Evolutionary history
Fossil record
Traces of the chemical 24-isopropylcholestane have been found in rocks formed .[22] This is a stable derivative of 24-isopropylcholesterol, which is thought to be produced by demosponges but not by eumetazoans ("true animals", i.e. cnidarians and bilaterians). Since choanoflagellates are thought to be animals' closest single-celled relatives, a team of scientists examinated the biochemistry and genes of one choanoflagellate species. They concluded that this species could not produce 24-isopropylcholesterol but that investigation of a wider range of choanoflagellates would be necessary in order to prove that the fossil 24-isopropylcholestane could only have been produced by demosponges.[23]
Well-preserved fossil sponges from about in the Ediacaran period have been found in the Doushantuo Formation. These fossils, which include spicules, pinacocytes, porocytes, archeocytes, sclerocytes and the internal cavity, have been classified as demosponges. Other probable demosponges have been found in the Early Cambrian Chengjiang fauna, from .[24] Silica spicules like those of demosponges have been reported from Nevada in rocks dated around .[25] Fossils of glass sponges have been found from around in rocks in Australia, China and Mongolia.[26] Calcium carbonate spicules of calcareous sponges have been found in Early Cambrian rocks from about in Australia. Freshwater sponges appear to be much younger, as the earliest known fossils date from the Mid-Eocene period about .[26]
Archaeocyathids, which some classify as a type of coralline sponge, are common in the Cambrian period from about , but apparently died out by the end of the Cambrian .[24]
Family tree
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Simplified family tree showing calcareous sponges
as closest to more complex animals[27] |
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as closest to more complex animals[28] |
In the 1990s sponges were widely regarded as a monophyletic group, in other words all of them descended from a common ancestor that was itself a sponge, and as the "sister-group" to all other metazoans (multi-celled animals), which themselves form a monophyletic group. On the other hand some 1990s analyses also revived the idea that animals' nearest evolutionary relatives are choanoflagellates, single-celled organisms very similar to sponges' choanocytes – which would imply that most Metazoa evolved from very sponge-like ancestors and therefore that sponges may not be monophyletic, as the same sponge-like ancestors may have given rise both to modern sponges and to non-sponge members of Metazoa.[27]
A study in 2001 based on comparisons of ribosome DNA concluded that sponges are not monophyletic because Calcareous sponges, those with calcium carbonate spicules, are at least as closely related to Eumetazoa ("true" animals, more complex than sponges) as to other types of sponge, and that the most fundamental division within sponges was between glass sponges and the rest.[27] In 2007 one analysis based on comparisons of RNA and another based mainly on comparison of spicules concluded that demosponges and glass sponges are more closely related to each other than either is to calcareous sponges, which in turn are more closely related to Eumetazoa.[29][26] A further analysis in 2007, based on comparison of nuclear DNA, excluding glass sponges and comb jellies, concluded that: Homoscleromorpha, a sub-group of demosponges, are most closely related to Eumetazoa; calcareous sponges are the next closest; the other demosponges are evolutionary "aunts" of these groups; and the chancelloriids, bag-like animals whose fossils are found in Cambrian rocks, may be sponges.[28]
The analyses described above conclude that sponges are closest to the ancestors of all Metazoa. However, another genetic comparison study in 2008 of 150 genes in each of 21 genera, ranging from fungi to humans but including only two species of sponge, suggests that comb jellies are the most basal lineage of the Metazoa sampled. If this is correct, either modern comb jellies developed their complex structures independently of other Metazoa, or sponges' ancestors were more complex and all known sponges are drastically simplified forms. The study recommended further analyses using a wider ranger of sponges and other simple Metazoa such as Placozoa.[30]
Archaeocyathids are very common fossils in rocks from the Early Cambrian about but are not found after the Late Cambrian. It has been suggested that they were produced by: sponges; cnidarians; algae; foraminiferans; a completely separate phylum of animals, Archaeocyatha; or even a completely separate kingdom of life, labelled Archaeata or Inferibionta. Since the 1990s archaeocyathids have been regarded as a distinctive group of sponges.[31]
