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Sponges
Sponges have thrived from their origination in the Precambrian through to today. They have changed little in the past 600 Million years, and are found throughout the fossil record.
They are of particular interest within this site because they were the prime source of the biogenic silica that formed the flints.
The following discussion covers:
- Introduction
- Origins of Sponges
- Appearance
- Morphology
- Feeding
- Reproduction
- Living (habitat)
- Taxonomic Classification
- Today, and Man's use
Introduction
"Sponge" is the common name for the Poriferan, who are members of the phylum Porifera. It is named for the pores with which every sponge is covered, 'porifera' meaning 'pore bearer'.
Sponges come in an incredible variety of colours and an amazing array of shapes. They probably achieved their greatest diversity during the Cretaceous and between 5 000 and 10 000 species live today - depending on which source you refer to. They are the simplest form of the multicellular organisms and all are aquatic, benthic organisms. Most are marine, with only 150 fresh water species known today. They are known to be present in all seas and in several lakes.
Sponges are divided into three distinct groups, as follows:
Hexactinellida, or glass sponges. These are characterized by siliceous spicules consisting of six rays intersecting at right angles.
Demospongia. This is by far the most diverse sponge group, although they are not well represented in the fossil record, as they do not possess skeletons that would easily fossilize.
Demosponge skeletons are composed of 'spongin' fibres and/or siliceous spicules.
Demosponges take on a variety of growth forms from encrusting sheets living beneath stones to branching stalks upright in the water column. They tend to be large and only exhibit the more complex 'leucon' grade of organization.
Calcarea. Members of the group Calcarea, the calcareous sponges, are the only sponges that possess spicules composed of calcium carbonate.
Sponge Origins
Poriferans have been numerous in the seas since the Precambrian era, and despite the paucity of the fossil record that far back, much can still be concluded from it. However, their origins are still uncertain.
An early branching event in the history of animals separated the sponges from the Metazoans, which were the ancestors of all other animals. Thereafter, sponges evolved independently, and since the Precambrian, they have been conspicuous members of many fossil communities. The number of described fossil genera exceeds 900, which is a tiny proportion of all that would have lived.
The most popular theory of the origin of sponges suggests that they might have derived from colonial 'choanoflagellate protozoans'. As the basal branch of the Metazoans, Poriferans have long been expected to be among the earliest undoubted animal fossils, those of the Vendian (or Ediacaran).
Until recently, however, no sponge body fossils had been identified or described from this fauna. However, in 1996, the first probable sponge, Paleophragmodictya, from the Ediacara of Australia was identified and described. Their specimens revealed a reticulating net of spicules in the sponge body wall.
Throughout most of the Phanerozoic, sponges were major contributors to reef formation.
Appearance
Although sponges are plant-like in appearance, they are in fact animals, although of a phylum that separated early on from the rest of the Animalia Kingdom. Many reef sponges resemble corals in shape and colour, but the difference is apparent on close inspection.
Despite being animals, sponges are almost always stationary and have no independent mobility. They live anchored to a hard surface.
Sponges come in infinite colours and shapes, and they vary in size from tubes only a few millimetres wide to over a metre tall, or two meters wide.
Informally, sponges can be divided into 2 types, encrusting or free-standing.
Encrusting sponges are similar to moss, and they tend to cover the surfaces of rocks in sheets or volcano-shaped mounds. These are more likely to grow in turbulent, rapidly-moving water.
Free-standing sponges have lots of 'inner volume' compared with their surface area, having a bag-like body with a central cavity.
Free-standing sponges have the greatest variety of shapes, from fans, cups and tubes to branching vases and candlesticks, and upright sheets reminiscent of elephant ears. They can grow to gigantic sizes - the tropical 'barrel sponge' can grow large enough to fit a person inside, and some can grow to over 2 metres in diameter. Even the cold Antarctic waters support the Scolymastra joubini, a barrel-like glass sponge that can grow to over 1 metre in height.
Morphology
Sponges lack symmetry, and unlike all other marine invertebrates, they have no true tissues or organs. Neither do they have nerve or muscle cells.
The body of a sponge is hollow, and it is composed of a simple aggregation of cells, between which there is little nervous coordination.
However, these cells perform a variety of bodily functions and appear to be more independent of each other than are the cells of other animals.
This gelatinous matrix is supported by an internal skeleton of spicules of silica, calcium carbonate, or fibrous protein known as 'spongin'.
The sponge's body encloses a vast network of chambers and canals that connect to the open pores on their surface.
