Porifera

Sponges - Porifera

asconoid sponge, Pov-Ray model

Above: a Pov-Ray model of a sponge. Sponges belong to the phylum Porifera, which literally many 'many pores' since the surface of a sponge is covered in minute pores that suck in water and nutrients, which the sponge filters before expelling the water from a large opening or osculum. Sponges are usually brightly coloured - red, orange, purple, green and yellow are common sponge colours, though deep sea forms are often dark brown and drab or glassy. The sponge above is definitely an individual - it has a single large opening, or osculum, which carries a jet of water out of the sponge's body, but many sponges form colonies of many individuals fused together with many oscula, in which case it is not very sensible to talk about an individual sponge (although the tissue surrounding each osculum may define an individual). Sponge bodies may be ball, vase, basket, cup or club-shaped like the one above, or they may be flat and encrusting or branching and tree-like. Indeed, the shape of a sponge is an adaptation to its environment - in rough waters, such as along a rocky coastline, sponges will be flat and encrusting, clinging tightly to any nook and cranny they can find in the rock surface, whilst in calm waters they tend to be more upright and present a larger surface to the water to enhance the rate at which they can sieve food from the water. Sponges may be tiny forms, which are easily overlooked, or they may be large enough for a man to stand inside! Sponges are very unusual animals, representing an off-shoot of the animal kingdom that evolved along its own lines, separate from the vast majority of animal types. It is instructive, therefore, to see how the sponge body is put together and how it works!

The simplest type of sponge is the asconoid type. This sponge has many minute pores opening directly into a single central cavity, or atrium (spongocoel), and one or more large oscula (singular: osculum). The sponge sucks water into the atrium through the many tiny incurrent or inhalent pores and pumps it out through the excurrent or exhalent osculum as a forceful jet of water that may travel 10 feet or more from a large sponge. As this water flows through the sponge body, food particles are filtered from it. These particles include mostly bacteria and other microscopic organisms and organic debris. The simplified anatomy of a small asconoid sponge is shown below (most sponges will contain far more cells and many more pores than the simplified model below):

Above: a single module from a type of sponge with the simplest canal architecture: the ascon (asconoid sponge). The asconoid sponge body consists of two layers of cells with a layer of gelatinous material (called mesogloea) sandwiched in-between. The outermost layer consists of flattened paving-slab like cells, called pinacocytes. These cells fit together tightly to form the outer surface or pinacoderm (a type of covering tissue or epidermis). They typically have wavy contours to strengthen the connections between them (by increasing the surface area along which the cells can be fastened together). Inside is a layer of so-called collar cells or choanocytes, each with one flagellum pointing toward the atrial cavity and surrounded by a ring of short appendages called cilia. The flagellum undulates or beats, expelling water out into the atrium and sucking it in through the pores that penetrate both cell layers. Each of these incurrent pores is a channel traveling through a cylindrical cell called a porocyte. Collectively, many thousands or millions of choanocytes can produce a powerful jet of water that leaves the atrium from the large excurrent osculum (or oscula if more than one is present). The choanocytes work very much like a group of organisms called choanoflagellates. Choanoflagellates may live as single cells or in multicellular colonies and are rather like protosponges - sponges lacking the pinacocytes and mesogloea and other packaging cells that make up the sponge body. The structure of a solitary choanoflagellate is shown below:

Above: the structure and function of a choanoflagellate. These microscopic creatures beat their flagellum, driving a current of water away from them and sucking in water from behind which passes between the collar of microvilli to fill the void left by the water pumped away. (Note: some refer to the appendages making the collar as cilia, others as microvilli, but although superficially similar these are very different organelles). The flagellum has wings or flanges
which help it push against the water more effectively (it must be remembered that on this microscopic scale water behaves as quite a thick sticky liquid). As water passes between the collar, potential food items, like bacteria, are sieved out and ingested by the cell. This is essentially similar to how a choanocyte in a sponge works (except that choanocytes lack the flanges on their flagella). Thus, a sponge's body contains thousands of microscopic pumps that also filter food from the water. It is thought that choanoflagellates are related to sponges, and that perhaps sponges evolved from colonies of these cells.

Another general cell type is typical of sponges - the amoebocyte. Amoebocytes are amoeboid cells that are free to crawl around (in and on) the body of the sponge in much the same way as do amoebae. (It would be interesting to examine the amoeboid locomotion of these cells in detail to see if it more closely resembles that of amoebae or of animal cells). Amoebocytes have a variety of functions, including maintaining the mesogloea (mesenchyme) and removing foreign organisms and debris from the sponge body. One of the main functions of the wandering amoebocytes is secretion of the sponge skeleton.

The mesenchyme is a transparent gelatinous matrix (mesogloea) containing free amoebocytes. The mesenchyme may be a collenchyma (meaning it has few cells and a lot of material between the cells), or a parenchyma (with a high cell density and little material between the cells as they are tightly packed together). The amoebocytes are free to wander about the sponge and fall into two main classes, lobopodous amoebocytes and collencytes. 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 (calcium carbonate, silicon or spongin protein). Lobopodous amoebocytes have lobopods - locomotory appendages or psedudopods that are blunt, rounded and finger-like - the so-called lobopod type of pseudopod. Collencytes have slender branching pseudopods (called filopods) and may form a syncytial network (a syncitium is a group of neighbouring cells whose cell membranes are fused together to form a continuous mass of cytoplasm. However, distinguishing between a syncitium and a collection of distinctly 'separate' cells with junctions linking them together is not easy!).

Archaeocytes are lobopodous amoebocytes. Archaeocytes are possibly undifferentiated cells and produce the sex cells (as may choanocytes in some sponges?) i.e. the spermatocytes and oocytes (sperm and egg cells) and are involved in regeneration since they can give rise to all other sponge cell types. Indeed, one of the striking characteristics of sponges is their ability to regenerate from a few cells - mince a sponge and each piece can grow back into a new sponge!

