Sponge

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This article is about the animal. For other uses, see Sponge (disambiguation).

The sponges or poriferans (from the Greek poros "pore" and ferro "to bear") are animals of the phylum Porifera. They are primitive, sessile, mostly marine, water dwelling filter feeders that pump water through their bodies to filter out particles of food matter. Sponges are among 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 Precambrian era, new species are still commonly discovered.

Anatomy

Sponges have several cell types:

Choanocytes (also known as "collar cells"), flagellated cells which function as the sponge's digestive system, 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 or 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 force water into the spongocoel through pores in the body wall. Choanocytes line the spongocoel and filter nutrients out of the water. Syconoid is a modification on this type with folds in the body wall and choanocytes lining canals within these folds. Leuconoid sponges lack a spongocoel and instead have flagellated chambers, containing choanocytes, which are led to and out of via canals.

Sponges have no real circulatory system, however the water current 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.

Taxonomy

Sponges represent the most primitive animals, and as such, lack many of the typical features of animals including nerves and locomotion. Because of this they are placed in the group Parazoa, which is a sister taxon to all higher animals (metazoa). Sponges share many characteristics with colonial protists, such as Volvox, though they have a higher degree of cell specialization and interdependence. However, if a sponge is placed in a blender, any surviving cells can reform a complete organism. If multiple sponges are blended together, each species will recombine independently (contrast animal chimera such as the geep).

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 three, a fourth 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. 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. Sponges have holes throughout their bodies in addition.

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

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.

Fossil sponges differ in size from 1 cm (0.4 inches) to more than 1 metre (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.

Reproduction

Sponges are able to reproduce sexually or asexually. Most sponges are monoecious. Sperm are formed from choanocytes; and oocytes derive from choanocytes in some sponges, and archaeocytes in others. Sperm are released into open water. Some sponges have oviparous fertilization where sperm and eggs meet in open water. In other species sperm are taken into the bodies of other sponges and fertilized inside; this is known as viviparous fertilization. The free-swimming larvae, known as parenchymula, serve as one of the main forms of dispersal for this largely sessile phylum. Asexual reproduction of sponges is through budding, where a small piece of sponge falls off of the main sponge and grows into a new one. Many freshwater sponges form small structures known as gemmules, which function as overwintering devices. These are similar to a bacterium's endospore. Gemmules are made up of amoebocytes surrounded by a layer of spicules and can survive conditions that would kill adult sponges. When the environment becomes less hostile, the gemmule resumes growing.

Ecology

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.

Skeleton as absorbent

In common usage, the term sponge is usually applied to the skeletons of these creatures alone, from which the animal matter has been removed by maceration and washing. The material of which these sponges are composed is spongin. 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 luffa sponge, also spelled "loofah," commonly sold for use in the kitchen or the shower, is not related to any animal sponge but is derived instead from the locules of a pepo fruit (Cucurbitaceae). Marine sponges come from fisheries in the Mediterranean and West Indies. The manufacture of rubber, plastic and cellulose based synthetic sponges has significantly reduced the commercial sponge fishing industry over recent years. Additionally, synthetic sponge products can be up to 10 percent more effective at retaining liquids, as compared to a natural sponge.

References

Porifera