Demospongiae (Demosponges)

(invertebrate zoology) A class of the phylum Porifera, including sponges with a skeleton of one-to four-rayed siliceous spicules, or of spongin fibers, or both.

(Demosponges)

Phylum: Porifera

Class: Demospongiae

Number of families: 80

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

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:

1. Subclass Homoscleromorpha, with one order, Homosclerophorida; one family; and about 60 species.
2. Subclass Tetractinomorpha, with four orders, Astrophorida (also known as Choristida), Chondrosida, Hadromerida, and Spirophorida; 22 families; and several hundred species.
3. Subclass Ceractinomorpha, with nine orders, Agelasida, Dendroceratida, Dictyoceratida, Halichondrida, Halisarcida, Haplosclerida, Poecilosclerida, Verongida, and Verticillitida; 57 families; and several thousand species.

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.

Physical characteristics

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 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.

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.

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.

Habitat

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.

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.

Reproductive biology

Some demosponges are hermaphroditic while others have distinct sexes. Their reproduction may be viviparous, oviparous, or asexual. Asexual reproduction occurs by means of budding, fragmentation, or the production of resistant globular bodies called gemmulae. Demosponge larvae are partly or completely ciliated, usually somewhat elongated blastulae (hollow larvae) or parenchymellae (solid larvae) about 300 µm long. The larvae swim or crawl around for a few hours or days at most, after which they settle on a substrate and metamorphose into an adult sponge.

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

Resources

Bergquist, Patricia R. Sponges. London: Hutchinson; Berkeley and Los Angeles: University of California Press, 1978.

Hooper, John N. A., and Rob W. M. van Soest, eds. Systema Porifera: A Guide to the Classification of Sponges. New York: Kluwer Academic/Plenum Publishers, 2002.

Hooper, John N. A., and Felix Wiedenmayer. "Porifera." In Zoological Catalogue of Australia. Vol. 12, edited by A. Wells. Melbourne, Australia: CSIRO, 1994.

Moss, David, and Graham Ackers, eds. The UCS Sponge Guide. Ross-on-Wye: The Underwater Conservation Society, 1982.

van Soest, Rob W. M., Bernard Picton, and Christine Morrow. Sponges of the North East Atlantic. [CD-ROM] World Biodiversity Database CD-ROM Series. Windows version 1.0. Amsterdam: Biodiversity Center of ETI, Multimedia Interactive Software, 2000.

[Article by: Wallie H. de Weerdt, PhD]

A class of the phylum Porifera, including sponges with a skeleton of one- to four-rayed siliceous spicules or of spongin fibers or both. Several genera lack a skeleton. The Demospongiae constitute the most abundant and widely distributed group of sponges, occurring in the sea from the tidal zone down to abyssal depths (at least to 18,000 ft or 5500 m). One family has invaded fresh water. The species vary in size from thin encrustations several centimeters in diameter to huge cake-shaped forms which may measure up to as much as 6.5 ft (2 m) in diameter. See also Porifera.

Comparative studies of the embryology and early attached stages of sponges of the class Demospongiae suggest at least two evolutionary lines within this group: the subclass Ceractinomorpha and the subclass Tetractinomorpha.