Sponge architectures

The flow of water through a sponge can follow various pathways, giving rise to different canal or aquiferous architectures.

Asconoid sponges (ascons) have the simplest arrangement. The water flows in via the pores, enters the main cavity of the sponge, or spongocoel, which is lined by choanocytes and then exits through the osculum: pores → spongocoel → osculum.

Syconoid sponges (sycons) have a more complicated body plan than asconoid sponges. Syconoids are vase-like with a single, terminal osculum. They have many finger-like out-pushings which form radial canals continuous with the central atrium (spongocoel). These external projections pack the surface and may be free and surrounded by sea-water or they may be covered by an outer epidermal covering which contains dermal pores, which open into the channels between the projections, called incurrent canals, which pass through pores called prosopyles into the radial canals which are lined by choanocytes. The radial canals then open into the spongocoel through internal ostia and the water flows through the osculum to the outside.

Leuconoid sponges (leucons) have the most complex structure. There is usually no central spongocoel, instead the sponge cavity is highly branched and divided into clusters of small round or oval chambers lined by choanoflagellates. Mesenchyme fills the spaces around these chambers. Leuconoid sponges have an indefinite form permeated by a maze of water channels. Nevertheless, a given unit of water only flows through one choanocyte chamber as the chambers operate in parallel rather than in series (there is little point trying to filter food from water that has already been filtered!). Most sponges are of the leuconoid type, as this permits the sponge to develop the most efficient water current and to attain a larger size (as it filters food from water more efficiently). The leuconoid architecture is shown in the diagrams below (click the images to enlarge). This figure was redrawn from Libbie Henrietta Hyman's classic text: The Invertebrates (Vol 1: Protozoa through Ctenophora). Note how the choanocytes are restricted to numerous small chambers formed by the two-fold evagination of the spongocoel.

Above: the principle types of sponge internal architecture. Asconoid sponges are the simplest and smallest and probably the most ancient in design. The whole inner surface of the asconoid is lined by choanocytes (the choanoderm) as indicated in red. The syconoid type represents an array of asconoid types arranged around the cylinder of the sponge (the incurrent pores are not shown, but the red layer is porous in all these sponge types). In type 2 syconoid sponges an additional layer of dermal tissue, perforated by large pores, covers the structure. The leuconoid type is the most advanced and consists of an array of syconoid units arranged around a cylinder. Only the layers shown in red, the choanocyte chambers, bear choanocytes, which line the inside only of each chamber. Syconoid and leuconoid sponges necessarily have a more complex system of pores and channels to convey water through the sponge. The arrangement is such that water only flows through one choanocyte chamber on its passage through the sponge, otherwise energy would be wasted in filtering water that has already been filtered. The more complex syconoid and leuconoid architectures are more efficient and so are able to achieve greater size - an asconoid becomes increasingly less efficient at larger size. Asconoid sponges are small, and rarely exceed 10 cm in height, but the more complex and efficient leuconoid sponges may reach two meters in height. The architectures shown above are all open architectures, in which water exits through a large central spongocoel cavity. Many sponges have solid architectures, in which a series of channels coalesce to expel water through one or more oscula and the large central spongocoel is essentially absent. Leuconoid sponges are the most common and their architecture may be complicated, such that the sponge's tissue becomes permeated by a complex maze of water channels.

Above: the architecture of a leuconoid sponge (after Hyman, 1940). In many sponges there is variable architecture, but the general pattern seen here involves water flow through the sponge as follows: ostia (o, dermal pores) → incurrent canal (i) → prosopyles (p) → choanocyte (flagellated) chambers (c) → excurrent canal (e) → spogocoel (s) → osculum (Os). This type of arrangement in which the choanocyte chambers connect directly to the excurrent canals by wide apertures is termed eurypylous.

Further modifications may occur, a narrow canal (aphodus, plural: aphodi) may connect each chamber to the excurrent canal, an arrangement termed aphodal: ostia → incurrent canal → prosopyles → choanocyte (flagellated) chambers → aphodus → excurrent canal → spongocoel → osculum. Finally the prosopyle may extend into a narrow canal, such that each chamber has a narrow inlet canal (prosodus, plural: prosodi) and a narrow outlet canal (aphodus) - an arrangement called diplodal: ostia → incurrent canal → prosodus → choanocyte (flagellated) chambers → aphodus → excurrent canal → spongocoel → osculum. In reality an individual sponge may show mixed or intermediate stages of canal architectures. Fort example, a single specimen of Scypha ciliata can show ascon, sycon and leucon types.

Sponges are generally modular organisms, made up of repeating aquiferous modules, and several oscula may be present, one per module. In some sponges, the true oscula themselves open into an atrium which opens to the outside water via a pseduosculum. Conversely, some sponges flatten out with a wide crater-like osculum and may become so wide and flat, with such a low oscular rim, that the osculum essentially disappears and excurrent canals then open directly to the outside. The ostia may aggregate into pore sieves in some sponges.

There are issues over naming the various parts of the aquiferous system of sponges. In particular the ostia are sometimes referred to as dermal pores, but these should not be confused with the pores of the ascon sponge which consist of porocytes.

There are important mechanical factors to consider when understanding the sponge aquiferous system. For example, in a leuconoid sponge, as water flows down through the incurrent canals it is siphoned off into adjacent choanocyte chambers and to maintain filtration pressure along its length, the cross-sectional area of the incurrent canals decreases, maintaining pressure as fluid volume contained within the incurrent system drops. Conversely, as more and more water is pumped into the excurrent canals, they increase their cross-sectional area so as not to generate increasing back pressure which will oppose further filtration (Larsen and Riisg�rd, 1993).

