Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'). They are sessile filter feeders that are bound to the seabed, and are one of the most ancient members of macrobenthos, with many historical species being important 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, 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 "collar cells".
Sponges may be the sister group to all other animals, although the evidence for this relationship remains inconclusive. Fossil evidence of primitive sponges such as Otavia dates to as early as the Tonian period (around 800 Mya). The branch of zoology that studies sponges is spongiology.
Etymology
The term sponge derives from the Ancient Greek word . The scientific name Porifera is a neuter plural of the Modern Latin term porifer, which comes from the roots porus meaning "pore, opening", and -fer meaning "bearing or carrying".
Overview
thumb|A [[Aplysina archeri|stove-pipe sponge]]
Sponges are similar to other animals in that they are multicellular heterotrophs that lack cell walls and produce sperm cells. Unlike other animals, they lack true tissues and organs. Some of them are radially symmetrical, but most are asymmetrical. The shapes of their bodies are adapted for maximal efficiency of water flow through the central cavity, where the water deposits nutrients and then leaves through a hole called the osculum. Their choanocyte cells are used to drive their water flow systems and capture most of their food. This, along with phylogenetic studies of ribosomal molecules, has been used as morphological evidence to suggest sponges are the sister group to the rest of animals. A great majority are marine (salt-water) species, ranging in habitat from tidal zones to depths exceeding , though the freshwater sponges inhabit all continents other than Antarctica. All adult sponges are sessile, meaning that they attach to an underwater surface and remain fixed in place (i.e., do not travel). While in their larval stage of life, they are motile.
Many sponges have a mesh-like endoskeleton composed of spicules made of either calcium carbonate or silicon dioxide (silica), embedded within spongin (a modified type of collagen).
Most sponges reproduce sexually, but they can also reproduce asexually. Sexually reproducing species release sperm cells into the water to fertilize ova released or retained by its mate or "mother"; the fertilized eggs develop into larvae 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 gemmules, "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 overfished 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. Dolphins have been observed using sponges as tools while foraging. 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.
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 cnidarians (jellyfish, etc.) and ctenophores (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 a thin fibrous mat, also known as a basal lamina.]]-->
Cell types
A sponge's body is hollow and 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. 18 distinct cell types have been identified. 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 external skin over all other parts of the mesohyl that are not covered by choanocytes, and the pinacocytes also digest food particles that are too large to enter the ostia,
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. |image=Porifera body structures 01.png |width=280 |image-width=280 |height=205
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Most sponges work rather like chimneys: they take in water at the bottom and eject it from the osculum at the top. Since ambient currents are faster at the top, the suction effect that they produce by Bernoulli's principle 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.
thumb|Sponge with calcium carbonate skeleton. 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, 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, 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 microtubules that run throughout the syncytium.]]
In waters where the supply of food particles is very poor, some species prey on crustaceans and other small animals. , a total of 137 species had been discovered. Most belong to the family Cladorhizidae, but a few members of the Guitarridae and Esperiopsidae are also carnivores. Most carnivorous sponges live in deep waters, up to , and the development of deep-ocean exploration techniques is expected to lead to the discovery of several more. However, Lycopodina hypogea has been found in relatively shallow depths of in Mediterranean sea caves. These cave-dwelling predators capture prey using filaments equipped with hooked spicules, and digest them by enveloping them with more threads over the course of a few days.
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. Sponges that host photosynthesizing organisms are most common in waters with relatively poor supplies of food particles and often have leafy shapes that maximize the amount of sunlight they collect.
The different sponge classes regenerate using distinct methods. but they are able to regenerate even from dissociated cells; after being subjected to chemical or mechanical tissue destruction, these dissociated cells reaggregate to begin the process of regeneration, and after reforming the epithelium, gradually reconstruct their anatomy, such as the water circulation system. This "whole-body regeneration" has been observed in demosponges, calcareous sponges, and homoscleromorphs. Cell death appears to have a limited role in whole-body regeneration, mainly occurring while the epithelium reforms.
