Hexactinellid sponges are sponges with a skeleton made of four- and/or six-pointed silaceous spicules, often referred to as glass sponges. They are usually classified along with other sponges in the phylum Porifera, but some researchers consider them sufficiently distinct to deserve their own phylum, Symplasma.
Glass sponges are relatively uncommon and are mostly found at depths from 450 to 900 metres (1,480 to 3,000 ft) although the species Oopsacas minuta has been found in shallow water, while others have been found much deeper. They are found in all oceans of the world, although they are particularly common in Antarctic waters.
They are more-or-less cup-shaped animals, ranging from 10 to 30 centimetres (3.9 to 12 in) in height, with sturdy lattice-like internal skeletons made up of fused spicules of silica. The body is relatively symmetrical, with a large central cavity that, in many species, opens to the outside through a sieve formed from the skeleton. Unlike other sponges, they tend to be present as individuals, rather than forming large fused colonies. They are generally pale in colour.
Much of the body is composed of syncitia, extensive regions of multinucleate cytoplasm. In particular, the epidermal cells of other sponges are absent, being replaced by a syncitial net of amoebocytes, through which the spicules penetrate. Unlike other sponges, they do not possess the ability to contract.
One ability they possess is a unique system for rapidly conducting electrical impulses across their bodies, making it possible for them to respond quickly to external stimuli. Glass sponges like "Venus' Flower Basket" have a tuft of fibers that extends outward like an inverted crown at the base of their skeleton. These fibers are 50 to 175 millimetres (2.0 to 6.9 in) long and about the thickness of a human hair. They work as optical fibers somewhat similar to those used in modern telecommunication networks.
Glass sponges are different from other sponges in a variety of other ways. For example, most of the cytoplasm is not divided into separate cells by walls but forms a syncytium or continuous
mass of cytoplasm with many nuclei (e.g., Reiswig and Mackie, 1983).
These creatures live for a very long time, but the exact age is hard to measure; one study based on modelling gave an estimated age of a specimen of Scolymastra joubini as 23,000 years, which is thought impossible, but is the basis for a listing of ~15,000 years in the AnAge Database. The shallow water occurrence of hexactinellids is rare world wide. In the Antarctic two species occur as shallow as 33 meters under the ice. In the Mediterranean one species occurs as shallow as 18 meters in a cave with deep water upwelling (Boury-Esnault & Vacelet (1994)
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