Sponge Reef - Characteristics of Hexactinellid Sponges

Characteristics of Hexactinellid Sponges

Hexactinellids, or “glassy” sponges are characterized by a rigid framework of spicules made of silica. Unlike other poriferans, hexactinellids do not possess the ability to contract. Another unique feature of glassy sponges is that their tissues are made up almost entirely of syncytia. In a syncytium there are many nuclei in a continuous cytoplasm; nuclei are not packaged in discrete cells.

As a result, the sponge has a distinctive electrical conduction system across its body. This allows the sponge to rapidly respond to disturbances such as a physical impact or excessive sediment in the water. The sponge’s response is to stop feeding. It will try to resume feeding after 20–30 minutes, but will stop again if the irritation is still present.

Hexactinellids are exclusively marine and are found throughout the world in deep (>1000 m) oceans. Individual sponges grow at a rate of 0–7 cm/year, and can live to be at least 220 years old. Little is known about hexactinellid sponge reproduction. Like all poriferans, the hexactinellids are filter feeders. They obtain nutrition from direct absorption of dissolved substances, and to a lesser extent from particulate materials. There are no known predators of healthy reef sponges. This is likely because the sponges possess very little organic tissue; the siliceous skeleton accounts for 90% of the sponge body weight.

Hexasterophoran sponges have spicules called hexactines that have six rays set at right angles. Orders within hexasterophora are classified by how tightly the spicules interlock with Lyssanctinosan spicules less tightly interlocked than those of Hexactinosan sponges.

The primary frame-building sponges are all members of the order Hexactinosa, and include the species Chonelasma/Heterochone calyx (chalice sponge), Aphrocallistes vastus (cloud sponge), and Farrea occa. Hexactinosan sponges have a rigid scaffolding of “fused” spicules that persists after the death of the sponge.

Other sponge species abundant on sponge reefs are members of the order Lyssactinosa (Rosselid sponges) and include Rhabdocalyptus dawsoni (boot sponge), Acanthascus platei, Acanthascus cactus and Staurocalyptus dowlingi. Rosselid sponges have a “woven” or “loose” siliceous skeleton that does not persist after the death of the sponge, and are capable of forming mats, but not reefs.