The Complexity of Organisation
Often, sponges are distinguished by the level of complexity exhibited in their organisation. Three such organisational forms have been identified. They are not however distinct, but are levels of complexity that grade one into the other.
ascon: The simplest form consists of a single tube two cell layers thick. Poriferans with this type of architecture are necessarily very small due to 'surface area to volume' constraints.
sycon: In order for a sponge to attain greater size, the sponge wall must be folded in on itself. A simple folding of the wall yields a sponge body with sycon organization.
leucon: The vast majority of sponges are organized in a more complex way. In this condition, the sponge body has folds that are themselves folded, resulting in a series of flagellated chambers connected by canals.
Water Circulation
The open pores on the surface of the sponge come in two types. The 'entry' cells are known as ostia, while the exit cells, which are bigger, are the oscula, often termed 'excurrent'. Specialised cells, the choanocytes, allow water into the ostia.
Water flows through the body of the sponge, entering at the ostia, circulating around the canals and chambers, then leaving from the oscula.
The flow of water through the sponge is unidirectional, driven by the beating of flagella which line the surface of the chambers.
The water movement through some sponges is aided by ambient currents passing over raised 'excurrent' openings, the oscula. This moving water creates an area of low pressure above the excurrent openings that assists in drawing water out of the sponge. Sponges are capable of regulating the amount of flow through their bodies by the constriction of various openings. The volume of water passing through a sponge can be enormous, up to 20 000 times its volume in a single 24-hour period.
Feeding
Sponges feed by drawing a current of seawater in through their entry pores, the oscia, filtering out food particles, then ejecting the water out through their exit pores, the oscula. This flowing water provides both food and oxygen, as well as being a means for waste removal.
In general, sponges feed by filtering bacteria from the water that passes through them. Some sponges trap about 90% of all bacteria in the water they filter, but others appear to be less efficient at capturing bacteria. These may specialize in feeding on other organisms or smaller bits of organic matter. Still other sponges harbour symbiotic unicellular algae such as green algae, dinoflagellates, or cyanobacteria, which supply them with food and allow salt accumulation for the skeleton.
Sponges are characterized by the possession of a feeding system unique among animals. Unlike all other animals, Poriferans do not have mouths. Instead, they simply pump water through their bodies, drawing it in and expelling it via pores
Cells in the sponge walls filter food from the water as it is pumped through the body.
The canals are lined with special net-like collar cells, which absorb the nutrients and oxygen from the water whilst expelling carbon dioxide and waste products into it.
Single cells digest the nutrients thus obtained.
Inside the sponge, the water flows through canals to a spacious chamber called a spongocoe. Deprived of its nutrients, the water is then expelled through the oscula.
The Carnivorous exception
Sponges of the family Cladorhizidae are unusual in that they typically feed by capturing and digesting whole animals. They capture small crustaceans with their spicules, which 'hook onto' and attach to crustacean exoskeletons which they may come in contact with the. Sponge cells then migrate around the helpless prey, and digestion takes place outside the cells - or 'extracellularly'.
Reproduction
In the Porifera, reproduction is both sexual, and asexual.Sexual Reproduction
Most Poriferans that reproduce by sexual means are hermaphroditic, so the same individual releases both sperm and eggs. They are released at different times to avoid self-fertilisation.
The sperm of sponges are frequently 'broadcast' into the water column. A sponge creates and concentrates the sperm, before expelling it from the excurrent (oscula) pores. The masses of sperms thus released are sometimes so dense that the sponge appears to be smoking. The sperm are then carried by the water current towards another sponge.
There, they are captured by the female cells, the choanocytes.
Inside the female, the sperm are transported to eggs by special. Fertilization occurs, and the resulting [zygotes] [fertilised eggs] develop into [ciliated] larvae.
Some sponges retain their larvae for some times, while others release them almost immediately. Because these larvae are coated with cilia (hairs), they can swim freely before attaching themselves to the substrates, which will become their permanent homes.
Once attached, the larvae develop into juvenile sponges.
Asexual Reproduction
Sponges that reproduce asexually produce either buds or, more often, gemmules, which are packets of several cells of various types inside a protective covering. Fresh water sponges of the Spongillidae often produce gemmules prior to winter. The development of these gemmules into adult sponges begins the following spring.
Some buds are resistant to drought, and can develop on their return to water.