Why the complexity?

This can best be understood in terms of the diameter of supply (DoS). The sponge must expel water far enough from itself such that it takes in very little of the water it exhaled, since exhausted water is depleted of oxygen and food and may contain toxic waste products. If the exhalent jet is fast enough then it will jet high enough to avoid all but trace amounts of such recirculation: the sponge has achieved a sufficient DoS (see diagram below). In a fast current, a flat encrusting sponge may have an infinite DoS, since the current will bring in a constant supply of fresh water whilst expelling waste water far downstream.

After Fry, W.G. , 1979. Taxonomy, the individual and the sponge. In Biology and Systematics of Colonial Organisms, Larwood, C and Rosen, B.R. (eds). Academic Press.

To increase DoS a sponge can raise the osculum by placing it at the end of a tall chimney or increase pumping pressure to increase the velocity of the exhalent jet. The pumping pressure of a choanocyte chamber is inversely proportional to its volume: a large single chamber (as in asconids) will generate a lower pressure than a series of smaller chambers arranged in parallel (as in a leuconoid sponge). Thus, the more complex architectures allow sponges to attain a greater size without compromising the DoS. Additionally, splitting the incurrent water and passing it through several smaller filtration chambers, rather than one large chamber, undoubtedly increases filtration efficiency in large sponges.

Most, if not all, sponges can regulate the diameter of the osculum and also regulate water pumping by contracting the sponge body to reduce the volume of the choanocyte chambers. Indeed, many sponges periodically contract so as to completely shut down pumping for transient intervals. Some sponges narrow the osculum aperture by general body contraction, others by contracting the pinacoderm around the osculum but some have specialised muscular sphincters. Generally, as a sponge increases pumping rate it increases opening of the osculum, so maintaining a constant exhalent jet velocity. The oscula may close in still water to prevent the sponge taking in waste water when the DoS is insufficient.

In still water some sponges will grow longer chimneys bearing the oscula at their summits, increasing DoS. In some sponges, several oscula may be grouped together to generate a combined jet which can push higher into the surrounding water (by reducing frictional losses in the compound jet) and oscula may even fuse into single chimneys. Fan-shaped sponges may further solve the problem by orienting into prevailing currents with the ostia on one surface (facing the incoming flow) and oscula on the other surface. Thus, the concept of DoS can go along way to making sense of the complexity of sponge architecture and shape.

The Sponge Skeleton

The sponge skeleton deserves special attention. The mesenchyme secretes and contains the skeleton. The skeleton consists of spicules and/or spongin fibres. The spicules may be principally calcium carbonate in calcareous sponges and are made of silica in siliceous sponges; spongin is a protein. The spicules (sclerites) are tiny (mostly microscopic) 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 (this topic gets very technical and has been much simplified!):

1. Monaxon spicules have a single axis, straight or curved. There are many types according to their shape and how they form. They may be lance-like, C-shaped, bow-shaped, thorny or knobbly, rod-shaped, twisted spirals or spiny. Some have a pointed end projecting to the exterior of the sponge, making its surface rough and spiny, presumably for protection.

2. Tetraxons (tetractines, quadriradiates) have four rays radiating from a common point. These rays are not in the same planes and may resemble jacks, though some of the rays may be reduced, absent or modified into discs.

3. Triaxons (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.

4. Polyaxons have several equal rays that radiate from a central point and may be star-shaped or resemble spiny spheres.

5. Spheres form from concentric growth around the centre.

6. The desma is a megasclere formed from a minute monaxon, triradiate or tetraxon spicule, forming a central structure upon which layers of silica are deposited. These deposits develop branches and tubercles. Desmas are usually united into a network to form a net-like skeleton (lithistid).

Spongin is a protein that forms a branching network. In some sponges, spongin often binds the siliceous spicules together. In the keratosan sponges the skeleton consists entirely of spongin (and embedded foreign particles).

Spicules are secreted by scleroblasts, which are a sub-class of amoebocyte. Silicoblasts are a type of scleroblast that secrete siliceous spicules in the siliceous sponges. Spongin is secreted by spongioblasts. The various spicule types (of which there are many sub-types I have not mentioned here) have different skeletal roles within the sponge body and the types present also depends upon species. I think that you get the point that the sponge skeleton is actually rather complex! The spicules make the tissue of many sponges very hard, prickly or stony and quite difficult to cut with a knife. Traditional bath sponges (not the artificial type!) have spongin skeletons and no mineral spicules, which gives them a springy and spongy texture.

The main function of the sponge skeleton is to support the aquiferous system and keep the water canals and chambers open. Individual choanocyte chambers may be supported by fibers connecting them to the spicules. The skeleton also functions to protect the sponge: they often give the sponge a tough texture and spiculues may guard openings such as the oscula and spicules may project out from the sponge surface to make them spiny or prickly. The spicules may be loosely embedded in the fibrous tissues of the sponge or cemented together. The exact arrangement and the degree to which the spicules interlock or work together are not always obvious. In forms that dwell on soft bottoms, long bundles of spicules may protrude from the basal end as rooting fibers. other forms may attach to hard surfaces by means of secretion, sometimes spongin, whilst in others the main body is buried in the sediment with chimneys to carry the openings clear.

