Demospongiae (Demosponges)

Members of Group Aditya Choirul Firdaus Azis Punjung Pinandito Dyah Palupi Ganjar Sayogo Utomo Juan Ghaviky Sagida Lalu Rizal Adit Pratama Luhur Wicaksono Marthin Julianto Sitorus

The Demospongia is by far the most diverse sponge group. Greater than 90 percent of the 5,000 known living sponge species are demosponges. This ratio is not maintained in the fossil record, where less than half of the known genera and families are demosponges. However, the vast majority of living demosponges do not possess skeletons that would easily fossilize, thus their fossil diversity, which peaks in the Cretaceous, is probably an enormous underestimate of their true diversity. As their great number of species would suggest, demosponges are found in many different environments, from warm high-energy intertidal settings to quiet cold abyssal depths. Indeed, all of the known freshwater poriferans are demosponges.

Demosponge skeletons are composed of spongin fibers and/or siliceous spicules, though one genus (Oscarella) has neither. Demosponge spicules, if present, are siliceous, have one to four rays not at right angles, and have axial canals that are triangular in cross section. Demosponges take on a variety of growth forms from encrusting sheets living beneath stones to branching stalks upright in the water column. They tend to be large and only exhibit the leucon grade of organization.

Demosponge systematics is an active area of research, and much is still to be learned. However, some rudimentary outlines can be made. The basal clade of the Demospongia is the Homoscleromorpha, characterized by the possession of a larva more reminiscent of that of the Calcarea than that of the rest of the Demospongia. Demosponges other than the Homoscleromorpha are split into two major groups, the Tetractinomorpha and the Ceractinomorpha. These two groups share characters that indicate common descent such as a distinctive larval type and the presence of spongin. Currently, the two groups are each characterized by distinctive types of microscleres, though some doubt still remains as to whether the distinctive microsclere types evolved only once in each group. Fossils of each of these groups is known from the Cambrian suggesting an early radiation of the major clades of demosponges. The Lithistida, a taxonomic grouping into which many of the fossil demosponges fall, is most certainly polyphyletic with members in both the Tetractinomorpha and the Ceractinomorpha.

Thumbnail Description Soft, elastic, but also tough, friable, or hard, frequently brightly colored sponges Varying in shape from encrusting, massive, tubes, or branches to cups or vases The body reinforced by spongin, siliceous (containing silica) spicules, or a combination of both

Physical Characteristic The demosponges as a group display a wide variety of shapes, colors, textures, skeletal architectures, and spicule morphology. There are species that are capable of hollowing out limestone, penetrating deep inside rocks, coral heads, and shells. Most demosponges have skeletons made of siliceous spicules, spongin fibers, or a combination of both; one group, however, has no skeleton at all. The architecture varies widely among the different groups; it may be reticulate (netlike), confused, radial (spreading outward from a common center), plumose (feathery) or axially compressed. The spicules are usually divided into two size categories (megascleres and microscleres) with a distinct morphology.

The order Poecilosclerida is the largest and most diverse order, with 25 families and several thousand species. Although this group displays a wide variety in form and skeletal architecture, it has a unique feature— chelae, which are meniscoid (crescent-shaped) microscleres with a curved shaft and recurved, winglike or broadly rounded structures at each end. These chelae are extremely diverse, and new ultrastructural characteristics are still being discovered.

The Dendroceratida, Dictyoceratida, and Verongida, also known as Keratosa, are sponges with a skeleton made up only of spongin without spicules. All commercial bath sponges belong to the Dictyoceratida. Taken together, these orders contain 10 families and about 450 species. The sponges are often rather tough and flexible; in one family, the Spongiidae, both the surface and the spongin fibers may be heavily coated with foreign spicules and detritus. Species of the order Verongida are easily noticed tube-, fan-, or vase-shaped sponges, frequently colored a deep sulphur yellow. When these sponges are damaged or exposed to air, their color changes rapidly to a deep purple or black.

The order Haplosclerida comprises 13 families and hundreds of species. All freshwater sponges belong to this order as the suborder Spongillina. They are frequently cushionshaped; however, encrusting, branching, tubular, vase-, and fan-shaped forms are also quite common. Their coloring is not very intense; most sponges in this order come in delicate shades of purple, lavender, light brown or blue. Most freshwater sponges are green. They are rather soft and easily squeezed except for species of the suborder Petrosina, which are firm and cannot be compressed. All haplosclerids have a netlike skeleton of smooth, single-rayed, one-pointed short megascleres bound together by different amounts of spongin. Most marine haplosclerids have no microscleres. Where microscleres are present, they are very simple in structure and none are unique to the order. The spicules of the Spongillina are more elaborate, with smooth or variably ornamented megascleres and several kinds of microscleres. The simple structure of the spicules, combined with a very high degree of variability in skeletal architecture in some species, make the marine Haplosclerida among the most difficult sponges to identify.

Scientific Classification of Demosponges

Evolution and systematics The demosponges originated in the Cambrian period and form the largest class of the phylum Porifera, containing about 85% of all described Holocene species. The class Demospongiae is divided into three subclasses (see Scientific Classification of Demosponges). The names of these subclasses have been in use for several decades. As of 2002, however, with the publication of Systema Porifera, several changes in classification have been made and definitions refined. These changes have made the subclasses more homogeneous, though still not completely so.

