Sponges are the most primitive multicellular organisms that possess no proper organs. All members of this phylum (Porifera) are permanently attached to another surface, such as rocks, corals, or shells. More than 10,000 species have been described to date. Although some species occur in freshwater, the vast majority are marine, living mainly in shallow tropical waters. A wide range of forms occur that are characterized by their different shapes and composition: some are tall, extending well into the water column, while others are low encrusting forms that spread out over a surface. Others, such as the Venus flower basket (Euplectella sp.), are tall and comprise an intricately-formed latticework arrangement, while many leuconoid sponges are goblet shaped. Despite their appearance, all sponges have a definite skeleton which provides a framework that supports the animal. Some are composed of a calcareous skeleton while others use silica for the same purpose. Still others are comprised of a softer spongy material known as spongin. In all species, however, this skeleton is made up of a complex arrangement of spicules, which are spiny strengthening rods with a crystalline appearance.
Sponges have an amazing power of regeneration. Many invertebrates are capable of growing new body parts that have been injured or snapped off the main body, but sponges are capable of growing into a new individual from even the tiniest fragment of the original body.
In cross section, most sponges consist of a convoluted outer wall that is liberally dotted with pores or openings of different sizes. These allow the free passage of water into the central part of the body, the atrium or spongocoel. Although water enters the body through a large number of openings, it always leaves through a single opening, the osculum. Some species, like the asconid sponges, are usually small and have a simple skeleton with a relatively large atrium. Others, however, have developed highly convoluted skeletons that not only maximize water intake, but also provide an opportunity for a maximum amount of oxygen and food particles to be absorbed. Each of the individual chambers in the spongocoel is lined with specialized collar cells or choanocytes, which consist of a flagellum encircled by a collar. These cells are responsible for producing the current of water through the sponge.
Sponges rely on large volumes of water passing through their bodies every day. All sponges feed by filtering tiny plankton from the water current. This same water also provides the animals with a continuous supply of oxygen and removes all body wastes as it leaves the sponge. In some ways, a sponge resembles a powerful water pump, drawing in water through its numerous pores and passing it through the body. Some of the larger species have been estimated to pump more than 5.3 gal (20 l) of water per day. The regular current is assured by countless numbers of tiny flagella that line the many chambers throughout the body wall.
Reproduction is a crucial event in the life history of all sponges, as it not only ensures the continuity of the species but also, for these immobile forms, represents the only chance the animal has of dispersing and finding a new home for itself. Once the sponge has settled on the substrate it has to remain there for the rest of its life. Sponges reproduce both by sexual and asexual means. In the latter, the simplest manner of producing new offspring is through the process known as branching or budding off, whereby the parent sponge produces a large number of tiny cells called gemmules, each of which is capable of developing into a new sponge. A simple sponge (for example, Leucosolenia sp.) sprouts horizontal branches which spread out over nearby rocks and give rise to a large colony of upright, vase-shaped individuals. Many sponges also produce asexual reproductive units known as gemmules that consist of a mass of food-filled cells surrounded by a protective coat strengthened by spicules. In such a state the cells are able to withstand periods of drought or food scarcity; when conditions improve once again the cells become active, break through the outer coat, and develop into a new sponge.
The process of sexual reproduction requires the production and release of large numbers of male sperm cells that are often released en masse in dense clouds. As these are transported by the water currents, some will enter other sponges of the same species. Here they will be trapped by the choanocytes from where they will be transported to the special egg chambers. Once there, fertilization may take place. The fertilized egg then goes through a process of division and transformation, developing eventually into a flagellated embryo known as an amphiblastula. When ready, this will be released from the parent sponge and will be carried away from the parent by the water current.
In their natural environment, sponges face a wide range of predators. Fish and sea slugs feed on a wide range of marine sponges. Some of the larger species have been indiscriminately harvested by humans for resale as ornamental souvenirs, while in some parts of the world the large spongin species have been collected for domestic purposes, the skeletons being used as elaborate bath sponges. Sponge fishing is still a major industry in many countries.
As they are dependent on clean, clear water, many sponges suffer as a result of sedimentation caused by inappropriate land-based activities such as agriculture and deforestation. The resulting runoff after rainfall, often with high levels of particulate matter, not only clouds the surrounding waters but also clog up the passageways in the sponges' skeleton. Aquatic and terrestrial-based pollutants represent a similar threat to these species.