The Case for the Paraphyletic Origins of Phylum Porifera

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Based on an observation of its common physical characteristics, Porifera were designated as a distinct phylum with a monophyletic origin. Because of corroborating evidence from fossil records and the fairly standard traits shared by Porifera, this phylum must have derived its attributes from a single ancestor. However, recent research strongly challenges the assertion that Porifera consist of a monophyletic origin. By evaluating the morphology, phylogenetic, and embryologic evidence, a comparison between the classes of Porifera and other metazoans reveals that there are multiple ancestors of this phylum.

The Origins of the Phylum Porifera

The term Porifera refers to sponges that are found in aquatic environments, much like the Eurasian Watermilfoil. Though sponges are among the oldest organisms in the world, their classification as a distinct phylum is a relatively recent phenomenon. During the 19th century, Robert Grant was the first to observe the unique morphology and physiology of sponges and develop a distinct Phylum for this group of organisms (Soest 2). While the documentation of extant sponges is incomplete, to date there are 8,500 species, 4 classes, 25 orders, 128 families, and 680 genera (2). An initial physical observation of Porifera led to initial views of Porifera as a monophyletic Phylum. Early analyses of the Cambrian fossil record documented the conception that many metazoan clades possessed a single common ancestor within a short period of time because the records appeared to document a time in which an explosion of life forms emerged (Minelli 28). Thus, biologists who utilized fossil records as a primary basis for the classification of species operated from the assumption that the origins of phyla were accurately reflected in the records. Yet, a closer evaluation reveals the problematic nature of these generalizations.

A Comparative Morphology of Phylum Porifera

The physical similarities of organisms within Phylum Porifera serve as the first argument for a single ancestry. Characteristics that are used to diagnose the characteristics of sponges and place them within the correct phylum are fairly simplistic to the physical observer. The first characteristic that is noted in sponges is their possession of a low degree of cellular organization (Lal 4-5). As a result, sponges have a porous body, from which their common name is derived. The second prominent feature of sponges is that they possess a canal system (5). Additionally, the skeletal makeup in sponges plays a significant role in their classification. Sponges feature skeletons of calcareous, siliceous spicules, or horny sponging fibers (5). Also, sponges commonly engage in asexual reproduction through gemmules while some sponges reproduce sexually through sperm and ova (5). Of these characteristics, the spine structure is most important for distinguishing different classes of Porifera.

Yet, even with clear physical distinctions, an examination of the morphology of Porifera demonstrates significant diversity within the phylum. For example, within the Demospongiae class, which accounts for 80 percent of all classified sponges, carnivorous sponges are unique because they are equipped to prey upon invertebrate animals (2). Yet, to adapt to this function, they lack aquiferous systems and choanocyte cells, two physical features that serve as diagnostic criteria for the Porifera Phylum (2). Thus, even within one class, there are incongruent morphologies that challenge the homogeneity of the Phylum.

Additionally, researchers have asserted that there are significant differences between the classes of Porifera. Similar to the Demospongiae Class, the Hexactinellida Class consists of sponges that posses hard spines and similar physical structures. However, a unique feature of Hexactinellida Porifera is that they possess soft tissues or syncytial and siliceous spines that posses a triaxonic symmetry (5). However, the Homoscleromorpha possess tetraxonic siliceous spicules while lacking the subdivision in mega and microscleres (Soest 2). Additionally, while the skeletal structure is key to the classification of sponges, some Homoscleromorpha are entirely lacking in skeletons (2). Further, the Calcarea Class is composed of sponges that possess mineral skeletons made of calcium carbonate and aquiferous systems that rage in the complexity of structure (7). Because they the wide physical distinctions between the Homoscleromorpha and Calcarea classes and the latter two classes, researchers have established the hypothesis that Demospongiae and Hexactinellida are a sister group to the classes Calcarea and Homoscleromorpha. Conducting a morphological evaluation of the Porifera Phylum reveals that there is physical evidence for its paraphyletic origins.

A Comparative Phylogenetic Profile of Phylum Porifera

Significant evidence for the paraphyletic characteristics of Porifera can be obtained through phylogenetic evidence. As previously discussed, the Cambrian fossil records initially served as the basis for establishing the monophyletic foundation of the phylum. Because the records were viewed as a complete account of metazoan phyla, it was believed that the evolution of the Porifera Phylum could be traced to its common ancestor in the records. Yet, developments in molecular biology demonstrate that several phyla evolved long before their development was documented by the Cambrian records. For example, molecular biologists revealed a divergence between protostomes and deuterostomes range between 1000 and 750 Ma (Minelli 28). Further, molecular studies confirm the contention that the origins of the Porifera Phylum were not accounted for in the records. For example, based on the alignment of 12060 amino acids encoded from 16 different animal taxa, researchers Rakas et al. determined that Bilateria, Cnidaria, and Porifera all possess multiple ancestors (28). Additionally, an evaluation of well-documented portions of the fossil evidence suggests that the Calcarea Class is a sister group with non-sponge metazoans, including Epitheliozoa (28). Additionally, genetic research determines that at least two metazoan lineages serve as the basis for Phylum Porifera (Borchiellini 176). The phylogenetic evidence further confirms the morphological evidence that there are further distinctions within and between the classes of Porifera.

A Comparative Embryology of Phylum Porifera

Embryological evidence provides a final validation that there are multiple ancestral origins of organisms classified within the Porifera Phylum. According to embryonic research on larval development in sponges, the process of gastrulation involves cellular reorganization that is similar to the process that is observed in other invertebrates (47). Yet, in Homoscleromorpha, the cinctoblastula forms a distinct process of multipolar egression of cells and eventually forms the external layer of the sponge (47). This difference in the early growth of sponges further distinguishes Homoscleromorpha from other classes of sponges. In most groups of sponges, there is homology between the external and internal cell layers (47). Further, the differentiation between the two main cell layers and the formation of an internal digestive cavity is a characteristic that distinguishes sponges from other metazoans (47). Thus, embryology confirms that the unique development of different classes of sponges points to a different ancestor for these divergent groups.

Conclusion

While sponges appear to possess easily identifiable morphological features and solid documentation in the fossil records, a closer evaluation of this phylum reveals that they are not entirely homogenous as a phylum. Further, the distinct characteristics that exist among and between different classes of sponges point to different ancestries from which these attributes are borrowed. As recent molecular research reveals, there are at least two ancestral groups that can be attributed to the four main classes of sponges. The differing embryology and evolutionary characteristics of these classes further point to their differentiating qualities. In the totality of the evidence, it is indisputable that the Phylum Porifera is paraphyletic.

Works Cited

Borchiellini, C., et al. “Sponge Paraphyly and the Origin of Metazoa.” Journal of Evolutionary Biology 14.1 (2001): 171-179. Print.

Lal, Sham Sunder, Practical Zoology Invertebrate. Meerut, ID: Global Media, 2010. Print.

Minelli, Alessandro. Perspectives in Animal Phylogeny and Evolution. Oxford: Oxford University Press, 2008. Print.

Soest, Rob, et al. "Global Diversity of Sponges (Porifera)." PLoS One 7.4 (2012): 1-22. ProQuest. Web. 21 Feb. 2014.