Sponge

Overview
The sponges  or poriferans (from Latin porus "pore" and ferre "to bear") are animals of the phylum Porifera. Porifera translates to "Pore-bearer". They are primitive, sessile, mostly marine, water dwelling, filter feeders that pump water through their bodies to filter out particles of food matter. Sponges represent the simplest of animals. With no true tissues (parazoa), they lack muscles, nerves, and internal organs. Their similarity to colonial choanoflagellates shows the probable evolutionary jump from unicellular to multicellular organisms. There are over 5,000 modern species of sponges known, and they can be found attached to surfaces anywhere from the intertidal zone to as deep as 8,500 m (29,000 feet) or further. Though the fossil record of sponges dates back to the Neoproterozoic Era, new species are still commonly discovered.

Anatomy and morphology
Sponges have several cell types:
 * 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 microvilli and are used to filter particles out of the water.  The beating of the choanocytes’ flagella creates the sponge’s water current.
 * Porocytes are tubular cells that make up the pores into the sponge body through the mesohyl.
 * Pinacocytes which form the pinacoderm, the outer epidermal layer of cells. This is the closest approach to true tissue in sponges
 * Myocytes are modified pinacocytes which control the size of the osculum and pore openings and thus the water flow.
 * 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.
 * Sclerocytes secrete calcareous siliceous spicules which reside in the mesohyl.
 * Spongocytes secrete spongin, collagen-like fibers which make up the mesohyl.
 * Collencytes secrete collagen.
 * Spicules are stiffened rods or spikes made of calcium carbonate or silica which are used for structure and defense.
 * Cells are arranged in a gelatinous non-cellular matrix called mesohyl.

Sponges have three body types: asconoid, syconoid, and leuconoid.

Asconoid sponges are tubular with a central shaft called the spongocoel. The beating of choanocyte flagella forces water into the spongocoel through pores in the body wall. Choanocytes line the spongocoel and filter nutrients out of the water.

Syconoid sponges are similar to asconoids. They have a tubular body with a single osculum, but the body wall is thicker and more complex than that of asconoids and contains choanocyte-lined radial canals that empty into the spongocoel. Water enters through a large number of dermal ostia into incurrent canals and then filters through tiny openings called prosopyles into the radial canals. There food is ingested by the choanocytes. Syconoids do not usually form highly branched colonies as asconoids do. During their development, syconoid sponges pass through an asconoid stage.

Leuconoid sponges lack a spongocoel and instead have flagellated chambers, containing choanocytes, which are led to and out of via canals.

Physiology
Sponges have no true circulatory system; instead, they create a water current which is used for circulation. Dissolved gases are brought to cells and enter the cells via simple diffusion. Metabolic wastes are also transferred to the water through diffusion. Sponges pump remarkable amounts of water. Leuconia, for example, is a small leuconoid sponge about 10 cm tall and 1 cm in diameter. It is estimated that water enters through more than 80,000 incurrent canals at a speed of 6cm per minute. However, because Leuconia has more than 2 million flagellated chambers whose combined diameter is much greater than that of the canals, water flow through chambers slows to 3.6cm per hour. Such a flow rate allows easy food capture by the collar cells. All water is expelled through a single osculum at a velocity of about 8.5 cm/second: a jet force capable of carrying waste products some distance away from the sponge.

Sponges have no respiratory or excretory organs; both functions occur by diffusion in individual cells. Contractile vacuoles are found in archaeocytes and choanocytes of freshwater sponges. The only visible activities and responses in sponges, other than propulsion of water, are slight alterations in shape and closing and opening of incurrent and excurrent pores, and these movements are slow.

Taxonomy
Sponges are classified as animals, despite the fact that they lack gastrulated embryos, extracellular digestive cavities, nerves, muscles, tissues, and obvious sensory structures, which all other animals possess. At one time, sponges were considered plants, as they appear to share the plant-like characteristic of being rooted to a single spot, although some species of sponge are motile. Unlike almost all plants, sponges do not photosynthesize, and they lack cellulose cell walls, which are common to all plants.

Long thought to be the most archaic branch of the animals, sponges are considered as useful models of the earliest multicellular ancestors of animals; although there is no evidence that they actually are, or that they are even descended from early animals. Sponge choanocytes (feeding cells) are likely to be an homologous cell type to choanoflagellates - a group of unicellular and colonial protists that are believed to be the immediate precursors of animals. Sponges appear to be, at best, a divergent side-group from the main animal line.

It has been suggested that the sponges are paraphyletic to the other animals. Otherwise they are sometimes treated as their own subkingdom, the Parazoa. Similar fossil animals known as Chancelloria are no longer regarded as sponges.

One phylogenetic hypothesis based on molecular analysis proposes that the phylum Porifera is itself paraphyletic, and should be split into two new phyla, the Calcarea and the Silicarea.

