Sponges (Porifera, Poriferans)
General Features
Sponges are either asymmetrical or exhibit fractal (branching) or radial symmetry. They are at the cellular grade of construction and have no organs (as such), no mouth or digestive tract and no nervous tissue. Radial forms have vase-like shapes, but most sponges are irregular or plant-like and of modular construction, forming flat, rounded or branching structures.
The sponge body is permeated by pores, canals and chambers through which a water current flows. Numerous channels permeate the interior; some or all of which are lined by choanocytes (collar cells). The incurrent openings (ostia) are numerous and small, whilst the excurrent openings (oscula) are few and large. The choanocytes resemble choanoflagellates and possess a single flagellum encircled at its base by a protoplasmic collar.
Most sponges have an internal skeleton, secreted by mesenchymal amoebocytes and composed of separate crystalline bodies (spicules) or organic fibres, or both, and may incorporate foreign particles. The spicules are made of carbonate in some sponges and of silicate in others.
All sponges are sessile and the adults are incapable of locomotion, and live fastened at their base to rocks, shells, timbers, corals, plants, etc. Some sponges are not attached and roll about freely on the bottom. Sponges are marine, except for the family Spongillidae that lives in ponds and lakes throughout the world. As sponges grow they spread or branch in a plant-like manner.
External Features
The Calcarea and the Hexactinellida contain the smallest and simplest sponges and have radial symmetry. The body is cylindroid, vase-like and hollow and fastened at one end, whilst narrowing to a single osculum at the other end. Many Choristida and Hadromerina (Demospongiae) are more-or-less spherical with an internal radiating structure. Most sponges, however, are irregular or have fractal symmetry. Sponges vary in size from small to large, and are sometimes cup-shaped, funnel-shaped, or fan-like (flabelliform), but are more often massive, encrusting and irregular or branching structures.
The sponge surface is studded with numerous pores and many larger oscula. The oscula are often borne on the ends of branches or projections or sunk into crater-like depressions. The form of sponges is highly dependent on the substratum and environmental conditions (especially water currents).
All sponges are sessile and the adults are incapable of locomotion, and live fastened at their base to rocks, shells, timbers, corals, plants, etc. Some sponges are not attached and roll about freely on the bottom. Sponges that live on muddy bottoms in deep waters, e.g. the hexactinellids, are fastened to the mud by root bundles of long spicules. Sponges may be stalked.
Sponges vary in height/diameter from a few mm to large rounded masses or upright growths of 1-2 m. Calcareous and siliceous sponges have a bristly and rough exterior, due to the presence of projecting spicules. Horny sponges and some other sponges, however, are slimy or smooth, or have a hard, leathery surface.
Most sponges are of a drab, flesh, or brownish colour, but some are bright orange, yellow, red, blue, violet or black, due to the presence of lipochrome pigment in some of the amoebocytes. Deep-water forms tend to be drab, whilst shallow water forms tend to be more brightly coloured.
Fresh-water sponges are brownish-green due to zoochlorellae. These sponges are encysting or slightly branching and attached to submerged objects and have monaxon siliceous spicules.
Structure
1. The asconoid is the simplest structural type of sponge. The asconoid has radial symmetry and is vase-like. A thin wall surrounds the large central cavity, or spongocoel, which opens at the summit via a single narrowed osculum. The wall consists of an outer monolayer of epidermis of thin, flat cells, and an inner epithelium comprised of a single-layer of choanocytes in loose contact with each other. A layer of mesenchyme (ectomesoderm) is sandwiched between these two epidermal layers and consists of amoebocytes and spicules embedded in a gelatinous matrix.
Numerous incurrent pores, the ostia, extend from the external surface to the spongocoel as intracellular pores; each forming a canal through a tubular cell called a porocyte. The choanocyte flagella generate a water current drawing water through the ostia and into the spongocoel and out through the osculum. This water current supplies the sponge with food and oxygen and removes waste.
The asconoid wall is simple with a complete and continuous choanocyte lining, interrupted only by porocytes. Very few sponges exhibit the asconoid structure.
2. The syconoid is a more complex structure formed by outpushings of the asconoid wall at regular intervals as finger-like projections or radial canals. There are two syconoid structural subtypes. In subtype I these projections may be free and surrounded on the outside by water along their whole length, in which case there are no definite incurrent channels. Alternatively, in subtype II the projections may fuse to form incurrent canals, tubular spaces between adjacent radial canals and the exterior has an epidermal lining. These incurrent canals open to the exterior between the blind outer ends of the radial canals via dermal pores or dermal ostia. The radial canals have a choanocyte lining and form, flagellated canals. The large spongocoel is lined by flat epithelium derived from the epidermis. The internal ostia are the openings of the radial canals into the spongocoel.
Syconoids are radial and vase-like with a single terminal ostium. The prosopyles are pores between the incurrent and radial canals and may correspond to the ostia of asconoids (intracellular? Apparently the prosopyles are intercellular spaces).
