Sponges are the simplest of multicellular organisms, in fact they don't even have organs! The highest level of organization in sponges is tissues. They are the only animal on the planet to use silicon as a supporting structure. About 5000 species of sponges have been described worldwide, but only 300 occur in freshwater. Only 27 species are found in the freshwaters of North America.
Sponges are often common and abundant, though many people don't even realize they occur in freshwaters. A sponge can be considered as a living filter, with the ability to create a current and drive water through itself. Larval sponges have flagella and are motile, but the adults are benthic, sessile filter-feeders. Adult sponges are either asymmetrical or radially symmetrical. Water flows into the sponge through openings termed ostia. The ostia lead to incurrent canals and chambers of specialized cells that extract food from the water. Choanocytes are flagellated cells that create the water current in the sponge and drive the water through the canals. The water flows out of the chambers via excurrent canals that drain into a central chamber called the atrium. Water leaves the atrium through a larger "hole" in the exterior of the sponge called the osculum. Most of the cells in a sponge are able to differentiate into any of the cell types, shifting their function as environmental conditions change. The shape of the sponges varies according to the water current and light availability. The surface of the sponge can either be flat, or very convoluted.
The skeleton of the sponge is made up of cells called spicules bound together with special collagen fibers called spongin. The spicules are made of calcium carbonate, silicon dioxide or collagen. The shape of spicules varies among species and is often used in taxonomy. They can be long or short, smooth or spiny, but are usually shaped like a needle.
The sensory system of sponges is very basic. They do not have any sensory cells, but will close their ostia if there is too much particulate matter in the water, or if they are touched.
Sponges obtain their oxygen through diffusion and get rid of excess water by using water expulsion vesicles. Material that is nondigestible is released into the excurrent water canals by reverse phagocytosis.
Sponges reproduce both asexually and sexually. In sexual reproduction, the sponges develop into males or females, but the gender of a sponge is not fixed; it may change from year to year if environmental conditions vary. In the spring "smoking sponges" can be seen. These are male sponges that are releasing sperm into the surrounding water. The sperm is brought into the female sponges by the canal system and taken up by the choanocytes. The sperm is then escorted by a choanocyte to an egg that needs to be fertilized. Freshwater sponges provide their larvae with nutrients and material for growth by a nurse cell. When the larvae are released they are covered with flagellated cells that enable them to disperse.
Some sponges also reproduce asexually by breaking off little pieces of themselves (fragmentation), but the most common manner of asexual reproduction is through the production of gemmules. Gemmules are formed by a mass of cells that is surrounded by a resistant coat made of spongin. The gemmule contains energy-rich materials and has a low metabolic rate until it germinates. They are usually produced in response to adverse environmental conditions and germinate when conditions improve.
Sponges feed by creating a water current through their canal system and filtering particles out of the water. Larger particles that pass through the sponge may be engulfed. Particles are then passed along to the digestive cells that deliver the nutritional constituents to other cells.
Sponges are preyed upon by fish, crayfish and snails. However, the sharp spicules in the skeleton of sponges irritate the mouth and digestive tract of most potential predators. Some sponges also produce toxic compounds that act as a further deterrent against predators.
Sponges are often mistaken for plants in freshwater systems because they usually contain algae symbionts within their cells that make them appear green. The algae and the sponge have a mutualistic association meaning that both benefit from living together. The algae provides sugars and other nutrients to the sponge, while the sponge supplies the algae with the nitrogen, phosphorus and carbon dioxide it need for growth.
Sponges are often hosts to other organisms as well. Protozoans, oligochaetes, nematodes, rotifers, bivalves, water mites and aquatic insects live within or attached to sponges.
There is an ongoing debate about whether sponges are truly colonial or simply individuals because of their ability to grow another sponge from a piece of the original.