Living habits
Sponges are the simplest form of multi-cellular animals and are a predominantly marine species. They are benthic [bottom-dwelling] creatures which usually live attached to hard substrates, in poorly illuminated habitats such as caves and crevices, in places where enough food can be found in order to allow it to survive and grow. In tropical seas, sponges compete with the corals in the construction of reefs. Many other small animals can be housed in their pores and canals.
Generally, sponges are [sessile], though it has been shown that some are able to move slowly (up to 4 mm per day) within aquaria. It is unknown whether this movement is important for sponge ecology under natural conditions.
Most sponges attach themselves to a hard surface, where they spend their lives. Usually, this is rock, but some sponges bore into the shells of bivalves, gastropods, and the colonial skeletons of corals by slowly etching away chips of calcareous material. Alternatively, a sponge may settle on a snail shell, which is already being occupied by a hermit crab. This unusual association of sponge and crab allows the sponge to move about, albeit in a piggyback fashion over which it has no control. (Described and studied by Floyd Sandford).
This is just one of the infinite examples of the interconnected web of life, where 'reuse and recycling' is pervasive.
Sponges can live in both calm, and turbulent, waters. If the sponge lives in areas subjected to strong wave action or to water currents, it can be flattened, globose or encrusting. This reduces its resistance to water, and saves it from being dragged away from its anchor. In stiller waters, sponges can acquire more convoluted, upstanding forms.
A notable exception to the more common marine species is the family Spongillidae, an extant group of fresh-water demosponges whose fossil record begins in the Cretaceous.
Sponges are ubiquitous, and can be found at all latitudes in all of the world's oceans, and at all depths from the inter-tidal to the deep-sea.
Taxonomic Classification
The Porifera are sometimes placed into the Subkingdom Parazoa of Animalia, while all the other animals are in the Metazoa.
As described above, they are divided into three distinct groups, the Hexactinellida (glass sponges), the Demospongia, and the Calcarea (calcareous sponges).
The classification of sponges once relied on the characters of the spicules and the fibres, since the outer shape and the colour vary with the habitat.
However, the classification is now more complex. The following extract, from the systematics page of the excellent Berkeley University description of Sponges, explains the development of the current taxonomic classification of sponges.
At one time, a diagnostic feature of the Porifera was the presence of spicules. As a result, certain fossil groups whose organization was consistent with that of living sponges were not placed within the phylum Porifera. In particular, groups with a solid calcareous skeleton such as the Archaeocyatha, chaetetids, sphinctozoans, stromatoporoids, and receptaculids were problematic. A great deal of insight into the phylogenetic affinities of these groups was gained with the discovery of more than 15 extant species of sponges having a solid calcareous skeleton. These species are diverse in form, and would be classified with the chaetetids, sphinctozoans and stromatoporoids if found as fossils. However, with the living material in hand, histological, cytological, and larval characteristics can be observed. This information suggests that these 15 species can readily be placed within the Calcarea and the Demospongia. This radically changes our view of poriferan phylogeny.
It is widely accepted among poriferan biologists that the Calcarea and the Demospongia are more closely related to each other than either is to the Hexactinellida. With the discovery of living chaetetids, stromatoporoids, and sphinctozoans, a fourth class was erected for these so-called sclerosponges. However, the Sclerospongia is not a natural monophyletic grouping and is thus being abandoned. The abundant fossil chaetetids, stromatoporoids, and sphinctozoans are probably part of the classes Demospongia and Calcarea, though some uncertainty still remains. The Archaeocyatha pose a special case. No living representative of this group has been discovered. Their organization is consistent with that of living sponges. The one phylogenetic analysis (carried out by Reitner) that included archaeocyaths with other sponges, grouped them as sisters to the demosponges. Therefore, although the taxonomic term Archaeocyatha is often accorded phylum status it is likely a sub-clade of the phylum Porifera, thereby violating the ranking system.
Today, and Man's usage
Although sponges were once the principle reef-builder, today, the corals dominate - an exception, rather than the rule. Sponges are certainly important ecological constituents of reef communities, but they do not commonly contribute to the construction of reef frameworks as has been the case during most of their history.
In the past - in the terms of Man's existance and his exploitation of the planet and its produce - fishing of Poriferans was widespread. The sponges were used as bath sponges, or for particular uses in painting, jewellery and surgery. Sponge fishing is now in decline as synthetic sponges are generally preferred.
Here endeth the section of the Rocks that make up Portsdown Hill. Please return to the Home page, or use the navigation links on your left.
For any comments, suggestions or contributions, please e-mail me at: portsdown@bbm.me.uk