The subclass Homoscleromorpha is a small and well-defined group of sponges with or without a skeleton, characterized by viviparous reproduction and a unique incubated cinctoblastula type of larva. If skeletal elements are present, they are relatively small, consisting of tetraxonic (four-rayed) siliceous spicules without a clear distinction between megascleres (large spicules) and microscleres (small spicules). The Tetractinomorpha have monaxonic (single-rayed) spicules in addition to large tetraxonic spicules; asterose (star-shaped) microscleres; a skeleton that is usually radial or axially compressed; predominantly oviparous reproduction and parenchymellar (solid) or blastular (hollow) larvae. Ceractinomorpha is the largest and most diverse subclass, with a wide variety of monactine megascleres and various kinds of microscleres, with the exception of asterose forms. In general, sponges in this subclass have skeletons made of spongin and spicules in different proportions, with a variety of skeletal structures. Their reproduction is predominantly viviparous and their larvae are parenchymellar.

The former class of Sclerospongiae, which was proposed in 1970 ("sclerosponges"), together with the former order Ceratoporellida, formed a polyphyletic (descended from more than one line of ancestors) group of coralline sponges that included several Holocene species as well as fossil sponges. The Sclerospongiae are hard, stony sponges with a rigid calcareous basal skeleton in addition to an otherwise "normal" demosponge type of skeleton and spicule complement. Since 1985 the class name Sclerospongiae has been discarded and its families reassigned to different orders on the basis of characteristics reflecting common ancestry.

Another polyphyletic group is the former order Lithistida, which included many fossil and several Holocene species characterized by a special type of spicules called desmas. Most species in this group were deep-water sponges. The evolutionary history of these sponges is still far from resolved; some appear to be related to the Astrophorida and others to the Hadromerida. Most taxa (categories) in this group, however, have been classified as an artificial fifth order (Lithistida) in the subclass Tetractinomorpha. Axinellida, another polyphyletic group, is no longer defined as an order. Its families have been reassigned to various orders of Tetractinomorpha and Ceractinomorpha.

Verticillitida consists of the fossil family Verticillitidae. It belongs to an unrelated assemblage of mainly calcified fossil sponges with chambered structures known as Sphinctozoa. One Holocene genus, Vaceletia, which has one known polymorphic species and possibly other "living fossil" species, has been assigned to this order.

Distribution The Astrophorida, Chondrosida, Hadromerida, Halichondrida, both the marine and freshwater Haplosclerida, the Homoscleromorpha, Poecilosclerida and most Spirophorida have a worldwide distribution. The Agelasida, Dictyoceratida, and the sclerosponges, however, are found mostly in the tropics. The Verticillitida; the spirophorid family Spirasigmidae; and two families of the Verongida, the Pseudoceratinidae and the Aplysinellidae, are restricted to the Indian and Pacific Oceans; while the Halisarcida, the dendroceratid family Dictyodendrillidae, and the dictyoceratid family Thorectidae are not found in the polar regions.

Habitats Most demosponges occur in all habitats at all depths. The Homoscleromorpha, Chondrosida, Agelasida, Dendroceratida, Halisarcida, and most Dictyoceratida occur mainly in the shallower parts of the oceans. The sclerosponges prefer cryptic (hidden) habitats.

Behavior Most demosponges are immobile animals attached at the base to a substrate, or surface on which they live. Some species, however, successfully compete with corals and other sponges for space by releasing toxic chemicals.

Reproduction 1. Asexual Reproduction a. External buds are small individuals that break off after attaining a certain size. b. Internal buds or gemmules are formed by archaeocytes that collect in mesohyl and are coated with tough spongin and spicules; they survive drought, freezing, etc. 2. Sexual Reproduction a. Most are monoecious with both male and female sex cells in one individual. b. Sperm arise from transformed choanocytes. c. In some Demospongiae and Calcarea, oocytes develop from choanocytes; others derive them from archaeocytes. d. Sponges provide nourishment to the zygote until it is released as a ciliated larva. e. In some, when one sponge releases sperm, they enter the pores of another. f. Choanocytes phagocytize the sperm and transfer them to carrier cells that carry sperm through mesohyl to oocytes. g. Some release both sperm and oocytes into water.

3. Regeneration and Somatic Embryonogenesis a. Sponges can regenerate wounded portions. b. Sponge fragments aggregate into new structures, this is somatic embryogenesis.

Feeding Ecology and Diet Like all other sponges, the Demospongiae are filter-feeders. One genus consists of carnivorous species that engulf and digest small crustaceans.

Conservation Status In response to the overfishing of commercial sponges, patrimonial interest, and rare and remarkable characteristics of certain sponges, eight Mediterranean sponges are protected under the Bern Convention of 1998, and an additional seven species are protected in Italy.

Significance to Humans Several species are of pharmacological interest because of the production of bioactive compounds with antiviral (spongothymidine) and antibacterial (polybrominated diphenyl ethers) properties. Mediterranean and Caribbean horny sponges have commercial value as bath sponges.

Species Accounts ,[object Object]

Demospongiae (Demosponges)

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