Its a common misconception that all sponges are soft & well spongy
An internal ‘skeleton’ is present in almost all sponges. There are two basic components - spicules and collagen fibers.
Spicules are elaborate crystals produced from compounds precipitated by scleroblast cells in the sponge tissue. Spicules are either calcereous (precipitated from calcium ions and carbonate ions) or siliceous (precipitated from silicate salts). The spicules serve at least two roles in the sponge: a kind of mesh-work internal skeleton, and, in some cases, as a protective device against predation.
The Callyspongia spp. in your tank has calcereous spicules..
Simon.
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