Hexactinellida (Glass Sponges)

(invertebrate zoology) A class of the phylum Porifera which includes sponges with a skeleton made up basically of hexactinal siliceous spicules.

(Glass sponges)

Phylum: Porifera

Class: Hexactinellida

Number of families: 17

Thumbnail description
Deepwater marine sponges with a glass skeleton, and typically six rays; unusual because of their multinucleate tissues and ability to conduct electrical signals in the absence of nerves

Evolution and systematics

Hexactinellids (glass sponges) are deepwater marine sponges that have skeletons of siliceous (glass) spicules with a distinctive triaxonic (cubic three-rayed) symmetry. Unlike the other two main classes of sponges (Calcarea and Demospongiae), glass sponges lack either a calcareous or organic skeleton. Furthermore, glass sponges are highly unusual in that their major tissue component is a giant "syncytium" (see below) that ramifies throughout the entire body, stretching like a cobweb over the glass skeleton. As their skeletons are both made of glass rather than calcium, early classification schemes grouped hexactinellids with the demosponges; however, at present hexactinellids are separated from cellular sponges (the Calcarea and Demospongiae) in the subphylum Symplasma because of their unique (syncytial) structure. Nevertheless, recent molecular evidence suggests that whereas modern hexactinellids are descended from the most ancient multicellular animals, they are more closely related to demosponges than either group is to the calcareous sponges or other metazoans. There are approximately 500 species of hexactinellids in two subclasses containing five orders, 17 families, and 118 genera.

Hexactinellids have left the oldest fossil record of multi-cellular animals on Earth. Their triaxonic spicules are known from the Late Proterozoic of Mongolia and China. The group thrived during the Middle Cambrian and reached its maximum radiation and diversity during the Cretaceous, when hexactinellids formed vast reefs in the Tethys Sea. Their fossilized skeletons now make up the stony outcrops upon which many castles are built from southern Spain through France, Germany, and Poland to Romania.

Physical characteristics

Within each of the two subclasses of hexactinellids are sponges with loose skeletons—spicules held together by living tissue—and sponges with fused skeletons. Sponges of both designs are essentially vase-shaped, with a large central or atrial cavity, usually with one opening, the osculum. The species with fused skeletons often have mittenlike or finger-like protrusions of the body wall, and some form platelike structures; these species have oscula on each of the projections. The tissue of hexactinellids generally is creamy yellow to white. Some animals are quite clean, so that the whiteness of their tissue looms out of the darkness of their deepwater habitats. Others tend to accumulate particulate matter on the outside and can look quite dirty. The diverse species of hexactinellids vary in length from 0.2 to 5 ft (0.5 cm to 1.5 m); many of the largest hexactinellids are as wide as they are tall.

The body wall is composed of three parts: both the inner and outer peripheral trabecular networks, and the feeding region, which is called the choanosome. Large incurrent and excurrent canals meet at the choanosome, where the fine, branchlike endings of the incurrent canals contact oval, flagellated chambers that create the feeding current through the sponge.

While sponges of other groups are constructed of single cells, each with a single nucleus, the greater part of the soft tissue in a hexactinellid consists of the trabecular reticulum, which contains thousands of nuclei and cytoplasm that is free to move as it is unimpeded by membrane barriers (the "syncytial" condition). The trabecular reticulum hangs from the skeleton in thin strands, resembling a cobweb, and stretches from the outermost layer, termed the dermal membrane, to the innermost layer, the atrial membrane. Single cells, specialized for particular functions, are also present. Cells are attached to one another and the trabecular reticulum by a unique type of attachment structure referred to as the "plugged junction," which is called so because it is slotted into the neck of cytoplasm between two regions, resembling a plug. It is not an extension of the lipo-protein cell membrane, but is a multilaminar proteinaceous structure. While it undoubtedly serves as a partial barrier to the free exchange of materials, electric currents can flow through it and transport vesicles are able to move through pores in its structure. Between cellular and syncytial components of the sponge there is a very thin collagen layer, the mesohyl. This layer is believed to be too thin for cells to migrate within, as is the case with other sponges. Instead, transport of nutrients and other materials appears to occur along vast networks of microtubules within the multinucleate tissue.

Distribution

Hexactinellid sponges are known at depths from 30 to 22,200 ft (9.14–6,770 m) in all oceans. There are no records of this class in freshwater. The fossil record suggests that their historical range was similar.

Habitat

The vast majority of hexactinellids live at depths greater than 1,000 ft (304.8 m). In a few coastal locations, however, such as Antarctica, the northeastern Pacific, New Zealand, and some caves in the Mediterranean, species are found at depths accessible by scuba divers. These habitats have in common cold water (35–52°F, or 2–11°C), relatively high levels of dissolved silica, and low light intensity. Although many hexactinellids require a firm substratum, such as rocks, for attachment, others grow on fused skeletons of dead sponges, and still others live over soft sediments. The latter group, though not numerous, support themselves on struts made of bundles of long spicules that project down into the sediment.