Flow of water is in through the dermal pores → incurrent canals → prosopyles → radial canals → internal ostia → spongocoel → osculum → outside.
Syconoid subtype I includes a few heterocoelous calcareous sponges, e.g. Sycon. In subtype II the epidermis and mesenchyme are spread over the outer surface to form a cortex (thin or thick) which may contain special cortical spicules and has definite dermal pores. The dermal pores open into either narrowed incurrent canals that have an irregular course and may branch and anastomose or into subdermal spaces that are the outer ends of the flagellated canals.
3. The most complex structural type of sponge is the leuconoid type. This is formed by continued outfolding of the choanocyte layer. The choanocytes are limited to clusters of small, round or oval chambers. Mesenchyme fills the spaces around the flagellated chambers. There is usually no spongocoel. Leuconoids have an irregular, indefinite form permeated by a maze of water channels.
The surface of the leuconoid sponge is usually covered by epidermis (not in hexactinellids) that is pierced by dermal pores and oscula. The flow of water is in through the dermal pores into either incurrent passages (branching irregularly through the mesenchyme) or into large subdermal spaces crossed by columns of spicules. Both incurrent passages and subdermal spaces open via prosopyle openings into small, rounded flagellated chambers. Water exits the flagellated chambers via the apopyle apertures into the excurrent channels that unite into larger and larger tubes that discharge to the outside through the oscula.
There are three structural subtypes. In the eurypylous system the apopyles open directly via wide mouths into the excurrent channels. In the aphodal system a narrow canal, or aphodus, connects the flagellated chamber to the excurrent channel. In the diplodal system, a narrow tube (prosodus) connects the incurrent channel and the flagellated chamber and there is also an aphodus connecting the chamber to the excurrent channel.
Often, the leuconoid body has an outer ectosome region devoid of flagellated chambers and an inner choanosome (endosome) containing clusters of flagellated chambers. The ectosome comprises a cortex or dermal membrane and subdermal spaces. The cortex differs from the choanosome in histology and spicule type or spicule arrangement. The dermal membrane consists of an epidermis and a thin mesenchyme stratum.
The leuconoid is characterised by the limitation of choanocytes to small chambers and the great development of its mesenchyme and the complexity of its incurrent and excurrent water passages.
Water flows in via the dermal ostia → subdermal spaces and incurrent channels → prosodus if present → prosopyle pores → flagellated chambers → apopyles → aphodus if present → excurrent canals → larger channels → out via oscula.
The leuconoid sponge may develop through the asconoid and syconoid stages, but most leuconoids develop from a rhagon stage. The rhagon is conical, tapering from a broad base to a single osculum at the summit. The rhagon possesses a spongocoel bordered by oval flagellated chambers opening into it by wide apopyles.
The majority of sponges are of the leuconoid type since this structure allows them to generate a more efficient water current and to attain a larger size. The leuconoid form probably evolved several times.
Histology
Sponges contain the following principal cell types: epidermal cells, muscle cells, gland cells, choanocytes and mesenchymal amoebocytes.
Some sponges possess an epidermis of large, flat, polygonal epithelial cells called pinacocytes. (Endo)pinacocytes line the incurrent canals and the spongocoel of syconoids and the larger canals and spaces of leuconoids. Pinacocytes are highly contractile and can greatly reduce the surface area of the sponge. Many sponges have a syncytial epidermis (epithelioid membrane). However, the Hexactinellida have no definite epidermis.
In asconoids, pore cells or porocytes are tubular cells pierced by an intracellular incurrent canal or pore. Pinacocytes are highly contractile and can close the pore by advancing a thin sheet of cytoplasm (pore diaphragm) from the edge to the centre at the outer end of the canal. The dermal pores of syconoids and leuconoids are of a disputed nature, but are closable. Freshwater sponges have dermal pores that begin as porocytes, each of which disappears later to leave a pore in the epidermis. The prosopyles are analogous to the incurrent pores of ascon sponges (but are they intracellular or extracellular?)
The mesenchyme is a transparent gelatinous matrix (mesogloea) containing free amoebocytes. The mesenchyme may be a collenchyma with few cells, or a parenchyma with a high cell density. The amoebocytes are free to wander about the sponge and fall into two main classes, lobopodous amoebocytes and colllencytes. Lobopodous amoebocytes include pigmented chromocytes, thesocytes that store food reserves and scleroblasts that secrete the skeleton. Scleroblasts are further divided into calcoblasts, silicoblasts and spongioblasts, depending on the nature of the skeletal material secreted. Collencytes have slender branching pseudopods and may form a syncytial network.
Archaeocytes are lobopodous amoebocytes each with a large nucleus, conspicuous nucleolus and sometimes containing cytoplasmic inclusions. Archaeocytes are possibly undifferentiated cells and produce the sex cells (as may choanocytes in some sponges?) and are involved in regeneration since they can give rise to all other sponge cell types.
Gland cells put out long strands to the sponge surface and possibly secret slime (amoebocytes also secrete slime).