It is difficult to fit chancelloriids into classifications of sponges or more complex animals. An analysis in 1996 concluded that they were closely related to sponges on the grounds that the detailed structure of chancellorid sclerites ("armor plates") is similar to that of fibers of spongin, a collagen protein, in modern keratose (horny) demosponges such as Darwinella.[33] However another analysis in 2002 concluded that chancelloriids are not sponges and may be intermediate between sponges and more complex animals, among other reasons because their skins were thicker and more tightly-connected than those of sponges.[34] In 2008 a detailed analysis of chancelloriids' sclerites concluded that they were very similar to those of halkieriids, mobile bilaterian animals that looked like slugs in chain mail and whose fossils are found in rocks from the very Early Cambrian to the Mid Cambrian – which would create a dilemma, as it is extremely unlikely that totally unrelated organisms could have developed such similar sclerites independently, but the huge difference in the structures of their bodies makes it hard to see how they could be closely related.[32]
Taxonomy
At present about 9,000 species have been identified in scientific publications. However some types of habitat, such as vertical rock and cave walls and galleries in rock and coral boulders, have been investigated very little, even in shallow seas.[11]
For a long time sponges were assigned to a separate subkingdom, Parazoa ("beside the animals"), separate from the Eumetazoa which formed the rest of the kingdom Animalia.[31] They are now classified as a phylum within Animalia, and divided into classes mainly according to the composition of their skeletons:[11][4]
- Hexactenellida (glass sponges) have silicate spicules, the largest of which have six rays and and may be individual or fused.[4] The main components of their bodies are syncytia in which large numbers of cell share a single external membrane.[11]
- Calcarea have skeletons made of calcite, a form of calcium carbonate, which may form separate spicules or large masses. All the cells have a single nucleus and membrane.[11]
- Most Demospongiae have silicate spicules or spongin fibers or both within their soft tissues. However a few also have massive external skeletons made of aragonite, another form of calcium carbonate.[11][4] All the cells have a single nucleus and membrane.[11]
- Archeocyatha are known only as fossils from the Cambrian period.[31]
In the 1970s sponges with calcium carbonate skeletons were assigned to a separate class, Sclerospongiae, otherwise known as "coralline sponges".[35] However in the 1980s it was found that these were all members of either the Calcarea or the Demospongiae.[36]
Although 90% of modern sponges are demosponges, fossilized remains of this type are less common than those of other types because their skeletons are composed of relatively soft spongin that does not fossilize well.[citation needed]
Basal lineage?
Sponges are among the simplest animals. They lack gastrulated embryos, extracellular digestive cavities, nerves, muscles, tissues, and obvious sensory structures, features possessed by all other animals. In addition, sponge choanocytes (feeding cells) appear to be a homologous to choanoflagellates, a group of unicellular and colonial protists that are believed to be the immediate precursors of animals. The traditional conclusion is that sponges are the basal lineage of the animals, and that features such as tissues developed after sponges and other animals diverged. Sponges were first assigned their own subkingdom, the Parazoa, but more recent molecular studies suggested that the sponges were paraphyletic to other animals, with the eumetazoa as a sister group to the most derived:[37]
Geological history
The fossil record of sponges is not abundant. Some fossil sponges have worldwide distribution, while others are restricted to certain areas. Sponge fossils such as Hydnoceras and Prismodictya are found in the Devonian rocks of New York state. In Europe the Jurassic limestone of the Swabian Alb are composed largely of sponge remains, some of which are well preserved. Many sponges are found in the Cretaceous Lower Greensand and Chalk Formations of England, and in rocks from the upper part of the Cretaceous period in France. A famous locality for fossil sponges is the Cretaceous Faringdon Sponge Gravels in Faringdon, Oxfordshire in England. An older sponge is the Cambrian Vauxia. Sponges have long been important agents of bioerosion in shells and carbonate rocks. Their borings extend back to the Ordovician in the fossil record.
Fossil sponges differ in size from 1 cm (0.4 inches) to more than 1 meter (3.3 feet). They vary greatly in shape, being commonly vase-shapes (such as Ventriculites), spherical (such as Porosphaera), saucer-shaped (such as Astraeospongia), pear-shaped (such as Siphonia), leaf-shaped (such as Elasmostoma), branching (such as Doryderma), irregular or encrusting.
Detailed identification of many fossil sponges relies on the study of thin sections.
Use
By dolphins
In 1997, use of sponges as a tool was described in bottlenose dolphins in Shark Bay. A dolphin will attach a marine sponge to its rostrum, which is presumably then used to protect it when searching for food in the sandy sea bottom.[38] The behaviour, known as sponging, has only been observed in this bay, and is almost exclusively shown by females. This is the only known case of tool use in