Behavior

Sponges are not noted for their complex behavior. Nevertheless, hexactinellids can respond to mechanical or electrical stimuli by instantly shutting down the feeding current. The explanation for this unusual ability lies in their possession of a trabecular reticulum, which acts like a nervous system, conveying impulses to all parts of the body. Sponges of other groups lack any such system and show no evidence of an ability to conduct electrical impulses. Electrical signals traveling at 0.07 in per second (0.26 cm per second) have been recorded from slabs of the body wall of R. dawsoni. It is presumed that when the signals reach the flagellated chambers, the pumping stops. No rhythmic pattern has been found in the cessation of pumping. It is thought that since glass sponges lack motile cells that would otherwise remove unwanted material from the sponge, shutting down the feeding current may prevent the clogging of the canal system with large amounts of debris.

Feeding ecology and diet

Like the majority of sponges, glass sponges are thought to filter food from the water that they pump through the choanosome. Two in situ studies confirm this to be true for sponges that lack debris on their outer surfaces. Studies comparing the content of inhaled and exhaled water showed that both Aphrocallistes vastus and Sericolophus hawaiicus retain particulate matter—mostly bacteria—and rely little on dissolved organic carbon. The results of one study that compared such water samples from a sponge covered in debris (Rhabdocalyptus dawsoni) suggest that particulate matter is not retained and that the sponge instead relies for nutrition entirely on the uptake of dissolved organic carbon. It is thought that the organisms coating the sponge produce sufficient organic carbon for themselves and their host. Nevertheless, R. dawsoni can phagocytose both bacteria and latex beads in laboratory preparations.

Laboratory experiments with R. dawsoni and Oopsacas minuta (also a rosellid sponge, but one with a clean exterior) have shown that uptake of particulates occurs in the trabecular syncytium near to and in the flagellated chambers, not in choanocytes, as is normally the case in cellular sponges. In hexactinellid sponges the structure equivalent to a choanocyte, the collar body, lacks a nucleus, and in most species examined so far the collar body is enveloped by extensions of the trabecular syncytium (the primary and secondary reticula), which do most of the particulate capture and uptake. The siliceous skeleton may protect hexactinellids from many predators, but at least one asteroid species is not deterred. Pteraster tesselatus frequently can be found digesting the soft tissues off the skeleton of R. dawsoni.

Reproductive biology

It is generally thought that most hexactinellids lack a seasonal reproductive period because of their deepwater habitat. Nevertheless, because of the difficulty in collecting and preserving these sponges there is little information on reproduction in most deepwater populations. Our knowledge of their development comes from studies done on only a handful of species. Hexactinellid sponges are viviparous. Eggs arise from cells within groups of archaeocytes, a type of pleuripotent cell found in all sponges. The first cell divisions that occur are equal and result in the formation of a hollow ball of cells (a blastula). Gastrulation (the formation of two cells layers during embryonic development) is said to occur by delamination. The larvae are top-shaped with a girdle, band, or cilia around their middle. The tissues are already syncytial, although it is not yet known how the multinucleate tissue arises. When the larva matures, it is released through the osculum. The species Oopsacas minuta, which is reproductive throughout the year, produces the only known live larvae. When studied in a laboratory setting larvae swim slowly to the surface of dishes in left-handed rotations (clockwise, as seen from the anterior pole); although they can swim for several days, they begin to settle and transform into juvenile sponges within 12 hours of release from the parent sponge.

Conservation status

In general, most hexactinellid sponges inhabit areas well out of the reach of human activity. However, on the continental shelf of the northeastern Pacific in British Columbia, reefs of hexactinellid sponges several city blocks in area have been damaged by trawlers. New legislation for the establishment of marine protected areas around these reefs is under development. No species of hexactinellid is listed by the IUCN.

Significance to humans

Euplectella aspergillum, which harbors a pair of crustaceans within its enclosed atrial cavity for life, is commonly given to newlyweds in Japan as a symbol of bonding.

Species accounts

Resources

Reiswig, Henry M. "Class Hexactinellida Schmidt, 1870." In Systema Porifera, A Guide to the Classification of Sponges. Vol. 2, Calcarea, Hexactinellida, Sphinctozoa, Archaeocyatha, Unrecognizable Taxa, and Index of Higher Taxa, edited by John N. A. Hooper and Rob W. M. Van Soest. New York: Kluwer Academic/Plenum Publishers, 2002.

