Discussion of Phylogenetic Relationships
Although the presence of three distinct groups among the extant sponges (classes Hexactinellida, Calcarea, and Demospongiae) is largely unchallenged, the evolutionary relationships among them are controversial. Gray (1867) was first to subdivide all sponges into ‘Porifera Silicea’ and ‘Porifera Calcarea’ based on the chemical composition of sponge spicules (silica vs. calcium carbonate), a view still advocated by some scholars (e.g., Böger 1988). More recently, Reid and Reiswig and Mackie (Reiswig and Mackie 1983) subdivided Porifera into ‘Cellularia’ (Demospongiae plus Calcarea) and ‘Symplasma’ (Hexactinellida), based on the syncytial nature of the choanoderm and pinacoderm in glass sponges (Hexactinellida). Unfortunately, neither chemical composition of spicules nor syncitial nature of glass sponges are likely phylogenetically informative. First, a diverse range of siliceous structures is known in different groups of unicellular eukaryotes, including choanoflagellates, the sister group of animals (Preisig 1994). Second, the syncytial tissue in glass sponges appears to be a derived trait within this group because the development of glass sponges starts with a cellular embryo (Leys, Cheung, and Boury-Esnault 2006).
Similarly, molecular studies have so far been also inconclusive in regard to poriferan relationships, although they suggested two interesting alternatives: 1) the paraphyly of sponges (Cavalier-Smith et al. 1996; Collins 1998; Adams, McInerney, and Kelly 1999; Borchiellini et al. 2001; Rokas, Kruger, Carroll 2005; Peterson et al. 2008) and b) the presence of a distinct fourth class of sponges: Homoscleromorpha (Borchiellini et al. 2004; Nichols 2005; Peterson et al. 2008). However, the latest and the largest molecular phylogenetic study supports the traditional view that sponges are monophyletic (Philippe et al., in print).
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