Glass Sponges (Hexactinellids)

Glass sponges have a skeleton consisting of a lattice of glass-like siliceous fibres. These strange sponges are deep-sea sponges, occurring at 100 m to 5000 m depth. Most are 10-30 cm tall, but some are over 1 metre. They are pale in colour and may have projecting spicules (which may be up to 30 cm long) which may give them a 'glass wool' like covering. In at least one form it would appear that the projecting glass fibres (the spicules) act as fibre-optic wires, transmitting light from a bioluminescent crustacean resident that lives inside the spongocoel, which must create a spectacular light display. It is quite possible that many glass sponges are bioluminescent. It has also been suggested that the glass spicules may be part of a light-sensing system, whether for releasing or receiving light, they are very good at transmitting light. These mysterious sponges are not very well studies and many glass-sponge wonders surely await discovery. One well known example is the Venus's flower basket - the beautiful glassy skeletons of the sponge Euplectella.

The internal structure of glass sponge tissues is very different from that of other sponges. They are covered by a dermal membrane, comprising a syncytium with underlying membrane of spongin (a type of collagen) and have a very open body plan with large open spaces, rather like a honeycomb. (A syncytium is a large multinucleate structure). The 'archaeocytes' are generally connected to one another by slender cytoplasmic bridges, forming a meshlike or trabecular syncytium that constitutes the bulk of the sponge body. This syncytium is continuous with choanablasts, forming a choanosyncytium. Each choanoblast gives off one or more stalk-like stolons that connect to collar bearing units, resembling choanocytes but lacking nuclei (there cytoplasm is continuous with that of the choanoblasts). Stolons may also connect these collar units to one another. This arrangement is reminiscent of certain colonial choanoflagellates, in which the cells are connected by stalk-like or root-like stolons. The only cells that are routinely seen separated from the syncytium are the scleroblasts, which secrete many of the spicules. All the other cells appear to be connected to one another by cytoplasmic bridges, though they may have rounded bodies, making them resemble distinct cells. It has been argued that glass sponges are so different to all other sponges that they should be in a separate phylum. However, their strange internal architecture stems from the tendency of their cells not to separate fully at cell division, but to remain in contact with one another by cytoplasmic bridges (which may be occluded by pore plates that allow certain materials to pass from 'cell' to 'cell' directly as the cytoplasm is continuous across the pores) with a single continuous cytoplasmic membrane surrounding the whole structure. Considering that some choanoflagellates are solitary, whilst others form colonies of connected individuals, I don't personally think that this difference is great enough to warrant placing glass sponges in a separate phylum.

A glass sponge.

Demosponges

Demosponges either have no spicules at all or they have the siliceous type. All are of the leuconoid type. There are at least three types, including the horny sponges (keratosans) which possess spongin skeletons but have no spicules (unless these are made of spongin), though they may incorporate rock grains and other materials into their bodies. The spongin forms a lattice or tree-like branching network throughout the keratosan sponge body. Keratosans are usually black in colour and have a smooth or warty leathery texture. They live attached to hard materials, glued in place by spongin secretion.

Calcareous sponges

These have calcareous spicules and include all the asconoid types, some syconoids and some leuconoids. The calcium carbonate spicules are usually separate but are sometimes fused into a network or are enclosed in calcareous cement. Projecting spicules often give these sponges a bristly texture.

Development of tissues in sponges

Sponges possess tissues (aggregates of cells) but few or no obvious organs. Contractile muscle cells called myocytes, along with the epidermal cells which may be contractile, permit limited movement and muscle cells may form a sphincter around the osculum which may close or open the osculum. This sphincter and the choanocyte chambers are the closest thing sponges have to 'organs'. In general they are said to be at the tissue-grade level of organisation and lack the complex organ systems that make up most animals. They do form definite structures, however, for example, long spicules may project to form an oscular fringe surrounding the osculum and a sieve membrane or sieve plate may cover the osculum. The alien-ness of sponges compared to other animals will become clear after studying aspects of their physiology below. Some biologists argue that sponges do not possess tissues and are at the 'cellular stage' of organisation. Though the central mesenchyme contains wandering cells, rather than cell aggregates as such, this is very similar to the connective tissue of vertebrates. Also the outer covering layer of cells form an epidermis (lacking a definite basement membrane), and the inner choanocyte layer form a choanoderm (also lacking a definite basement membrane). A dermis is a type of tissue that serves to cover a surface (whether internally or externally). It may well be that these cell layers are not connected by electrical gap junctions, as in many animals, but some sponges possess syncytia - fusions of cells into a large multicellular mass, e.g. the meshlike tissue of glass sponges (hexactinellids) is syncytial. Indeed, many biologists use the word 'tissue' to describe sponge material. Personally, I consider sponges to be made up of tissues, albeit tissues in which the cells are mostly only loosely integrated.

Physiology - Movement

Adult sponges are immotile and exhibit limited movement. The only movements exhibited by some sponges are changes in the porocytes which may open or close the incurrent pores. Most sponges, however, are capable of local or general contraction due to forces produced by the pinacocytes, desmacytes or the myocytes. Such contractions are especially noticeable in some ascons. Most calcareous sponges exhibit slight or no contractility. Many Desmospongiae can contract their whole surface as a result of contractions of fibre cells in the cortex and along the main channels. Sphincters control the aperture sizes of the oscula. Dermal pores can open and close as a result of sphincter action or changes in the encircling cytoplasm, except in glass sponges in which the pores appear non-contractile but remain open. Indeed, most glass sponges seem incapable of movement.

Physiology - Sensitivity

Sponges have no nervous system and no specialised sensory cells. Disturbances result in general body contraction. Oscula close on exposure to air, in response to injury, lack of oxygen, chemical irritation, extremes of temperature and sometimes also to touch. The oscula also close when the sponge is in a small volume of water that is inadequate for waste removal. The dermal pores are less responsive, but close on injury to the sponge. Apparently neither of the
behaviours of oscula, dermal pores and choanocytes is correlated (?). Reactions are slow, taking place over one to several minutes. Obnoxious stimuli are transmitted either not at all or at up to 3-4 mm maximum.

