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					Biology of Invertebrates                                             Spring 2010
Assignment: Sponge Phylogeny                                         Due W 2/10
Homework # 2
Please answer the questions and submit a 1-2 page typewritten response. Feel free to work in
groups and share ideas. However, everyone should compose his/her own answers.
Def. Monophyletic taxon : a group of species that evolved from a common ancestor and that
includes all descendants of that ancestor. By definition every valid clade forms a monophyletic
taxon.
Def. Polyphyletic grouping: an incorrect grouping of species containing species that descended
from 2 or more different ancestors. Typically the error results from use of a convergent character
as the basis for grouping species.
Def. Paraphyletic grouping: an incorrect grouping of species sharing an immediate ancestor
but not including all descendants of that ancestor. Typically this results from use of an ancestral
character as a basis for grouping species.

Overview
In most invertebrate textbooks including ours, sponges are grouped as a single monophyletic
phylum, the Porifera, The group comprises sedentary filter-feeding organisms with a well
defined aquiferous system: a complex network of water-conducting canals and chambers, and
special flagellated cells called choanocytes that pump the water and catch food particles.
Organisms displaying these characteristics are in fact quite different. Some sponges have a
calcareous skeleton (Class Calcarea or Calcispongia), while others have silicious (glass) spicules
(Silicea: Class Demospongiae and Class Hexactinellida). Some glass sponges have a largely
syncitial (i.e. multinucliate) histological organization that is very different from the external
pinacoderm and the internal choanoderm cell layers found in other sponge groups. These
differences have led a number of zoologists to question the monophyly of the Phylum Porifera.
Recent studies employing molecular methods have added to the debate about sponge phylogeny
as shown by the cladograms on pg. 2. Please answer the two questions below on the basis of this
new evidence.
Q.1. Examine the cladogram depicted in figure 1, on pg. 2 of this assignment. According to this
analysis, is the Phylum Porifera monophyletic? Is it Polyphyletic or Paraphyletic or both?
Describe what evidence that helped you reach your conclusion.
Q.2. Figure 2 consists of a cladogram showing the traditional view of sponge phylogeny (a) and
another showing the proposed molecular phylogeny (b). Answer the following question, taking
into account the basic morphological characteristics of sponges described above and in the
textbook (Ch 4): What are the implications of the new sponge phylogeny to our understanding of
the ancestry of sponges and other animal phyla? In answering this question you should consider
(a) whether according to the molecular phylogeny choanoflagellates are still a good model for
the protistan ancestor of animals in general and (b) whether the first metazoans may have been
microsuspension or macrophagous feeders (i.e. eating detritus or plants).

Q.3. How should the taxonomy of sponges should be changed to take into account the new
molecular evidence?
Figure 1. Cladistic analysis of 32 metazoan taxa and 10 sponge species, rooted (i.e. outgroup) on the
plant Arabidopsis and the yeast Saccaromyces. (from Sperling and Peterson, 2007). The term Metazoa
means all animals; Epitheliozoa refers to animals with true epithelia (including Homoscleromorph
sponges but excluding all other sponges); Eumetazoa refers to animals with organ systems.




Figure 2. These cladograms show conflicting hypotheses about sponge phylogeny. The
traditional monophyletic view (a) has Demonspongia (glass sponges) and Calcispongia
(calcarea) sharing a common ancestor that possessed a water-canal feeding system. Thus the
water canal is a shared derived character that defines the monophyletic phylum Porifera. As you
may notice, this phylogeny provides no information on the feeding mode of the Eumetazoan-
Poriferan ancestor (note the question mark). In other words, we cannot know for certain whether
the ancestor was microphagous (i.e. a filter feeder with single cells digesting food) or
macrophagous (gut). The molecular phylogeny (b) shows a conflicting view about sponge
evolution. Moreover, the position of the sponge groups in this phylogeny allows researchers to
draw some inferences about the feeding mode of the Eumetazoan-Poriferan ancestor.
Q.1. Examine the cladogram depicted in figure 1, on pg. 2 of this assignment. According to this
analysis, is the Phylum Porifera monophyletic? Is it Polyphyletic or Paraphyletic or both?
Describe what evidence that helped you reach your conclusion.

According to the newly published phylogeny shown in Figure 1, the Porifera is no longer
monophyletic. The Calcispongia and Homoscleromorpha share a more recent ancestor with
other animals than with Demonspongia and other sponges. In fact, careful reading of the
cladogram reveals that the common ancestor of Calcispongia and Homoscleromorpha (ancestor
#2) , which lived after Demospongia had split from the lineage, was also the ancestor of all the
other Metazoa (i.e. Epitheliozoa). Therefore, the current phylum Porifera that includes only
Calcisponges, Demosponges and Homoscleromorpha violates the cladistics requirement that all
descendants of that ancestor are included in the taxon; in other words all animals would have to
be included in the Porifera to make the grouping monophyletic.

One could make the case that in addition to being paraphyletic the Porifera is now polyphyletic.
This is because it would include the Calcispongia and Homoscleromorpha whose recent ancestor
is #2 with the Demospongia whose recent shared ancestor was #1.
Q.2. Figure 2 consists of a cladogram showing the traditional view of sponge phylogeny (a) and
another showing the proposed molecular phylogeny (b). Answer the following question, taking
into account the basic morphological characteristics of sponges described above and in the
textbook (Ch 4): What are the implications of the new sponge phylogeny to our understanding of
the ancestry of sponges and other animal phyla? In answering this question you should consider
(a) whether according to the molecular phylogeny choanoflagellates are still a good model for
the protistan ancestor of animals in general and (b) whether the first metazoans may have been
microsuspension or macrophagous feeders (i.e. eating detritus or plants).

Cladogram 2B shows that Demosponges are the sister group of the Calcisponge-Eumetazoan
lineage. Since there are sponge-like animals on both sides of the first node, we can infer that the
common ancestor represented by that node was also a sponge-like animal. Since the ancestor
was a sponge like animals and microphage was present in both of its descendent lineages we can
infer further that the ancestor of the Demosponge : Calcisponge-Eumetaoan clades was also a
microsuspension feeder, most likely a sponge like animal with choanocyte cells for feeding.
Thus, the phylogeny in 2B provides insights into what the ancestor of sponges and all other
metazoans may have been like. It also reinforces the view that the ancestor of all multicellular
animals was a colonial choanoflagellate. No such inferences could be made from Cladogram 2A
because sponges occupied only one side of the branching point; it was believed that the water-
canal system was a shared derived character of the sponge lineage only.

Q.3. How should the taxonomy of sponges be changed to take into account the new molecular
evidence? Obviously there can no longer be a single phylum that includes sponges and only
sponges. The Demospongia are a valid monophyletic clade (since its only descendents are
Demospongia) that can be made into a new monophyletic phylum, separate from the
Calcispongia taxon, which includes Homoscleromorpha and all other metazoan descendants of
the Calcisponge-like ancestor.

				
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Description: Biology of Invertebrates sponge