Boury-Esnault, N., S. Efremova, C. Bézac, and J. Vacelet. "Reproduction of a Hexactinellid Sponge: First Description of Gastrulation by Cellular Delamination in the Porifera." Invertebrate Reproduction and Devevelopment 35, no. 3 (1999): 187–201.

Conway, K. W., M. Krautter, J. V. Barrie, and M. Neuweiler. "Hexactinellid Sponge Reefs on the Canadian Continental Shelf: A Unique 'Living Fossil.'" Geoscience Canada 28, no. 2 (2001): 71–78.

Lawn, I. D., G. O. Mackie, and G. Silver. "Conduction System in a Sponge." Science 211 (1981): 1169–1171.

Leys, Sally P. "The Choanosome of Hexactinellid Sponges" Invertebrate Biology 118 (1999): 221–235. ——. "Cytoskeletal Architecture and Organelle Transport in Giant Syncytia Formed by Fusion of Hexactinellid Sponge Tissues." Biological Bulletin 188 (1995): 241–254.

Leys, Sally P., and N. R. J. Lauzon. "Hexactinellid Sponge Ecology: Growth Rates and Seasonality in Deep Water Sponges." Journal of Experimental Marine Biology and Ecology 230 (1998): 111–129.

Leys, Sally P., and G. O. Mackie. "Electrical Recording from a Glass Sponge." Nature 387 (1997): 29–30.

Mackie, G. O., and C. L. Singla. "Studies on Hexactinellid Sponges. I. Histology of Rhabdocalyptus dawsoni (Lambe, 1873)." Philosophical Transactions of the Royal Society of London B 301 (1983): 365–400.

Mackie, G. O., Lawn, I. D., and M. Pavans de Ceccatty. "Studies on Hexactinellid Sponges. II. Excitability, Conduction and Coordination of Responses in Rhabdocalyptus dawsoni (Lambe, 1873)." PhilosophicalTransactions of the Royal Society of London B 301 (1983): 401–418.

Perez, T. "La Rétention de Particules par une Éponge Hexactinellide, Oopsacas minuta (Leucopsacasidae): Le Rôle du Réticulum." Comptes Rendus de l'Academie des Sciences de Paris 4 (1996): 1–29.

Reiswig, Henry M. "Histology of Hexactinellida (Porifera)." Colloques Internationaux de Centre Natnional de Recherche Scientifique 291 (1979): 173–180.

Reiswig, Henry M., and G. O. Mackie. "Studies on Hexactinellid Sponges. III. The Taxonomic Status of Hexactinellida within the Porifera." Philosophical Transactions of the Royal Society of London B 301 (1983): 419–428.

Reitner, J., and D. Mehl. "Early Paleozoic Diversification of Sponges: New Data and Evidences." Geologisch-Paläontologische Mitteilungen Innsbrück 20 (1995): 335–347.

Schultz, F. E. "Report on the Hexactinellida Collected by H.M.S. Challenger during the Years 1873–1876." Report on Scientific Research of the Challenger, Zooolgy 21 (1887): 1–513. ——. "On the Structure and Arrangement of the Soft Parts in Euplectella aspergillum." Royal Society of Edinburgh Transactions 29 (1880): 661–673.

Wyeth, R. C., Leys, S. P., and G. O. Mackie. "Use of Sandwich Cultures for the Study of Feeding in the Hexactinellid Sponge Rhabdocalyptus dawsoni (Lambe, 1892)." Acta Zoologica 77, no. 3 (1998): 227–232.

The Sponge Project. "Recent Hexactinellid Sponge Reefs on the Continental Shelf of British Columbia, Canada." April 22, 2003 [June 11, 2003].

[Article by: Sally P. Leys, PhD; Henry M. Reiswig, PhD]

A class of sponges whose skeletons are made of siliceous hexactine spicules. These exclusively marine sponges are widely distributed in modern oceans. Their fossil record extends from the late Precambrian to the Recent. The basic spicule type of the class is a triaxial hexactine, in which the three pairs of opposed rays areat right angles to each other and lie along one of the three axes of a cube. Proximal ray ends and axial filaments meetat the center of the cube. These principal spicules and variants of that form make up skeletons of the sponges.

Recent hexactinellid sponges are chiefly upper bathyal marine animals and are most common indepths of 200–2000 m (660–6560 ft), although many species are known to inhabit lower bathyal depths. Living hexactinellid sponges are commonly goblet- or vase-shaped, although branched, massive, tubular, or ropy-appearing spongesalso occur in the class. Many have root tufts of long spicules that anchor them in place and support them above the sea floor.

The following classification is a combination of ones used in living and fossil sponges. .

Class Hexactinellida

     Subclass Hexasterophora

          Order Lyssacinosida

          Order Hexactinosida

          Order Lychniscosida

     Subclass Amphidiscophora

          Order Reticulosa

          Order Amphidiscosa

          Order Hemidiscosa