Sponge identification is very difficult. The current classification of Phylum Porifera is based mainly on features of the organic and inorganic skeletons, which are imprecise and there are therefore many exceptions to the rules.
Features used to identify the Porifera include:
- spongin fibres (thick strands of smooth collagen deposited in concentric layers that may form very complex meshes)
- collagen filaments (microscopic protein strands often in tangled bundles)
- mineral skeleton composed of discrete and or fused spicules composed of calcium carbonate (calcitic spicules) or silicon dioxide (siliceous spicules). Some species even have a basal skeleton of solid limestone (termed hypercalcified).
There are three classes of sponges within the phlyum:
- Calcarea (with calcitic spicules)
- Hexactinellida (with siliceous spicules)
- Demospongiae (with siliceous spicules and fibrous skeletons)
There are 25 orders, 127 families and 700 genera currently accepted in the living fauna, and about 8,500 named species worldwide. This is probably about half the number of living species, with many still awaiting discovery, naming and scientific description.
Taxonomic classification to order, family, genus and species level is based mainly on skeletal structure and composition, but increasingly on non-skeletal characters such as mode of reproduction, cellular characters, larval morphology etc. Nevertheless, the current classification is as complex as the diversity of sponges themselves. Sponge taxonomy is not for the faint-hearted because some characters, such as microsclere spicules, can be frequently lost; other characters, such is incorporation of foreign spicules and detritus into the skeleton confound; and other characters, such as some spicule geometries, can be modified to such an extent that appear to differ only subtly from completely unrelated species.
Over the past decade sponge taxonomy has progressed substantially, including the development of more comprehensive text books and online resources, but unfortunately taxonomic keys still have only limited value. Essentially sponge taxonomy is best approached from the species level upwards, not the other direction.
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