Little is known about the biodiversity within this phylum. In two previous studies we have shown that the yet monotypic Placozoa actually consists of several, genetically very different lineages that represent different species, genera or even higher taxonomic units [1,2]. The most suitable genetic marker to characterize the diversity within the Placozoa is the mitochondrial ribosomal large subunit (16S). With this marker a total of 16 different 16S haplotypes were identified distributed among seven clearly separated clades (Fig.1). In addition to 16S we are sequencing nuclear marker genes and complete mitochondrial genomes to robustly reconstruct placozoan relationships.


legend: Fig. 1. Unexpected systematical diversity in the phylum Placozoa, which was formerly assumed to be monotypic. Shown is a ML phylogram (based on a fragment of the large mitochondrial ribosomal RNA, 16S) of 16 different placozoans. Until now seven genetically highly different clades (I-VII) have been identified. Although sampling efforts have been extended in the last years current knowledge on placozoan biodiversity is still limited and more samples are urgently needed. See Eitel & Schierwater, 2010 [1].

We find the molecular characterizations of distinct genetic lineages alone insufficient to erect new taxa in the Placozoa. Thus, we are also studying morphological, developmental and ecological features. As one example for a distinct morphological feature the so-called ‘concave disc’ is shown in Fig. 2 (see [3]).

legend: Fig. 2. Identification of a new morphological feature in placozoans, the concave discs (cd; TEM and SEM imaging). These discoidal structures are interposed between the cell edges in the upper epithelium. Concave discs are found in six out of ten morphologically characterized placozoan lineages, e.g. in the ‘PAN’ [a] and ‘TUN-B’ clonal lineages [b]. The function of the concave discs is yet unknown. See Guidi et al., 2011 [3]

1. Eitel M, Schierwater B (2010) The phylogeography of the Placozoa suggests a taxon rich phylum in tropical and subtropical waters. Molecular Ecology 19: 2315–2327.
2. Voigt O, Collins AG, Pearse VB, Pearse JS, Ender A, et al. (2004) Placozoa -- no longer a phylum of one. Current Biology 14: R944-945.
3. Guidi L, Eitel M, Cesarini E, Schierwater B, Balsamo M (2011) Ultrastructural analyses support different morphological lineages in the Placozoa, Grell 1971. Journal of Morphology 272: 371-378.

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• Bernd Schierwater (This email address is being protected from spambots. You need JavaScript enabled to view it.)

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