The osculum is the most conductive area, especially in the direction away from the osculum, and the oscular rim is the most sensitive part. In the freshwater sponge Ephydatia each osculum is borne on the summit of a chimney-like tube. If the oscular rim is stimulated, a signal is transmitted down the chimney causing the chimney to contract or collapse.

In glass sponges, the cells are connected to one another as a syncytium to form cobweb-like meshes and electrical signals can travel quite rapidly from one part of the sponge to another, presumably via the cytoplasmic bridges connecting the 'cells' together.

Physiology - Canal System

In the well-studied sponge Leucandra, the speed of the oscula excurrent has been measured at 8.5 cm per second, and travels as a jet for up to 25-50 cm. A Leucandra sponge 10 cm high and 1 cm in diameter has some 2.25 million flagellated chambers and filters 22.5 litres of water per day. On this basis a large sponge would filter about 100 litres per day. The flow rate inside the flagellated chambers is very slow at about 0.01 mm per second due to a slight
pressure gradient regulated by the sizes of the oscula apertures. This slow flow rate gives sufficient time for the filtration process, which removes food particles from the water.

The flagella do not beat in coordination. Each undergoes a spiral undulation that travels from the base to the tip and ensures that the water current travels in a single direction. The collars point towards the apopyle (the opening of the flagellated chamber to the excurrent canal in non-asconoid sponges with complex architecture) and the resulting current exits through the larger aperture. Smaller flagellated chambers are more efficient since they permit less
stagnation.

In leuconoids the water current is very slow in the flagellated chambers, allowing time for filtering and exchange of materials to occur. The current then speeds up as it leaves the sponge.

Physiology - Nutrition

Sponges are filter feeders and sieve out microorganisms and organic debris from the water flowing through the flagellated chambers. Dissolved nutrients are possibly also taken up directly by the sponge (?). Food particles in the water current adsorb to or are filtered by the choanocyte collars and ingested into the cytoplasm. The microvilli of the collars of all sponges seem to be connected to one another by strands of mucus or glycoprotein, forming a very fine mesh capable of filtering bacteria from the water. In the large choanocytes the particles maybe wholly digested, but are usually either partially digested or passed straight onto the amoebocytes of the mesenchyme for intracellular digestion. The smaller choanocytes of other sponges possibly have no digestive role but pass food directly onto the amoebocytes (?). It is also possible that amoebocytes receive some particles absorbed directly through the walls of the incurrent passages (?). As they are motile (in all but glass sponges) the amoebocytes may transport food around the sponge body and pass it on to other cells (?). The amoebocytes eject waste that leaves the sponge via the excurrents. Other cell types also appear capable of capturing and ingesting food particles directly, should they come into contact with them (such as the cells lining the canals). The collar units and choanoblasts of glass sponges are continuous with the trabecular (mesh-like or web-like) syncytium that forms the bulk of the sponge and nutrients may pass directly from the collar units to the rest of the sponge tissue via the connecting cytoplasmic bridges. Sponges can also absorb dissolved nutrients directly from the sea water, though in many sponges bacteria constitute the main food item, and some sponges can live entirely off bacteria.

Physiology - Respiration

Sponges are aerobic. Sycon consumes 0.16 - 0.04 cubic cm of oxygen per gram of fresh-weight per hour, larger specimens consuming less oxygen per unit mass than smaller ones, as is the case for animals generally. Oxygen consumption is reduced by 80% when the oscula are closed and this is compensated for by supernormal oxygen consumption when the oscula reopen.

Physiology - Excretion

Ammonia is excreted and waste-laden amoebocytes are possibly discharged (?). Gland cells may put out long strands to the sponge surface and may secrete slime. Amoebocytes also secrete slime. Some sponges exude mucus or slime that has an unpleasant odour and may cause irritation to the skin on contact.

Ecology

Many animals live on or in sponges, exploiting their sessile habit. Coelenterates, bryozoans and barnacles may grow on the surface of sponges, whilst annelids and crustaceans often inhabit the water canals. In one large specimen of loggerhead sponge (Spheciospongia) were found 16 352 Synalpheus shrimps. Shrimps of the family Stenopidae, especially Spongicola, live in pairs in the spongocoel of glass sponges like Euplectella and Hyalonema. The shrimps
enter the sponge when young and grow such that they can no longer escape through the oscular sieve plate and must remain trapped alive within the sponge.

Sponges of the family Suberitidae grow on the snail shells inhabited by hermit crabs. The sponge grows to enclose the shell, the shell is dissolved and the crab occupies a spiral cavity in the sponge, which is lined by smooth fibrous tissue. The larvae of this sponge possibly only grow when they land upon a shell containing a hermit crab (?).

Freshwater sponges (and marine?) may be parasitised by mites. The female mites lay eggs in the sponge�s tissue.Crabs, such as Dromia and others, break off pieces of living sponge (and other objects) and hold them over their backs with their last pair of legs, for camouflage. Some crabs, e.g. from the family Majidae, stick sponges, algae and hydroids onto their back and legs with an adhesive secretion. These implanted organisms may continue to grow. Sponges are seldom eaten since they are prickly and have a bad taste/odour. Some crustacean inhabitants feed on sponge tissue as parasites. Some nudibranchs (sea slugs and their kin) also eat sponges. Sponges may smother and kill other sessile animals, as they grow, including oysters. Boring sponges may kill barnacles and other shelled animals by slowly boring through their shells.