Glass sponges are also able to regenerate; Rhabdocalyptus dawsoni is able to regenerate at a rate of per day, filling in from the atrial, then dermal side, and closing up to form a scar. This rate is up to 40 times faster than growth of new tissue in the species, which may be due to the cytoplasmic method glass sponges use, as opposed to the slower cellular method in other sponges.
Regeneration is noted to be faster than normal growth, as seen in Aplysilla rosea. Additionally, regeneration at the base appears to occur faster than that occurring at the apex (top) of the sponge, at least in Cinachyrella cf. cavernosa.
Reproduction
Asexual
thumb|right|The [[freshwater sponge Spongilla lacustris]]
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. 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 through a fine cloth to regenerate. The gemmules then become dormant, and in this state can survive cold, drying out, lack of oxygen and extreme variations in salinity. 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.
Meiosis
The cytological progression of porifera oogenesis and spermatogenesis (gametogenesis) is very similar to that of other metazoa. Most of the genes from the classic set of meiotic genes, including genes for DNA recombination and double-strand break repair, that are conserved in eukaryotes are expressed in the sponges (e.g. Geodia hentscheli and Geodia phlegraei). Demosponge Samus anonymus (up to 50 m), hexactinellid Scleroplegma lanterna (~100–600 m), hexactinellid Aulocalyx irregularis (~550–915 m), lithistid demosponge Neoaulaxinia persicum (~500–1700 m)]]
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 per year and, if that rate is constant, specimens 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. 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.
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Sponges have several cell types:
- Archaeocytes (or amoebocytes) have many functions; they are totipotent cells which can transform into sclerocytes, spongocytes, or collencytes. They also have a role in nutrient transport and sexual reproduction.
- Cells are arranged in a gelatinous non-cellular matrix called mesohyll
Sponges have three body types: asconoid, syconoid, and leuconoid.
- Choanocytes (also known as "collar cells") function as the sponge's digestive system, and are remarkably similar to the protistan choanoflagellates. The collars are composed of many microvilli and are used to filter particles out of the water. The beating of the choanocytes' flagella creates the sponge's water current.
- Collencytes secrete collagen.
- Myocytes are modified pinacocytes which control the size of the osculum and pore openings and thus the water flow.
- Pinacocytes which form the pinacoderm, the outer epidermal layer of cells. This is the closest approach to true tissue in sponges
- Porocytes are tubular cells that make up the sponge's pores.
- Sclerocytes secrete calcareous siliceous spicules which reside in the mesohyl.
- Spongocytes secrete spongin, collagen-like fibers which make up the mesohyl.
- Spicules are stiffened rods or spikes made of calcium carbonate or silica which are used for structure and defense.
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Ecology
Habitats
thumb|[[Euplectella aspergillum is a deep ocean glass sponge, seen here at a depth of off the coast of California]]
Sponges are worldwide in their distribution, living in a wide range of ocean habitats, from the polar regions to the tropics.
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. Glass sponges 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. Demosponges and calcareous sponges are abundant and diverse in shallower non-polar waters.
The different classes of sponge live in different ranges of habitat:
:{|class="wikitable"
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! Class !! Water type and another study found that high levels of coral predation did predict the presence of chemically defended species.
Glass sponges produce no toxic chemicals, and live in very deep water where predators are rare.]]
Spongeflies, also known as spongillaflies (Neuroptera, Sisyridae), are specialist predators of freshwater sponges. The female lays her eggs on vegetation overhanging water. The larvae hatch and drop into the water where they seek out sponges to feed on. They use their elongated mouthparts to pierce the sponge and suck the fluids within. The larvae of some species cling to the surface of the sponge while others take refuge in the sponge's internal cavities. The fully grown larvae leave the water and spin a cocoon in which to pupate.
Bioerosion
The Caribbean chicken-liver sponge Chondrilla nucula secretes toxins that kill coral polyps, allowing the sponges to grow over the coral skeletons. The rust-colored bands are caused by a cyanobacterium, but it is unknown whether this organism actually causes the disease.
