The body plan of members of the Phylum Porifera, known as sponges, is perhaps the simplest among living animals. Sponges contain few different cell types, and these are not organized into the distinct tissues that characterize members of other animal phyla. Sponges are also relatively uniform in manner of life—they are all sessile, aquatic, and feed on particles suspended in water. Despite this simplicity of form and function, sponges are an important component of animal diversity in marine and freshwater habitats (with at least 5,000 living species)—and they have been since they appeared as fossils in the late Pre-cambrian. Sponges play vital ecological roles in many aquatic habitats, especially coral reefs, and because they are thought to be the most primitive living animals, they also play an important role in studies of animal phylogeny.
Living sponges are divided into three classes: the Calcarea, Demospongiae, and Hexa-ctinellida. Members of the first two groups make up the vast majority of known species, and much of what follows will describe these groups. Hexactinellids are quite different and will be discussed separately.
Although some species are consistently tube- or vase-shaped, the bodies of many
The Internal Anatomy of a Demosponge
Note: Arrows show water flow patterns through the body. The outer surface of the sponge is covered with pinacocytes. Water enters canals through ring-shaped pinacocytes, propelled by the flagellae of numerous choanocytes (in this sponge, the choanocytes are organized into distinct chambers). Used water is passed into the spongocoel and eventually out the osculum.
sponges lack any obvious symmetry, taking the form of irregular encrustations or lumpy or branching masses of tissue. This irregularity masks a consistent organization at the cellular level. Sponges are organized around a system of canals through which they pump a unidirectional water current (see Figure 1). The outer surface of the body is lined by a single layer of flattened cells, the pinacocytes. Some pinacocytes are ring-shaped, with the central hole serving as a pore through which water enters the sponge's water-pumping system. The canals of this system may be lined in part with more pinacocytes, but they also include regions lined with distinctive cells called choanocytes (see Figure 2). Each choanocyte bears a collar of microvilli from which protrudes a single flagellum. These fla-gellae provide the motive force for pumping water through the sponge. The choanocyte-lined canals eventually lead to a central chamber, the spongocoel, which opens to the outside by one or more large openings, or oscula.
Between the outer pinacocyte and inner choanocyte layers lies a thin, gelatinous matrix, the mesohyl. The mesohyl contains mobile cells, the amoebocytes, and also structures that contribute to the shape and firmness of the
sponge—for example, collagen fibers and mineralized spicules. The composition of these structural elements varies among sponges and plays an important role in their systematics. For example, members of the Class Calcarea have spicules made largely of calcium carbonate; in demosponges the spicules are siliceous. Spicules come in an astonishing array of shapes and sizes. The mesohyl of some demosponges also includes particularly robust arrays of collagen fibers that form a meshlike network, called spongin. When the body of one of these sponges is dried and cleaned, all that remains is the spongin. This material has been used for centuries by humans as "bath sponge."
Water pumped through the sponge by the choanocytes provides food and oxygen, as well as a medium for carrying out metabolic wastes. As water passes over the choanocytes, sus pended particles are captured on their microvilli and taken into the cells, where they are digested. Very small particles, including bacteria, can be removed from the water in this way. Used water passes into the spongocoel and out the oscula. Because sponge canal systems may be very convoluted and contain large numbers of choanocytes, sponges can process water at enormous rates. Some sponges also take advantage of ambient currents to help drive water through their canal systems.
Unlike other animals, sponges have no internal systems for transporting food, respiratory gases, or metabolic wastes. Most cells are in direct contact with or at least very near the water passing over the sponge or through the canal system, and each cell exchanges material with this water independently.
Most sponges are hermaphroditic. Adults typically release sperm into the seawater; sperm are captured by choanocytes of neighboring sponges and transported to the mesohyl, where they fertilize eggs. Embryos are brooded in the mesohyl until they are released as larvae that settle to the bottom and metamorphose into juvenile sponges.
The above applies to the most frequently encountered sponges, members of the Calcarea and Demospongiae. Calcareous sponges are all marine. Demosponges, which include most living sponges, occur in both marine and freshwater habitats. They are often brightly colored and may reach several meters in diameter. Some demosponges that contain substantial amounts of spongin are harvested for sale as bath sponges. A few species bore into calcareous substrates and are important eroders of coral reefs. Demosponges are also actively studied, because many contain bioactive compounds that may be useful as drugs.
A few demosponges have evolved an unusual body form—a massive calcareous basal skeleton on which rests a thin film of sponge tissue containing siliceous spicules. When living species with this body form were first discovered, in cryptic tropical marine habitats in the 1960s, they were accorded status as a separate class, the Sclerospongiae. The discovery of living "sclerosponges" allowed paleontologists to recognize previously problematic fossil taxa—archeocyaths, chaetitids, sphincto-zoans, and stromatoporoids—as sponges. Members of these groups were extremely important reef-building organisms in the Paleozoic and Mesozoic eras.
Hexactinellid sponges are so peculiar that some argue they merit placement in a separate phylum. Their spicules are characteristically six-pointed and siliceous, and they are sometimes fused to form a rigid internal skeleton. Spicules are mostly surrounded by a few large syncitia—multinucleate cells—rather than the typical sponge construction of layers of mononucleate cells. Even the choanocytes, otherwise characteristic of all sponges, are unique in form. Although mostly separate from the rest of the syncitial body, they contain no nuclei. Hexactinellids are exclusively marine in habitat.
Sponges are thought to have evolved from protists called choanoflagellates, which closely resemble choanocytes. Phylogenetic analyses suggest that sponges share a common ancestor with the rest of the metazoans. Relationships among the three living classes of sponges are not well resolved.
See also: Evolutionary Biodiversity; Protoctists Bibliography
Bergquist, Patricia R. 1978. Sponges. Berkeley: University of California Press; De Vos, Louis, et al. 1991. Atlas of Sponge Morphology. Washington, DC: Smithsonian Institution Press; Nielsen, Claus. 2001. Animal Evolution: Interrelationships of the Living Phyla. London: Oxford University Press.
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