Methods of Classification
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
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.