Collaboration with other organisms
In addition to hosting photosynthesizing endosymbionts,
Shrimps of the genus Synalpheus form colonies in sponges, and each shrimp species inhabits a different sponge species, making Synalpheus one of the most diverse crustacean 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.
Sponge loop
thumb|upright=1.8|right|Sponge loop hypothesis. Steps of the sponge loop pathway: (1) corals and algae release exudates as [[dissolved organic matter (DOM), (2) sponges take up DOM, (3) sponges release detrital particulate organic matter (POM), (4) sponge detritus (POM) is taken up by sponge-associated and free-living detritivores.]]
Most sponges are detritivores which filter organic debris particles and microscopic life forms from ocean water. In particular, sponges occupy an important role as detritivores in coral reef food webs by recycling detritus to higher trophic levels. 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. Coral-derived organic matter could also be indirectly transferred to sponges via bacteria, which can also consume coral mucus.]]
Besides a one to one symbiotic relationship, it is possible for a host to become symbiotic with a microbial consortium, resulting in a diverse sponge microbiome. Sponges are able to host a wide range of microbial communities that can also be very specific. The microbial communities that form a symbiotic relationship with the sponge can amount to as much as 35% of the biomass of its host.
The term for this specific symbiotic relationship, where a microbial consortia pairs with a host is called a holobiotic relationship. The sponge as well as the microbial community associated with it will produce a large range of secondary metabolites 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. For a long time thereafter, sponges were assigned to subkingdom Parazoa ("beside the animals") separated from the Eumetazoa which formed the rest of the kingdom Animalia. 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,
<!-- thumb|A sponge in [[Papua New Guinea]] -->
Sponges are divided into classes mainly according to the composition of their skeletons:!! Spicules
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! Homoscleromorpha
|Single nucleus, single external membrane||Silica ||In many species ||Never ||Sylleibid or leuconoid
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Phylogeny
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The phylogeny of sponges has been debated heavily since the advent of phylogenetics. Originally thought to be the sister group to all other animal phyla, there is now considerable evidence that Ctenophora may hold that title instead. Additionally, the monophyly of the phylum is now under question. Several studies have concluded that all other animals emerged from within the sponges, and usually recover that the calcareous sponges and Homoscleromorpha are closer to other animals than to demosponges. The internal relationships of Porifera have proven to be less uncertain. A close relationship of Homoscleromorpha and Calcarea has been recovered in nearly all studies, whether or not they support sponge or eumetazoan monophyly. An unsubstantiated 2002 report exists of spicules in rocks dated around . 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. The Ediacaran record of sponges also contains two other genera: the stem-hexactinellid Helicolocellus from the Dengying Formation and the possible stem-archaeocyathan Arimasia from the Nama Group. These genera are both from the "Nama assemblage" of Ediacaran biota, although whether this is due to a genuine lack beforehand or preservational bias is uncertain. Fossils of glass sponges have been found from around in rocks in Australia, China, and Mongolia. Early Cambrian sponges from Mexico belonging to the genus Kiwetinokia show evidence of fusion of several smaller spicules to form a single large spicule. Calcium carbonate spicules of calcareous sponges have been found in Early Cambrian rocks from about in Australia. Other probable demosponges have been found in the Early Cambrian Chengjiang fauna, from . Fossils found in the Canadian Northwest Territories dating to may be sponges; if this finding is confirmed, it suggests the first animals appeared before the Neoproterozoic oxygenation event.
thumb|left|Oxygen content of the atmosphere over the last billion years. If confirmed, the discovery of fossilized sponges dating to 890 million years ago would predate the Neoproterozoic Oxygenation Event.
Freshwater sponges appear to be much younger, as the earliest known fossils date from the Mid-Eocene period about .
The earliest sponge symbionts are known from the early Silurian.
A chemical tracer is 24-isopropyl cholestane, which is a stable derivative of 24-isopropyl cholesterol, which is said 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 examined the biochemistry and genes 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.
Archaeocyathids, 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 .