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Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the Eumetazoa. They are sessile that are bound to the seabed, and are one of the most ancient members of macrobenthos, with many historical species being important sponge reef-building organisms.
Sponges are multicellular organisms consisting of jelly-like mesohyl sandwiched between two thin layers of cells, and usually have tube-like bodies full of pores and channels that allow water to circulate through them. They have unspecialized cells that can transform into other types and that often migrate between the main cell layers and the mesohyl in the process. They do not have complex nervous system, digestive or circulatory systems. Instead, most rely on maintaining a constant water flow through their bodies to obtain food and oxygen and to remove wastes, usually via flagella movements of the so-called "".
Sponges are believed to have been the first outgroup to branch off the evolutionary tree from the Urmetazoan, with fossil evidence of primitive sponges such as Otavia from as early as the Tonian geologic period (around 800 Mya). The branch of zoology that studies sponges is spongiology.
Many sponges have internal skeletons of spicules (skeletal-like fragments of calcium carbonate or silicon dioxide), and/or spongin (a modified type of collagen protein). An internal gelatinous matrix called mesohyl functions as an endoskeleton, and it is the only skeleton in soft sponges that encrust such hard surfaces as rocks. More commonly, the mesohyl is stiffened by mineral sponge spicule, by spongin fibers, or both. Most sponges (over 90% of all known species) are , which have the widest range of habitats (including all freshwater ones); they use spongin, silica spicules, or both, and some species have calcium carbonate . Calcareans have calcium carbonate spicules and, in some species, calcium carbonate exoskeletons; they are restricted to relatively shallow marine waters where production of calcium carbonate is easiest. The fragile glass sponge use "scaffolding" of silica spicules and are restricted to polar regions or ocean depths where predators are rare. Fossils of all of these types have been found in rocks dated from . In addition , whose fossils are common in rocks from , are now regarded as a type of sponge. The smallest class of extant sponges are homoscleromorphs, which either have calcium carbonate spicules like the calcereans or are aspiculate, and found in shaded marine environments like caves and overhangs.
Although most of the approximately 5,000–10,000 known species of sponges feed on bacteria and other microscopic food in the water, some host photosynthesis microorganisms as , and these alliances often produce more food and oxygen than they consume. A few species of sponges that live in food-poor environments have evolved as that prey mainly on small .
Most sponges reproduce sexually, but they can also reproduce asexually. Sexually reproducing species release sperm cells into the water to fertilize ovum released or retained by its mate or "mother"; the fertilized eggs develop into which swim off in search of places to settle. Sponges are known for regenerating from fragments that are broken off, although this only works if the fragments include the right types of cells. Some species reproduce by budding. When environmental conditions become less hospitable to the sponges, for example as temperatures drop, many freshwater species and a few marine ones produce , "survival pods" of unspecialized cells that remain dormant until conditions improve; they then either form completely new sponges or recolonize the skeletons of their parents.
The few species of demosponge that have entirely soft fibrous skeletons with no hard elements have been used by humans over thousands of years for several purposes, including as padding and as cleaning tools. By the 1950s, though, these had been overfishing so heavily that the industry almost collapsed, and most sponge-like materials are now synthetic. Sponges and their microscopic endosymbionts are now being researched as possible sources of medicines for treating a wide range of diseases. have been observed using sponges as tools while foraging.
Even if a few sponges are able to produce mucus – which acts as a microbial barrier in all other animals – no sponge with the ability to secrete a functional mucus layer has been recorded. Without such a mucus layer their living tissue is covered by a layer of microbial symbionts, which can contribute up to 40–50% of the sponge wet mass. This inability to prevent microbes from penetrating their porous tissue could be a major reason why they have never evolved a more complex anatomy.
Like (jellyfish, etc.) and Ctenophora (comb jellies), and unlike all other known metazoans, sponges' bodies consist of a non-living jelly-like mass (mesohyl) sandwiched between two main layers of cells. Cnidarians and ctenophores have simple nervous systems, and their cell layers are bound by internal connections and by being mounted on a basement membrane (thin fibrous mat, also known as "basal lamina"). Sponges do not have a nervous system similar to that of vertebrates but may have one that is quite different. Their middle jelly-like layers have large and varied populations of cells, and some types of cells in their outer layers may move into the middle layer and change their functions.
