Sponge: Encyclopedia - Sponge

Sponge

Calcarea
Hexactinellida
Demospongiae

The sponges or poriferans (from Latin porus "pore" and ferre "to bear") are animals of the phylum Porifera. They are primitive, sessile, mostly marine, waterdwelling filter feeders that pump water through their matrix to filter out particulates of food matter. Sponges are among the simplest of animals, with partially differentiated tissues but without muscles, nerves, or internal organs. In some ways they are closer to being cell colonies than 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.

The structure of a sponge is simple: it is shaped like a tube, with one end stuck to a rock or other object and an open end, the osculum, open to the environment. The spongocoel, or interior of the sponge, is composed of walls perforated with microscopic pores that allow water to flow through the spongocoel.

Sponge - Anatomy

Sponges comprise only four types of cells:

• Choanocytes (also known as "collar cells"), which line the spongocoel and function as the sponge's digestive system, are remarkably similar to the protistan choanoflagellates.
• Porocytes are tubular cells that make up the pores.
• Flat epidermal cells line the outside of the sponge and form its skin.
• Amoebocytes live between the choanocytes and the epidermis. They carry out many of the sponge's functions, such as transport of nutrients, secretion of the spicules, and production of gametes.

Sponges have only three body types: asconoid, syconoid, and leuconoid. The former two are both vase-like, asconoid being the most primitive with a single chamber, syconoid's are more complex with primary folds in its body wall, while leuconoid are the most advanced taking a lumpy shape much like a rock and have secondary folds. Leuconoid's also have the highest concentration of collar cells (which line the chambers collecting food).

Image:Sponge1.jpg

Although they are animals, sponges may also be considered multi-cellular colonies. A sponge can be placed in a blender and any remaining living cells will reform themselves into another sponge. If multiple sponges are blended together, each species will recombine independently (contrast animal chimera such as the geep).

It is thought that the earliest multicellular life on Earth was a sponge-like creature. The earliest known multicelled animal fossils are sponges from China that are roughly 600,000,000 years old. Sponges have not been as extensively studied as some other phyla and there may be some surprises still to be found. For example, it has recently been shown that some sponges are not sessile and can move to more favorable locations as rapidly as a few centimeters a day. Another sponge, Venus' Flower Basket, has some newly discovered uses involving fiber optics.

Sponge - Taxonomy

Sponges are divided into classes based on the type of spicules in their skeleton. The three classes of sponges are bony (Calcarea), glass (Hexactenellida), 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.^[1]

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.

Sponge - Reproduction

Sponges are able to reproduce sexually or asexually. In sexual reproduction, sperm is released from a sponge and floats through the water, where it is taken into a sponge with eggs and internal fertilization occurs. This produces a motile larval stage of sponge. 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. When a sponge is in a hostile environment, sponges can also form small structures known as gemmules. 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.

Sponge - Ecology

Modern sponges are predominantly marine, with some species adapted to freshwater environments, ranging 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. Their bodies are porous and they feed by filtering micro-organisms from the water. They lack any internal organs, a nervous system, or circulatory and digestive systems, such as are found in the higher invertebrate animals.

Adult sponges live in an attached position, and tend to lack any means of locomotion. The greatest numbers of sponges are usually to be found where a firm means of fastening is provided, such as on a rocky 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.

Sponge - 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 loofah or luffa sponge, commonly sold for use in the kitchen or the shower, is not related to any animal sponge but is derived instead from a vegetable. 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.

Sponge - 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 Farringdon Sponge Gravels in Farringdon, 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), 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.

Adapted from the Wikipedia article "Sponge", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki