- 1 Introduction
- 2 Types of Sponges
- 3 Biology of Sponges
- 4 The life of a sponge
- 5 Community and Ecological Interaction of Sponges
- 6 References
Sponges may have been the first animal on Earth. A 2016 study showed a connection between a molecule produced by sponges and a similar molecule found in rocks hundreds of millions of years old. 
Types of Sponges
There are currently around 5,000 known sponge species. These include four main classes, glass sponges, calcareous sponges, demosponges, and homoscleromorpha.
Approximately 90% of all sponge species are in this class.They are often brightly colored, unlike the dull glass and calcareous sponges. Demosponges can grow quite large, to over 2 meters in height. They have varied skeletal structures which can be made up of spongin fibers, spicules of silica or calcium carbonate, or any combination of these.  The common household sponge is a dried demosponge. 
Scientists have identified around 400 species of calcareous sponges. They can be found living on coral reefs in the shallow waters of tropical regions. Their skeletal structure is made up of large spicules of calcium carbonate. Generally, they are not as large or brightly colored as demosponges. 
Glass sponges are not found in coral reefs. Instead, they exist mainly in deep Arctic waters. They are composed of silica, the same chemical compound that makes up glass. Their spicules are fused in large complicated patterns. 
The class Homoscleromorpha is the most recent class identified. It used to be a part of the Demosponges. There are less than 100 known species in this class. These sponges are usually found in shallow waters. Unlike the other classes, spicules are formed in both the sclerocytes and the epithelial cells and do not form a distinct pattern or structure.
Biology of Sponges
Sponges can be found in many different shapes and sizes. One distinction is between encrusting and free-standing sponges. An encrusting sponge is one the conforms to a hard surface. Free-standing sponges include the tube, vase, and barrel varieties. These are sponges that have an inner space with a larger volume than the amount of surface area on the outside.
Sponges are made up of three layers.The outside layer consists of pinacocytes.The inside layer consists of choanocytes, or collar cells. These cells have flagellum that move water containing oxygen and nutrients throughout the sponge. The space between these layers is called the mesenchyme. This is a layer containing the skeleton of the sponge and some cells. 
Ostia and Osculum
Water flow is essential to the functioning of a sponge as they receive water and nutrients from this continual flow. Water comes into the sponge through ostia, small pores in the surface of the sponge. The water is moved through a series of canals by the flagellum of choanocytes, and then it exits through seperate pores, called oscula.
The skeleton of the sponge can be made up of a combination of either spongin fibers (collagen threads) ,collagen filaments (complex protein bundles), or spicules. Spicules can be composed of either calcium carbonate or silicon dioxide and are produced in sponge cells called sclerocytes. They can be further divided using a complicated classification system but one main classification is in size. Spicules are either large(megascleres) or small (microscleres). 
The sponge has no nervous system but it does have genes that are comparable to those in human neurons.Researchers have discovered around 25 sponge genes that are comparable to those in human neurons and have observed protein behaviors that mimic interactions of proteins at human nerve synapses.
The life of a sponge
Sponges gain oxygen from the water through diffusion. The water enters through small pores called ostia and then is distributed throughout the sponge from areas of high concentration to areas of low concentration. The water flows throughout the sponge with the help of choanocytes, cells with flagella for movement. This simple process works without a more complex respiratory system because all cells of a sponge are within close proximity of exchange sites. 
A study of the Red Sea coral reef sponge Negombata magnifica reveals the distribution of oxygen use. In this sponge, about 25.1% of oxygen was used for water pumping. Water pumping and maintenance together accounted for about 74% of all oxygen usage. The remaining 26% was used for other processes, mainly growth. This suggests that oxygen usage could contribute to controlling growth of sponges on the coral reef. 
Sponges are completely sessile past their first few days after fertilization. Once the sponge is attached it does not move from that surface. 
Sponges are hermaphroditic though they usually only release sperm or eggs at one time. Sperm is released and travels through the water until it lands within the interior of a sponge with eggs. The eggs are fertilized and then spend a short amount of time in the water until anchoring to a hard surface and beginning growth. 
Sponges can also reproduce asexually. This process is characterized by the creation of gemmules, tiny cells that branch off of the sponge and have the potential to form a genetically identical sponge. More complex gemmules, also contain an outer layer containing spicules that protects the cells and nutrients contained on the inside. This equips these cells to survive harsh conditions and only develop when the period of disturbance has passed. 
The respiration process detailed above also captures microorganisms and detritus in the water, to be digested by the sponge. Sponges are basically capable of digesting any biological waste that is small enough to be absorbed by their filtration mechanisms, so sponges rarely have trouble harvesting food. If any Ostia become clogged by slightly larger particles, like sand or large detritus, archaeocytes migrate to the blocked area and clear it before respiration and food collection can become impacted.
Sponges are found most abundantly in coral reefs. Demosponges are the primary reef inhabitants, while Glass sponges are never seen. Most sponges will attach to a hard substrate, as corals do, but some have complex subterranean growth that moors them to a ground like a taproot. Because of this variety and the simplistic feeding requirements of sponges, they can be found at almost any microbiome within a reef.