Large horny sponges live for 50+ years. Smaller sponges live for several months or years. Freshwater sponges are necessarily seasonal. One study aged a deep sea sponge by dating the giant basal spicules, which may be up to 3 m long and inferred the age of the sponge to be around 1100 years! (https://www.sciencedirect.com/science/article/abs/pii/S0009254112000277)

Different sponge morphologies suite different waters. Sponges in calm or silty water have elevated oscula. Sponges that live in rough waters are low and encrusting. Flabellate (fan-shaped) and lamellate (vertical and sheet-like) sponges thrive in water with a constant current, and grow with their inhalent surface facing the current and their oscula point downstream. Deep-water sponges living on muddy bottoms have anchoring root tufts or long projections
of spicules. Most marine sponges live in shallow water, from the tidal zone down to 50-m depth, and do not live in brackish water. The glass sponges are deep-water sponges. Some desmosponges live in the deep ocean oozes. Spongin skeletons prevail in tropical / subtropical waters, whilst mineral skeletons (of calcium or silica) are more abundant in cold waters.

Reproduction

Asexual Reproduction: Some sponges constrict off the ends of branches, which then round up into a ball and regenerate. This may occur regularly or under adverse conditions. All freshwater, and some marine, sponges form gemmules. A gemmule is a group of amoebocytes (archaeocytes?) with other amoebocytes forming a covering layer (of columnar cells) that secretes thick and hard inner and outer membranes. The amoebocytes receive glyco- or lipoprotein food reserves from special nurse cells (trophocytes). Scleroblasts may deposit spicules between the membranes. The result is a round ball of archaeocytes, trophocytes and columnar cells, with a micropyle (pore) outlet.

The freshwater sponges produce gemmules in autumn and then die. The gemmules are able to resist winter drying and freezing and hatch in spring. Upon hatching, cells stream out through the micropyle and form a young sponge in about one week.

In marine gemmule producing sponges, the gemmule consists of an amoebocyte aggregation (archaeocytes?) with a flagellated columnar dermal covering and the gemmule becomes a free-living flagellated larva that swims as the flagella beat. The larva attaches near its posterior non-flagellated pole, loses its flagella, and develops into a young sponge.

Sexual Reproduction: All sponges can reproduce sexually. Ova and spermatozoa are produced, possibly from archaeocytes, other amoebocytes or from choanocytes (?). The egg mother cell (ovocyte) is an enlarged amoebocyte that grows either by engulfing other amoebocytes or from nutrients supplied to it by trophocytes. And then undergoes maturation divisions (the cell divides into daughter cells that mature into egg cells (ova)).

Each sperm mother cell is an enlarged amoebocyte, or possibly a transformed choanocyte since whole flagellated chambers have been observed to transform into spermatozoa. The sperm mother cell becomes covered by one or more flattened covering cells, which derive from divisions of the sperm mother cell or from other amoebocytes. The whole structure is called a spermatocyst. The enclosed spermatogonium undergoes 2-3 divisions to produce spermatocytes.

The sperm are ejected in the out-going water current, often forming milky, smoke-like jets that may extend for several metres above the oscular vent. In most species it appears that the eggs are also ejected, in which case fertilisation occurs in the external water, but in some species, the sperm enter the recipient sponge in the water current and fertilisation is internal. In the Calcarea (calcareous sponges), the sperm enters a choanocyte nurse cell. The nurse cell then fuses with the egg, releasing the contained sperm. In the species Reniera the sperm enters an amoebocyte, which transfers the sperm to the egg.

The fertilised egg undergoes unequal holoblastic cleavage to produce a blastula (a hollow ball of cells), inside the recipient sponge. The blastula is flagellated and exits the sponge via the osculum. This larva swims for several hours before attaching and developing into a sponge.

Most sponges are hermaphroditic (they produce both eggs and sperm), but some are dioecious (have separate sexes). Hermaphrodite individuals often produce eggs and sperm at different times, whilst some produce sperm apically and eggs basally. Shallow water forms reproduce seasonally.

Above: three tubular demosponges, one of which is ejecting sperm in the excurrent jetting from its osculum (a 'smoking sponge'). These sponges may be 1.5 m or more in height and can eject sperm for several metres. Sponges that live rooted in soft sediment benefit from being tall as this prevents them being easily buried by shifting sediments. Click image to enlarge.

A barrel sponge releasing sperm.

The shapes of sponges

Sponges appear in a huge variety of forms: spherical, conical, club-shaped, vase-like, tubular, goblet or cup-like, encrusting sheets, upright sheets, plate-like, fans and treelike forms, etc. Sometimes individuals of the same species, but growing in different locations, may show very different shapes, although each species tends to adopt a particular range of shapes. There are several physical reasons why sponges are the shapes they are:

1. Treelike forms branch in such a way as to maximise the filtration of water. Just as an oak tree has to maximise the light it intercepts and the carbon dioxide it removes from the atmosphere, so a sponge has to maximise the efficiency with it extracts food from water by filtration. If two branches of an oak tree are too close together, then their leaves may overlap and the shaded leaves may receive insufficient light for photosynthesis and they are a waste to the plant.
Additionally, the leaves of one branch absorb carbon dioxide from the atmosphere and create a zone of air around the branch depleted in carbon dioxide which is only slowly replaced by diffusion in slow air, but is replaced more quickly in windy conditions. With two branches too close together the branches will compete for carbon dioxide and most likely there will not be enough to go around. This would be the case if the crown was a solid ball with no branches - all parts of the crown would be competing for insufficient resources and this would be wasteful. To overcome this problem, tress branch according to one of several optimum branching patterns (the exact solution used depends upon species, which is one reason why different species have different branching patterns) which spreads the branches apart so that they do not compete too much with one another and the tree can maximise its exploitation of light and carbon dioxide without wasting resources (like wood).