Other types of cells live and move within the mesohyl:
Many larval sponges possess neuron-less that are based on . They mediate phototaxic behavior.
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 Spider web that contains most of the cell types. This tissue is a syncytium that in some ways behaves like many cells that share a single external Cell membrane, and in others like a single cell with multiple cell nucleus.
Although the layers of and 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.
The simplest body structure in sponges is a tube or vase shape known as "asconoid", but this severely limits the size of the animal. The body structure is characterized by a stalk-like spongocoel surrounded by a single layer of choanocytes. If it is simply scaled up, the ratio of its volume to surface area 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 in diameter.
Some sponges overcome this limitation by adopting the "syconoid" structure, in which the body wall is . 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 a few centimeters in diameter.
The "leuconoid" 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 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 are similar to the leuconid structure.
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. For example, in Leuconia, a small leuconoid sponge about tall and 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.
[[File:Porifera calcifying 01.png|thumb|Sponge with calcium carbonate skeleton.
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Some sponges also secrete that lie completely outside their organic components. For example, ("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 that form the animals' skins.
It used to be claimed that 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. Collar bodies digest food and distribute it wrapped in vesicles that are transported by dynein "motor" molecules along bundles of that run throughout the syncytium.
Sponges' cells absorb oxygen by diffusion from water into cells as water flows through body, 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.
Most known carnivorous sponges have completely lost the water flow system and . However, the genus Chondrocladia uses a highly modified water flow system to inflate balloon-like structures that are used for capturing prey.
A recently discovered carnivorous sponge that lives near hydrothermal vents hosts Methanotrophic bacteria and digests some of them.
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. make gemmules by wrapping shells of spongin, often reinforced with spicules, round clusters of that are full of nutrients. Freshwater gemmules may also include photosynthesizing symbionts. (2025). 9780471358374, John Wiley and Sons. ISBN 9780471358374 The gemmules then become dormant, and in this state can survive cold, drying out, lack of oxygen and extreme variations in salinity. Freshwater gemmules often do not revive until the temperature drops, stays cold for a few months and then reaches a near-"normal" level. When a gemmule germinates, the archeocytes round the outside of the cluster transform into , 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. 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.
A few species release fertilized eggs into the water, but most retain the eggs until they hatch. By retaining the eggs, the parents can transfer symbiotic microorganisms directly to their offspring through vertical transmission, while the species who release their eggs into the water has to acquire symbionts horizontally (a combination of both is probably most common, where larvae with vertically transmitted symbionts also acquire others horizontally). There are four types of larvae, but all are lecithotrophic (non-feeding) balls of cells with an outer layer of cells whose flagella 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.
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.
Sponges contain very similar to those that contain the "recipe" for the post-synapse 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.
Sponges are more abundant but less diverse in temperate waters than in tropical waters, possibly because organisms that prey on sponges are more abundant in tropical waters. are the most common in polar waters and in the depths of temperate and tropical seas, as their very porous construction enables them to extract food from these resource-poor waters with the minimum of effort. and Calcarea are abundant and diverse in shallower non-polar waters. (1996). 9780521336659, Cambridge University Press. ISBN 9780521336659
The different classes of sponge live in different ranges of habitat:
A few species, including the Caribbean fire sponge Tedania ignis, cause a severe rash in humans who handle them. Turtles and some fish feed mainly on sponges. It is often said that sponges produce against such predators. However, experiments have been unable to establish a relationship between the toxicity of chemicals produced 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. However, some studies have shown fish showing a preference for non-chemically-defended sponges, and another study found that high levels of coral predation did predict the presence of chemically defended species.
produce no toxic chemicals, and live in very deep water where predators are rare.
of the genus Synalpheus form colonies in sponges, and each shrimp species inhabits a different sponge species, making Synalpheus one of the most diverse genera. Specifically, Synalpheus regalis utilizes the sponge not only as a food source, but also as a defense against other shrimp and predators. As many as 16,000 individuals inhabit a single loggerhead sponge, feeding off the larger particles that collect on the sponge as it filters the ocean to feed itself. Other crustaceans such as hermit crabs commonly have a specific species of sponge, Pseudospongosorites, grow on them as both the sponge and crab occupy gastropod shells until the crab and sponge outgrow the shell, eventually resulting in the crab using the sponge's body as protection instead of the shell until the crab finds a suitable replacement shell.
The hypothesis has been made that coral reef sponges facilitate the transfer of coral-derived organic matter to their associated detritivores via the production of sponge detritus, as shown in the diagram. Several sponge species are able to convert coral-derived DOM into sponge detritus,Rix L, de Goeij JM, Mueller CE, Struck U and others (2016) "Coral mucus fuels the sponge loop in warm- and coldwater coral reef ecosystems". Sci Rep, 6: 18715. and transfer organic matter produced by corals further up the reef food web. Corals release organic matter as both dissolved and particulate mucus, as well as cellular material such as expelled Symbiodinium.
Organic matter could be transferred from corals to sponges by all these pathways, but DOM likely makes up the largest fraction, as the majority (56 to 80%) of coral mucus dissolves in the water column, and coral loss of fixed carbon due to expulsion of Symbiodinium is typically negligible (0.01%) compared with mucus release (up to ~40%). Coral-derived organic matter could also be indirectly transferred to sponges via bacteria, which can also consume coral mucus.
The term for this specific symbiotic relationship, where a microbial consortia pairs with a host is called a Holobiont. The sponge as well as the microbial community associated with it will produce a large range of secondary that help protect it against predators through mechanisms such as chemical defense. The sponge holobiont is an example of the concept of nested ecosystems. Environmental factors act at multiple scales to alter microbiome, holobiont, community, and ecosystem scale processes. Thus, factors that alter microbiome functioning can lead to changes at the holobiont, community, or even ecosystem level and vice versa, illustrating the necessity of considering multiple scales when evaluating functioning in nested ecosystems.
Some of these relationships include endosymbionts within bacteriocyte cells, and cyanobacteria or microalgae found below the pinacoderm cell layer where they are able to receive the highest amount of light, used for phototrophy. They can host over 50 different microbial phyla and candidate phyla, including Alphaprotoebacteria, Actinomycetota, Chloroflexota, Nitrospirota, "Cyanobacteria", the taxa Gamma-, the candidate phylum Poribacteria, and Thaumarchaea.
The phylum Porifera is further divided into classes mainly according to the composition of their :
In the 1970s, sponges with massive calcium carbonate skeletons were assigned to a separate class, Sclerospongiae, otherwise known as "coralline sponges". (cited by MGG.rsmas.miami.edu). However, in the 1980s, it was found that these were all members of either the Calcarea or the Demospongiae.
So far scientific publications have identified about 9,000 poriferan species, of which about 400 are glass sponges, about 500 are calcareous species, and the rest are demosponges. 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, and may harbor many more species.
Sponges are divided into classes mainly according to the composition of their : These are arranged in evolutionary order as shown below in ascending order of their evolution from top to bottom:
| ! Class !! Type of cells!! Sponge spicule !! Spongin fibers !! Massive exoskeleton !! Body form |
Freshwater sponges appear to be much younger, as the earliest known fossils date from the Mid-Eocene period about . Although about 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. The earliest sponge symbionts are known from the Llandovery epoch.
A chemical tracer is 24-isopropyl cholestane, which is a stable derivative of 24-isopropyl cholesterol, which is said to be produced by but not by ("true animals", i.e. and ). Since are thought to be animals' closest single-celled relatives, a team of scientists examined the biochemistry and of one choanoflagellate species. They concluded that this species could not produce 24-isopropyl cholesterol but that investigation of a wider range of choanoflagellates would be necessary in order to prove that the fossil 24-isopropyl cholestane could only have been produced by demosponges. Although a previous publication reported traces of the chemical 24-isopropyl cholestane in ancient rocks dating to , recent research using a much more accurately dated rock series has revealed that these biomarkers only appear before the end of the Marinoan glaciation approximately , and that "Biomarker analysis has yet to reveal any convincing evidence for ancient sponges pre-dating the first globally extensive Neoproterozoic glacial episode (the Sturtian, ~ in Oman)". While it has been argued that this 'sponge biomarker' could have originated from marine algae, recent research suggests that the algae's ability to produce this biomarker evolved only in the Carboniferous; as such, the biomarker remains strongly supportive of the presence of demosponges in the Cryogenian.
, which some classify as a type of coralline sponge, are very common fossils in rocks from the Early Cambrian about , but apparently died out by the end of the Cambrian . It has been suggested that they were produced by: sponges; ; algae; ; a completely separate phylum of animals, Archaeocyatha; or even a completely separate kingdom of life, labeled Archaeata or Inferibionta. Since the 1990s, archaeocyathids have been regarded as a distinctive group of sponges.
It is difficult to fit 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) such as Darwinella. 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. free text at In 2008, a detailed analysis of chancelloriids' sclerites concluded that they were very similar to those of , mobile bilaterian animals that looked like in chain mail and whose fossils are found in rocks from the very Early Cambrian to the Mid Cambrian. If this is correct, it 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.
Analyses since 2001 have concluded that Eumetazoa (more complex than sponges) are more closely related to particular groups of sponges than to other sponge groups. Such conclusions imply that sponges are not monophyletic, because the last common ancestor of all sponges would also be a direct ancestor of the Eumetazoa, which are not sponges. A study in 2001 based on comparisons of ribosome DNA concluded that the most fundamental division within sponges was between and the rest, and that Eumetazoa are more closely related to Calcarea (those with calcium carbonate spicules) than to other types of sponge. 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 the calcareous sponges, which in turn are more closely related to Eumetazoa.
Other anatomical and biochemical evidence links the Eumetazoa with Homoscleromorpha, a sub-group of demosponges. A comparison in 2007 of cell nucleus DNA, excluding glass sponges and Ctenophora, concluded that:
The analyses described above concluded that sponges are closest to the ancestors of all Metazoa, of all multi-celled animals including both sponges and more complex groups. However, another comparison in 2008 of 150 genes in each of 21 genera, ranging from fungi to humans but including only two species of sponge, suggested that ctenophora (ctenophora) are the most basal lineage of the Metazoa included in the sample. 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 range of sponges and other simple Metazoa such as Placozoa.
However, reanalysis of the data showed that the computer algorithms used for analysis were misled by the presence of specific ctenophore genes that were markedly different from those of other species, leaving sponges as either the sister group to all other animals, or an ancestral paraphyletic grade. 'Family trees' constructed using a combination of all available data – morphological, developmental and molecular – concluded that the sponges are in fact a monophyletic group, and with the form the sister group to the bilaterians.
A very large and internally consistent alignment of 1,719 proteins at the metazoan scale, published in 2017, showed that (i) sponges – represented by Homoscleromorpha, Calcarea, Hexactinellida, and Demospongiae – are monophyletic, (ii) sponges are sister-group to all other multicellular animals, (iii) ctenophores emerge as the second-earliest branching animal lineage, and (iv) placozoans emerge as the third animal lineage, followed by Planulozoa.
In March 2021, scientists from Dublin found additional evidence that sponges are the sister group to all other animals, while in May 2023, Schultz et al. found patterns of irreversible change in genome synteny that provide strong evidence that ctenophora are the sister group to all other animals instead.
Many objects with sponge-like textures are now made of substances not derived from poriferans. Synthetic sponges include personal and household cleaning tools, , and contraceptive sponges. Typical materials used are cellulose foam, polyurethane foam, and less frequently, silicone foam.
The luffa "sponge", also spelled loofah, which is commonly sold for use in the kitchen or the shower, is not derived from an animal but mainly from the fibrous "skeleton" of the Luffa aegyptiaca ( Luffa aegyptiaca, Cucurbitaceae).
Lacking any protective shell or means of escape, sponges have evolved to synthesize a variety of unusual compounds. One such class is the oxidized fatty acid derivatives called . Members of this family have been found to have anti-cancer, anti-bacterial and anti-fungal properties. One example isolated from the Okinawan Plakortis sponges, plakoridine A, has shown potential as a cytotoxin to murine lymphoma cells.
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