Definitions

loufah sponge

Sponge

[spuhnj]
'' The sponges or poriferans (from Latin porus "pore" and ferre "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 represent the simplest of animals.These are non-motile animals attached to some solid support.They are covered with a hard outer layer or skeleton. 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. However, recent genomic studies suggest they are not the most ancient lineage of animals, but may instead be secondarily simplified.

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:

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

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

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

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. Their 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.

It should be noted that these 3 body grades are useful only in describing morphology, and not in classifying sponge species, although the asconoid and syconoid construction is present in Calcarea only.

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 of the family Cladorhizidae (order Poecilosclerida, class Demospongiae) are species usually found in deep water, but also in littoral caves in the Mediterranean (Asbestopluma hypogea), that have become carnivorous, using a strategy that has much in common with what is found in carnivorous plants such as sundew. When small crustaceans comes in contact with their surface, they get captured by a sticky substance, or in the case of the Mediterranean species by spicules modified into raised hook-shaped spines, and then digested by migrating cells which soon covers the prey. This lifestyle has caused the loss of their aquiferous system and the choanocytes, resulting in forms like the ping-pong tree sponge (Chondrocladia lampadiglobus), which don't look like typical sponges.

Taxonomy

Sponges are traditionally 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, 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.

Sponge taxonomy is an area of active research, with molecular studies improving our understanding of their relationship with other animals.

Basal lineage?

Sponges are among the simplest animals. They lack gastrulated embryos, extracellular digestive cavities, nerves, muscles, tissues, and obvious sensory structures, features possessed by all other animals. In addition, sponge choanocytes (feeding cells) appear to be a homologous to choanoflagellates, a group of unicellular and colonial protists that are believed to be the immediate precursors of animals. The traditional conclusion is that sponges are the basal lineage of the animals, and that features such as tissues developed after sponges and other animals diverged. Sponges were first assigned their own subkingdom, the Parazoa, but more recent molecular studies suggested that the sponges were paraphyletic to other animals, with the eumetazoa as a sister group to the most derived:

Either way, sponges have long been considered useful models of the earliest multicellular ancestors of animals.

...or secondarily simplified?

However, a phylogenomic study in 2008 of 150 genes in 21 genera suggests that the ctenophora are the most basal lineage of the 21 taxa sampled, and that sponges—or at least those lines of sponges investigated so far—are not primitive, but secondarily simplified, having lost tissues and other eumetazoan characteristics from their common ancestor.

Geological history

The fossil record of sponges is not abundant. 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 Alb 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

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 number of 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, and this is seen only in the freshwater sponge family, the Spongillidae. An asexually reproduced sponge has exactly the same genetic material as the parent.

In sexual reproduction, sperm are dispersed by water currents and enter neighboring sponges. All sponges of a particular species release their sperm at approximately the same time. Fertilization occurs internally, in the mesohyl. Fertilized oocytes develop within the mesohyl. Cleavage stages are highly varied within and between groups, sometimes even within a single species. Larval development usually involves an odd type of morphogenetic movement termed an inversion of layers. When this occurs in some species (for example, in Sycon coactum), the larva flips into the choanocyte chamber, and then can emerge via the water canal system and out through the osculum.

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.

Use

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.

By humans

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, which is 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.

Bibliography

References

Further reading

  • Bergquist, P. R. 1998. "The Porifera" (pp. 10-27), in D. T. Anderson (ed.) Invertebrate Zoology. (A brief treatment)
  • Bergquist, P. R. 1978. Sponges Hutchinson, London.

See also

External links

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