It is possible to use a computer to solve the mathematical problem of supply and predict the optimum branching patterns. This has been done for sponges (using the diffusion equation to model the diffusion of food particles into the volume of water around the sponge that the sponge can exploit by sucking this water through its body). The assumption is that sponge tissue will not grow into regions of water where it finds insufficient food (such as too close to other sponge tissue which have already removed the food from the water). These models correctly predict many sponge-like shapes, including the branching tree-like forms. One also has to consider whether or not the sponge lives in the open where there are strong currents to bring in fresh food rapidly, or whether it lives in a sheltered region of more stagnant water.

Another factor which effects sponge shape is the diameter of supply. This diameter is the furthest that a sponge can eject water from itself before some of that water recirculates and gets taken up again by the sponge, assuming stagnant conditions. Ideally, the sponge would take up fresh water without mixing in any of the water it has already filtered - filtered water has had the food and much of the oxygen removed from it by the sponge and is now of little use
and hence the further the sponge can eject it, the better, as then it is less likely to return or is thoroughly mixed and diluted with fresh water. In strong currents, this is not a problem, the currents carry away the waste water and bring in fresh water. Such a sponge may be an encrusting film that coats the surface of a rock and puts up very short chimneys, each with an osculum on top. In more stagnant water, however, the sponge may have taller chimneys
through which to eject its waste, which may help the ejected water reach the currents higher above the surface (a boundary layer of stationary or slow-moving fluid always covers the surface of objects, and the depth of this boundary layer increases in more stagnant conditions and reduces in strong currents; above the boundary layer is the turbulent layer where currents mix the water). Factories use the same principle - taller chimneys carry the waste higher into the winds where they carried further away.

A third factor determining sponge shape is mechanical. A tall fan-shaped sponge may be swept away in strong currents, whereas a slender conical sponge or a low-lying encrusting sponge stuck to the rocks over a large surface, are more likely to survive. On the other hand, in milder currents, the flat encrusting form may be trapped inside the boundary layer, away from the currents higher up that bring in fresh water, oxygen and food.

Different species are best adapted to different conditions, however, the fact that sponges can alter their shape to some degree proves that sponges are sensitive creatures - able to respond to their environment and grow to a shape that best suites the environment. For example, the sponge Hymeniacidon perleve grows as a thin film that encrusts the surface and puts up a number of very short chimneys, but sometimes it can grow tall chimneys, which may branch or
fuse into a single large cluster of chimneys (for greater strength allowing them to grow taller). Presumably, these different forms are adaptations to the local environment. Other sponges may grow on elongated stalks, enabling them to reach the currents in the turbulent zone and perhaps to avoid sucking up silt from the sediment, which may clog their pores.

Above: a model of a goblet sponge - several species adopt a cup-like form, complete with stalk and attachment disc. This is one example of an open sponge architecture, in which the outflowing water enters a common spongocoel chamber before being expelled through the osculum. Other sponges have a solid architecture, in which canals permeate the whole body and coalesce to open directly into one or more oscula. Click the image to enlarge.

Regeneration

Sponges have very high regenerative powers. Any piece can regenerate into a whole sponge, but the process is slow, requiring months or years for the new sponge to reach full size. If a sponge is broken into cells and cell-clumps, then the amoebocytes aggregate to form a reunition mass. Some reunition masses contain collar cells without collars and various types of amoebocyte. Some of the amoebocytes form an epidermis and a whole sponge is reformed. Reunition masses composed entirely of choanocytes cannot reform a sponge. Cells from different species, mixed together, may temporarily form a reunition mass before separating.

In adverse conditions, many marine sponges and freshwater sponges, collapse and disintegrate to leave a reduction body remnant comprised of a covering epidermis and an internal amoebocyte mass with partially de-differentiated choanocytes. This will grow into a sponge when and if favourable conditions return.

Infaunal Sponges

A peculiar type of sponge that is little known and little studies is the infaunal sponge that lives mostly buried in soft sediment at the bottom of the sea or below reef slopes on the continental margin. For example, the model sponge shown below (click thumbnails to enlarge) is similar to species of Oceanapia consists of a dark brown spheroidal central bulb, some 6 cm by 4 cm, buried beneath 5-10 cm of sediment. From this bulb extend 4-9 tubes or siphons, 6-28 cm in length and about 1 cm in diameter and whose white tips extend 3-8 cm above the sediment surface. In Oceanapia peltata, found off the Colombian coast, these siphons may bear a number of stacked horizontal discs partially enclosing them (not shown). Also from the bulb extend a number of tubular 'roots' which penetrate more or less vertically into the sediment to a distance of several cm. The description given is intended to assist your imagination, but infaunal sponges come in a diverse variety of shapes and forms. Experiments with Oceanapia have shown that the tips of the siphons are porous and draw water in to a series of channels that permeate the non-porous bulb. Food particles are filtered from the water and then the water is expelled into the sediment by the root-like excurrent tubes (which bear pores of an uncertain nature as it is hard to collect the specimens without damaging the roots). In life, these sponges are firm and somewhat elastic, but they become brittle when dry. The secret nature of
these sponges and the technical difficulties involved in collecting intact specimens has hampered the study of these sponges. Remember, that despite their appearance, texture and limited movements, sponges are animals!

Above: Some deep-sea sponges are carnivorous! This is Cladorhiza, a carnivorous genus of sponges. This is a (monaxonid) demosoponge with monaxon megascleres. Further examples of these bizarre carnivorous sponges include the filamentous Asbestopluma (resembling a mass of crystalline filaments), Chondrocladia lampadiglobus (with a central stem bearing stalked globules) and Chondrocladia lyra (resembling an 'alien' harp). These sponges have surfaces which are sticky, sometimes by means of a layer of tiny protruding hook-like spicules, or perhaps by glue-like secretions. Small animals, such as shrimp, get accidentally trapped when they contact the sponge and the nearby cells of the sponge then move to enclose the prey in a digestive cavity! This Cladorhiza has a long stem which embeds in the bottom ooze. The umbrella-like spines possibly serve to both prevent the sponge sinking too deep into the ooze and to catch prey.

Above: a model of an infaunal sponge such as Oceanapia. These sponges vary considerably in form between species and many undoubtedly remain unknown to science. Siphons emerge above the sediment which buries the bulb-like body and root-like appendages extend into the substrate.

Below: the sponge as seen in its natural habitat - with only the porous tips of the siphons visible above the sediment.

An interesting paper by Cerrano et al. (2007) discusses the inclusion of foreign objects into the bodies of certain sponges, such as the infaunal sponge Oceanapia fistulosa. Some sponges deliberately incorporate particles of detritus into their own bodies. These particles may come from the 'snow' of detritus that constantly falls to the sea bed or from substrate particles. Infaunal sponges, and certain sponges or fragments of sponges that are normally attached to solid substrates but become dislodged must ensure that they maintain the correct orientation to prevent sediment from clogging their aquiferous system. They may do this by incorporating particles of sediment into their basal portions, acting as 'ballast' and stabilise the sponge body in the correct orientation.The anchoring basal strands of an infaunal sponge like Oceanapia are particularly active in taking in the larger foreign particles (those above about 2 mm are preferred) and may reach a length of 15-20 cm (the globular sponge body may reach 5-15 cm in diameter) and 1 cm in diameter, but these tend to be narrower, longer and more numerous in finer-grained sediment.

This type of particle inclusion seems non-specific, but other parts of sponges may incorporate selective particles, selected on the basis of size and/or mineral composition. When debris falls upon a sponge, the pinacocytes may remove it by wave-like movements of their cell membranes or they may engulf the particles and sort them. Acceptable particles may then be incorporated into the underlying tissues to aid in skeletal support or to give the sponge body toughness. Some species produce no spicules of their own and incorporate foreign spicules and other particles into their spongin skeleton or other matrix should they lack spongin also. Some sponges incorporate silica in the form of quartz, which seems detrimental to some species but beneficial to others. Indeed, some sponges dissolve internalised quartz as a source of silica and in Chondrosia silica switches on the genes for collagen synthesis.

Individuality in sponges?

Many sponges are colonial - many tubes, each bearing an osculum vent may be fused together at their bases to form a single mass. It may be that each unit, comprising a single osculum and surrounding tissue is one individual and that one sponge produced new individuals that failed to separate completely. On the other hand, it is known that some sponges if placed next to one another will fuse together to form a single individual, whilst others may fuse temporarily before rejecting one another, in which case a space (zone of non-coalescence) appears between them. It turns out that usually only sponges of the same species and the same strain will fuse together. A sponge will fuse with pieces of itself - if a piece of tissue is removed and then placed near to the source sponge, or inside a hole cut into the sponge (an autograft), then the fragment will fuse with its parent body. If a graft (say a cube of tissue) from another sponge is placed inside a hole cut into the recipient sponge, then only if the graft was from another sponge of the same strain will it fuse with the recipient. If the graft is from a different strain, even one of the same species, then rejection will occur - the graft will not fuse and may become black and necrotic and shrink away as tissue from the recipient grows into the wound to replace the rejected graft. Other times the graft will grow and enlarge at the expense of the recipient sponge. Biochemicals, called sponge factors, are known to be secreted by sponge tissues when a graft is introduced, if the strains are incompatible, then these factors will reduce the adhesiveness of the cells to one another and the graft will be unable to stick and fuse to the recipient. Whether we define an individual sponge as a physically separate sponge (even if one grew from a fragment of the other) or whether we define the compatible strain as the individual is a matter of opinion.

One sponge or three? Click image to enlarge.

The sponge spicule skeleton is a truly remarkable structure that ought to be the envy of engineers. It is interesting to look at sections of sponges and see how the various spicules fit together like mechano into diverse large-scale meshworks designed to take the weight of the animal, give its tissues hardness and protect it against predators, and all with minimum material costs and light-weight construction. Megascleres are larger and often mesh together to form the bulk skeleton; microscleres are smaller and generally free in the tissues, e.g. microcalthrops - a small calthrops. Triaenes are tetraxons with one long ray, called a rhsbdome and 3 smaller clads that form the cap-like cladome.

Typical examples of spicule (sclerite) types are illustrated below with 3D computer-generated models:

Two-rayed diaxon.

One-rayed monaxon.

Triaene tetraxon

Amphidisk

Six-rayed triaxon (6 points and 3 axes)

Triaxon (3 rays/axes)

Pentaradiate (5 points and 3 axes)

Tetraxon (4 axes/rays)

Tetraxon (calthrops - equal rays)

An open cubical cage. Several such subunits may be joined into a larger cube unit.

Above and below: spicules.

Above: gemmules.

Above: spongin fibers.

References

Cerrano, C., Calcinai, B., Di Camillo, C.G., Valisano, L. and Bavestrello, G. 2007. How and why do sponges incorporate foreign material? Strategies in porifera. Porifera research: Biodiversity, Innovation and Sustainability: 239-246.

Fry, W.G. 1979. Taxonomy, the individual and the sponge, in Biology and Systematics of Colonial Organisms, Larwood, G. and Rosen, B.R. (eds.). Academic Press.

Hyman, L.H. 1940. the invertebrates: Protozoa through Ctenophora. McGraw-Hill Book Company, Inc. New York and London.

Larsen, P.S. and Riisgård, H.U. 1993. The sponge pump. J. Theor. Biol. 168: 53-63.

Minchin, E.A., Fowler, G.H. and Bourne, G.C, 1900. A treatise on zoology: part II - Porifera and Coelentera. Lankester, E.R. (ed.). Adam & Charles Black, London.