Desmacytes (fibre cells) are found in layers in the cortex and around larger internal channels, and are common in the Desmospongiae. Myocytes are fusiform muscle cells that resemble smooth muscle cells and usually form a sphincter at the osculum and other openings.
Choanocytes (collar cells) are rounded or oval cells with their base resting on the mesenchyme. Their free end bears a transparent contractile collar encircling the base of the long single flagellum. Choanocytes are larger in the Calcarea.
Skeleton
The mesenchyme secretes and contains the skeleton. The skeleton consists of spicules and/or spongin fibres. The spicules (sclerites) are crystalline bodies and each is a spine or a number of spines radiating from a point. Each spicule consists of an organic axis surrounded by calcium carbonate or hydrated silica. Megascleres are the larger spicules that from the main supporting framework. Microscleres are smaller flesh spicules strewn throughout the mesenchyme. However, such a size distinction does not hold for calcareous sponges and some other groups.
Spicules are classified according to the number of spines or axes as follows.
- Monaxon spicules have a single axis, straight or curved. The style (monactinal monaxon) results from growth in one direction only and is often rounded at one end (strongylote end) and pointed (oxeate) at the other. Tylostyles are styles that have the broad end knobbed. Acanthostyles are styles that are covered with thorny processes. The pointed end of each style usually projects to the exterior of the sponge. Diactinal monaxons (diactines, rhabds) result from growth in both directions from a central point. Oxeas are pointed at each end, tornotes are lance-headed at each end, strongyles are rounded at each end and tylotes have a pinhead like knob at each end. Microscleres have the prefix micro-, e.g. microrhabds, microxeas, microstrongyles. The Sigmatophora and Poecilosclerina have curved types of diactinal miscroscleres, including C-shaped sigmas and bow-shaped toxas and chelas with recurved hooks, plates or flukes at each end. Isochelas are chelas with both ends alike, whilst anisochelas have unlike ends. Sigmaspires are spirally twisted sigmas. Streptasters are short, spiny, microscleric monaxons and include spirally twisted spirasters, rod-shaped sanidasters, plesioasters that have a few spines radiating from a very short axis, and amphiasters that have spines at each end.
- Tetraxons (tetractines, quadriradiates) have four rays radiating from a common point. These rays are not in the same planes. Tetraxons may lose 1-3 of their rays. Calthrops tetraxons have equal rays. Microcalthrops are microscleric calthrops. Triaenes have one elongated ray, the rhabdome, and three short rays (the cladi or clads) constitute the cladome. Developmental loss of one cladi results in a diaene spicule. The cladome may be a simple or scalloped disc instead of separate cladi. If such a disc occurs at both ends of the rhabdome, then the resulting spicule is a birotular spicule or amphidisc. The triradiate or triactinal spicule is the most common spicule type in calcareous sponges and is a variety of triaene in which the rhabdome is lost to leave a cladome of three rays that lie almost in one plane.
- Triaxon (hexactinal spicules) have three axes crossing at right-angles to give six rays, some of which may be lost or reduced or branched or curved and may have spines or knobs, etc. These spicules occur only in the class Hexactinellida.
- Polyaxons have several equal rays that radiate from a central point. Asters are microscleres (euasters as opposed to streptasters) and include oxyasters that have a small centre and long pointed rays; strongylasters that have a small centre and long rays with rounded ends; and tylasters that have a small centre and long rays with knobbed ends. Polyaxons with a large centre and short rays are the spherasters, with definite rays, and the sterrasters in which the rays are small projections from the spherical surface.
- Spheres form from concentric growth around the centre.
- The desma is a megasclere formed from a minute monaxon, triradiate or tetraxon spicule, forming the central crepis, upon which layers of silica are deposited. These deposits develop branches and tubercles. Desmas are named after the crepis shape, as monocrepid, tricrepid or tetracrepid. Desmas are usually united into a network to form a reticulated skeleton (lithistid).
Spongin is a protein that forms a branching network. In the Monoaxonida, spongin often binds the siliceous spicules together. In the Keratosa the skeleton consists entirely of spongin (and embedded foreign particles).
Spicules are secreted by scleroblasts. This secretion process is best known for calcareous spicules. A monaxon spicule begins inside a binucleate cell as an organic axial thread between the nuclei, which is coated in calcium carbonate within a clear space inside the scleroblast. The nuclei draw apart as the spicule lengthens and the cell separates into two daughters. One daughter is the thickener and is situated at the outer end of the spicule, which is usually the projecting end. The other daughter is the founder and is situated at the inner end of the spicule and deposits most of the spicule as it moves inward, its secretion is supplemented by that of the thickener. Upon completion of the spicule, both cells wander off into the mesogloea (the founder leaves first).
Triradiate calcareous spicules are formed by three scleroblasts that come together in a trefoil. Each scleroblast divides into a founder at the inner end and a thickener at the outer end and three spicules are secreted joined together as the founders move towards the tips. The three thickeners remain awhile at the junction as they thicken the spicule. Quadriradiate spicules are formed by an additional scleroblast adding a 4th ray to a triradiate spicule.