Class Calcarea

There are about 400 described species of sponges in the Calcarea group. (Wörheide, 2002)

Calcarea sponges are found throughout the oceans, but are mainly in temperate areas. (Wörheide, 2002)

Exclusively marine and mainly in temperate regions, Calcarea sponges are usually found in shallower, sheltered waters less than 1000 m. In tropical regions they are associated with coral reefs. (Wörheide, 2002)

Calcarea is thought to be most primitive group of sponges. It is the only class with asconoid and syconoid construction, and has sponges with leuconoid construction as well. The Hexactinellida and Demospongiae groups have only leuconoid forms. Calcarea is divided into two groups, Calcinea and Calcaronea, differentiated by the larvae and location of the nuclei in the choanocytes. Calcinea is further divided to two orders and nine families, and Calcaronea has two orders and ten families. Taxonomic resolution is still being revised. (Brusca and Brusca, 2003; Wörheide, 2002)

Calcarea is the only class with asconoid and syconoid construction. All others have leuconoid construction. The calcium carbonate spicules are only megascleres, or large structural spicules. Other groups of sponges have microscleres, which are smaller reinforcing spicules. Most Calcarea are 10 cm less in height, and are dull in color, although some colorful species are known. (Barnes, 1987; Brusca and Brusca, 2003; Wörheide, 2002)

Reproduction occurs asexually by budding and sexually. Development of fertilized eggs takes place within the sponge. The larval stage has outer flagellated cells, often with spicules. The young break out of the parent's mesohyl, and become free swimming larvae, but not for more than two days.

Sponges have different amoeboid cells in the mesohyl. Acheocytes are large cells with large nuclei. These cells are totipotent, meaning they can develop into any cell type. Sclerocytes, also in the mesohyl, accumulate calcium to produce spicules. Three sclerocytes will fuse to form spicules in intercellular spaces. (Barnes, 1987; Brusca and Brusca, 2003)

Members of Porifera are sessile since they are attached to the substrate. However, some sponges may move as amoeboid cells at the base move. (Brusca and Brusca, 2003)

Sponges will react by closing ostia or oscula, either because of direct physical stimulation or when suspended particles within the sponge are too large or highly concentrated. However, there are no known nerve structures. However, some sponges may respond to electrical impulses. (Bamfield Marine Sciences Centre, 2004)

Because of the simple cells that make up sponges, cell types and water currents used by the sponges to obtain food have been extensively studied. Sponges in general use flagellated cells called choanocyte cells to create a current. Choanocytes are located in the interior part of the sponge. In the asconoid structure, the water is drawn in through the ostium (outer pores), goes through the spongocoel or atrium, and out the osculum (the opening in the top of the sponge). Outer pores are 50 micrometers or less, so larger particles and animals are not ingested.

As food or particles are moved through the sponges, amoemoid cells surround and engulf it (pinocytosis and phagocytosis). Particles are caught in the collar part of the choanocyte cells. (Brusca and Brusca, 2003)

Sponges are preyed on by many animals. Spicules, and other compounds, including potential biotoxins, probably discourage most predators. (Brusca and Brusca, 2003)

Sponges in general may make up a significant portion of the benthic biomass. For example, in Antarctica, at depths of 100-200 m, 75 per cent of the benthic biomass are sponges. (Brusca and Brusca, 2003)

Sponges have been harvested for centuries by many civilizations. Compounds produced by sponges are being explored for pharmaceuticals. (Brusca and Brusca, 2003)

Sediments effect sponges although they are resistant to hydrocarbons (including detergents) and heavy metals. Particular species have been overharvested. (Brusca and Brusca, 2003)