As sessile creatures, sponges have to compete for growing space with plants and corals. As many sponges aren't photosynthetic, they can often be found in crevices and shady areas where corals and plants have more difficulty living. Sponges have been known to grow on every surface imaginable in reefs, including sand, mollusc shells, and on top of hard corals.
Microbes such as bacteria, archaea, microalgae, and fungi live in close contact with sponges. They can make up around 40% of a sponge's volume and interact with the sponge in a variety of ways.
Community and Ecological Interaction of Sponges
Sponges are sessile, so they cannot escape any predators. They instead make themselves as unappetizing as possible. All sponges have spicules, which are akin to glass capillary tubes, making it very dangerous for an unprepared mouth to try and eat a sponge. Some sponges contain toxins that make them taste disgusting, as well. Common sponge predators include sea turtles, wrasses, parrotfish, and nudibranchs, all of which have evolved special features that allow them to work around the sponge's defenses.
Another form of "Predation" common in sponges is infection. Many bacteria and viruses can enter the sponges through its Ostia when it takes in food, and take up residence there. Sponges have specialized cells to counter this, forming a very basic immune system. When an infection is detected, these cells migrate to the affected area and secrete chemicals that prevent cell movement, preventing the infection from spreading. Eventually, the immune system cells will secrete a toxin that kills all cells in the affected area, hopefully eradicating the disease.
Many sponges have photosynthetic algae in their cells, like corals. The sponges can get up to 40% of their oxygen from these cells in return for having to compete for well-lit spaces.
Sponges don't need to compete for as many resources as other creatures do, since they get oxygen and nutrients from their filtration. However, as sessile organisms, sponges primarily compete for space. As mentioned previously, most sponges don't have to compete for sunlit areas, and can live in rocky crevices. Many sponges can shed their spicules, which produces an result not dissimilar to surrounding a bed with legos before falling asleep. It makes it very difficult for anything to approach the sponge on the ground, and downright impossible for some creatures to moor there and begin growing. Other sponges can produce water-soluble toxins that form an unpleasant cloud of chemicals around them, but this is generally inefficient if there is a high amount of water flow around them, as the chemical will be dispersed too quickly.
The filtration feeding that sponges use is actually a vital part of explaining Darwin's Paradox  (the question of how such great biodiversity and large populations can proliferate in such a low-nutrient environment. As various dead things decompose, sponge filtration catches much of their relevant nutrients before the detritus can be swept out into the greater ocean current system.  Furthermore, sponges shed their cells basically constantly, depositing these nutrient where the lowest levels of the trophic chain can absorb them and cycle them back into the system.
Like many reef organisms, sponges can damage the reef if they are allowed to overpopulate it. The most common way that this happens involves the depletion of the sponge's natural predators (as usual). More specifically, many common species of sponges counteract predation by outgrowing their predators, instead of making themselves unappetizing. These sponges grow and reproduce extremely quickly, but are normally kept at bay by their predators. When these predators are depleted by overfishing, the sponge populations explode, producing sponge reefs, an otherwise rare ecosystem.
Humans have used sponges for a variety of purposes over the years, from paint pigments to the modern absorption material. Humans used to harvest sponges, but today most sponge-based products use synthetic sponges. However, humans are still damaging sponge populations with weighted nets and even taut fishing lines, which can slice sponges apart by accident. As a rule, anything that can affect the ocean floor will have a negative impact on sponges, as they have never developed protection against impacts.
Sponges are not as directly affected by changing temperatures as coral reefs. In fact, bleached coral makes an excellent site for a sponge to grow, and sponge reefs can form in the wake of coral death (algae reefs tend to be more common because they reproduce more quickly). However, climate change has also been linked to a rise in sponge pathogens. This is especially bad because the sponges that are most vulnerable to pathogens are the largest sponges, because they filter larger amounts of water. These larger sponges are common habitats for reef fish and other organisms, so their death can have an especially serious impact on a reef community.
- Gold, David A., et al. "Sterol and genomic analyses validate the sponge biomarker hypothesis." Proceedings of the National Academy of Sciences (2016): 201512614. http://www.pnas.org/content/early/2016/02/17/1512614113.abstract
- Advances in Sponge Science: Phylogeny, Systematics, Ecology edited by Mikel A. Becerro http://books.google.com/books?id=d8P44czALJAC&pg=PA38&lpg=PA38&dq=homoscleromorpha+sponges&source=bl&ots=RsvjlPcnKg&sig=6yHzdt9wRMZRUy--O7XfuOzKl1w&hl=en&sa=X&ei=1OELU8fYI8SIkQeOv4H4Cg&ved=0CCkQ6AEwATgK#v=onepage&q&f=false
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- Michael W. Taylor, et al. Sponge-Associated Microorganisms. (http://mmbr.asm.org/content/71/2/295.full)
- Jasper M. de Goeij, Dick van Oevelen, Mark J. A. Vermeij, Ronald Osinga, Jack J. Middelburg, Anton F. P. M. de Goeij, and Wim Admiraal. Surviving in a Marine Desert: The Sponge Loop Retains Resources Within Coral Reefs. Science 4 October 2013: 342 (6154), 108-110. [DOI:10.1126/science.1241981] http://www.sciencemag.org/content/342/6154/108.full