Sponges are divided into classes based on the type of spicules in their skeleton. The three classes of sponges are bony (Calcarea), glass (Hexactenellida), and spongin (Demospongiae). Some taxonomists have suggested a fourth class, Sclerospongiae, of coralline sponges, but the modern consensus is that coralline sponges have arisen several times and are not closely related. In addition to these four, a fifth and extinct class has been proposed: Archaeocyatha. While these ancient animals have been phylogenetically vague for years, the current general consensus is that they were a type of sponge.

Although 90% of modern sponges are demosponges, fossilized remains of this type are less common than those of other types because their skeletons are composed of relatively soft spongin that does not fossilize well. The fossil Archaeocyantha may also belong here, though their skeletons are solid rather than separated into spicules.

Geological history


The fossil record of sponges is not abundant, except in a few scattered localities. Some fossil sponges have worldwide distribution, while others are restricted to certain areas. Sponge fossils such as Hydnoceras and Prismodictya are found in the Devonian rocks of New York state. In Europe the Jurassic limestone of the Swabian Alps are composed largely of sponge remains, some of which are well preserved. Many sponges are found in the Cretaceous Lower Greensand and Chalk Formations of England, and in rocks from the upper part of the Cretaceous period in France. A famous locality for fossil sponges is the Cretaceous Faringdon Sponge Gravels in Faringdon, Oxfordshire in England. An older sponge is the Cambrian Vauxia. Sponges have long been important agents of bioerosion in shells and carbonate rocks. Their borings extend back to the Ordovician in the fossil record.

Fossil sponges differ in size from 1 cm (0.4 inches) to more than 1 meter (3.3 feet). They vary greatly in shape, being commonly vase-shapes (such as Ventriculites), spherical (such as Porosphaera), saucer-shaped (such as Astraeospongia), pear-shaped (such as Siphonia), leaf-shaped (such as Elasmostoma), branching (such as Doryderma), irregular or encrusting.

Detailed identification of many fossil sponges relies on the study of thin sections.

Ecology and Reproduction


Modern sponges are predominantly marine, with some 150 species adapted to freshwater environments. Their habitats range from the inter-tidal zone to depths of 6,000 metres (19,680 feet). Certain types of sponges are limited in the range of depths at which they are found. Sponges are worldwide in their distribution, and range from waters of the polar regions to the tropical regions. Sponges are most abundant in both numbers of individuals and species in warmer waters.

Adult sponges are largely sessile, and live in an attached position. However, it has been noted that certain sponges can move slowly by directing their water current in a certain direction with myocytes. The greatest numbers of sponges are usually to be found where a firm means of fastening is provided, such as on a rocky ocean bottom. Some kinds of sponges are able to attach themselves to soft sediment by means of a root-like base. Sponges also live in quiet clear waters, because if the sediment is agitated by wave action or by currents, it tends to block the pores of the animal, lessening its ability to feed and survive.

Recent evidence suggests that a new disease called Aplysina red band syndrome (ARBS) is threatening sponges in the Caribbean. Aplysina red band syndrome causes Aplysina to develop one or more rust-coloured leading edges to their structure, sometimes with a surrounding area of necrotic tissue so that the lesion causes a contiguous band around some or all of the sponge's branch.

Reproduction
Sponges can reproduce sexually or asexually.

Asexual reproduction is through internal and external budding. External budding occurs when the parent sponge grows a bud on the outside of its body. This will either break away or stay connected. Internal budding occurs when archaeocytes collect in the mesohyl and become surrounded by spongin. The internal bud is called a gemmule. An asexually reproduced sponge has exactly the same genetic material as the parent.

In sexual reproduction, sperm are dispersed by water currents and enter neighbouring sponges. All sponges of a particular species release their sperm at approximately the same time. Fertilization occurs internally, in the mesohyl. Flagellated zygotes develop and then leave the parent sponge to settle somewhere else.

Although sponges are hermaphroditic (both male and female), they are not self-fertile. Most sponges are sequential hermaphrodites, capable of producing eggs or sperm, but not both at the same time.

By dolphins
In 1997, use of sponges as a tool was described in Bottlenose Dolphins in Shark Bay. A dolphin will attach a marine sponge to its rostrum, which is presumably then used to protect it when searching for food in the sandy sea bottom. The behaviour, known as sponging, has only been observed in this bay, and is almost exclusively shown by females. This is the only known case of tool use in marine mammals outside of Sea Otters. An elaborate study in 2005 showed that mothers most likely teach the behaviour to their daughters.

Skeleton as absorbent
In common usage, the term sponge is applied to the skeleton of the animal, from which the tissue has been removed by maceration and washing, leaving just the spongin scaffolding. Calcareous and siliceous sponges are too harsh for similar use. Commercial sponges are derived from various species and come in many grades, from fine soft "lamb's wool" sponges to the coarse grades used for washing cars.

The manufacture of rubber-, plastic- and cellulose-based synthetic sponges has significantly reduced the commercial sponge fishing industry in recent years.

The luffa "sponge", also spelled "loofah," commonly sold for use in the kitchen or the shower, is not derived from an animal sponge, but from the locules of a gourd (Cucurbitaceae).

Antibiotic compounds
Sponges have medicinal potential due to the presence of antimicrobial compounds in either the sponge itself or their microbial symbionts.