Currently, as known to scientists, there are approximately five thousand species of sponges, rendering this phylum one of the most biodiverse collections of organisms, despite their simplicity of composition. For the past two centuries, scientists have disputed the means of classification for porifera. In the hierarchial classification of sponge taxonomy, an entire class has been dissolved in the past few years. It is difficult to delineate between species due to to simple physical features. Therefore, it is critical that meticulous techniques of classification are maintained and developed for utilization by scientists.

Jane Fromont, a curator of the Western Australian Museum, was an extremely helpful contributer to our page. The majorityof the information on this particular page was derived from her paper, written along with Patricia Bergquist, entitled, "Structural Characters and Their Use in Sponge Taxonomy: When Is a Sigma Not a Sigma?" We thank Dr. Fromont for her help and assistance throughout the development of our web site.

There are several different methods which may be applied. Structural methods essentially analyze the spicules of a species. Genera are structured due to similarities. However, such a method is extremely general. By simply using the spicules of a species, complications may arise. For species to be identical, there must be perfectly identical genetic connections.

Cladisitic methods search for characters that are standard throughout two or more organisms. In resembling eachother, there may be advanced or simple characters. Those with advanced characters, referred to as synapomorphies, are often more accurate in ensuring a monophyletic group. Advanced characters are derived from a common ancestor, thus indicating a relationship. Primitive characters, referred to as symplesiomorphies, indicate a paraphyletic group. When advanced characteristics are similar between species, the relationship is closer than if the characteristics are paraphyletic. For example, all sponges have sterol complements. However, if two species have the exact same sterol complement, they are more closely related.

However, how does one distinguish between advanced or simple characters? Henning, a scientist who established many of the criteria pertaining to cladistic classification, used an out-group rule. The rule is stated "If one takes a given character in members of a monophyletic group, and looks for its homologue in a sister group (that is a group more closely related to it than any other group), then if the character occurs in both, then it is plesiomorphous (a primitive characteristic) for the group first considered. It is absent in the sister group and unique to the group under study, it is an apomorphous (advanced) character." When there is a apomorphous character, the members of this group share a common ancestor and may be accurately distributed on the geologic timeline and taxonomic tree. To complement the structural cladistic methods of classification, scientists also apply geologic record, biogeographical facts, and other characteristics exclusive to a specific collection of species.

Currently, however, interest is shifting from the usage of structural and physical characteristics to classify porifera. Terms are confusing, and the results are not as definite. When discussing spicules, scientists must consider both microscleres, spicules that involve individual structures projecting from the sponge, and megascleres, spicules that create a skeletal structure. As technology improves, chemicals and sterol complements are providing answers. However, the old methods were the basis for the developments of modern times.