The Placozoa Database

This is the smallest of all animal databases yet; it contains a single valid species, Trichoplax adhaerens (Fig. 1). A second species that is sometimes mentioned, Treptoplax reptans, has never been found again after its first and only report and is not an accepted species rather than most likely a fragmentated Trichoplax [1].

With only one species in the phylum the latter knows only three taxonomic ranks: Phylum (Placozoa), genus (Trichoplax), and species (T. adhaerens). This actual situation is soon to change radically. Genetic analyses have suggested that Placozoa harbor at least seven major species clades which may represent different orders with multiple families inside (Fig. 2; [2-4]).

The estimated total number of placozoan species is at least 80 species [5], but the absence of documented morphological and ecological differences and the lack of life-cycle information still hinder the assignment of new species and higher taxonomic units.

The placozoan species database will be updated immediately whenever a relevant manuscript becomes accepted. Please send relevant information to Bernd Schierwater.


Fig. 1: Photograph of Trichoplax adhaerens, F.E. Schulze (1883).




Fig. 2: Unexpected diversity has been found in the phylum Placozoa, formerly assumed to be monotypic. Shown is a MP phylogram of placozoans based on combined SSU and LSU data. The sampling has been very limited so far and in order to unravel biodiversity of this unique phylum, more and extended studies are urgently needed. See Voigt et al. [2].

How does the phylum relate to other animal phyla?

Historically placozoans have been placed at the base of the metazoan tree of life because of the very simple morphology with only five described cell types and the lack of any kind of ECM, nerve cells, and axes. Early molecular phylogenetic studies based on small and large ribosomal subunits (18S and 28S) have placed Trichoplax at various positions in different close relationships to (i) Cnidaria, (ii) Ctenophora, and (iii) even in close relationship to bilateria (Schierwater et al. 2009a). A recent study using a group of genes from the completed nuclear genome placed Trichoplax basal to Eumetazoa with Porifera branching off first (Srivastava et al. 2008). In contrast to this commonly proposed ‘Porifera-basal-model’ another comprehensive study found an early evolutionary split into two sister clades: the ‘bilateria’ (or triploblasts) and the ‘non-bilaterian’ (or diploblasts). The later comprises the basal metazoan Placozoa, Porifera, Cnidaria and Ctenophora, with plaozoans being basal within this clade (Fig. 3). Most recently this picture has been changed twice within a few months using hundreds of species with each tens to hundreds of genes from a growing mass of EST data. These studies placed either Porifera (Philippe et al. 2009) or Ctenophora (Hejnol et al. 2009) at the base of the Metazoa. Both trees, however, seem to hav very low or even no support at the relevant clades (Siddall 2009, in press). It becomes more and more obvious that even thousands of genes might not be able to find a final answer to one of the most important questions in evolutionary biology: the ancestral metazoan phylum. The answer has to be searched for using a combination of ALL available data from various fields including morphology, molecular morphology (secondary structure of various molecules), mitochondrial genome data, comparative evo devo studies and also from whole genome comparisons. At present the only valid bootstrap support suggests a basal position of Placozoa within the diploblats (Siddall 2009, in press).


Fig. 3: Maximum Likelihood Phylogenetic Tree of Metazoan Relationships Using the Concatenated Data Matrix. In this tree Placozoa groups basal within the diploblasts (like in many other studies) but diploblasts and triploblasts occur as sister groups, a hypothesis that is controversially discussed. From Schierwater B, Eitel M, Jakob W, Osigus HJ, Hadrys H, Dellaporta S, Kolokotronis SO, DeSalle R (2009b) Concatenated Molecular and Morphological Analysis Sheds Light on Early Metazoan Evolution and Fuels a Modern “Urmetazoon” Hypothesis. PlosBiology 7(1): e1000020.

Placozoa: Biology and History

Placozoa occur in the littoral of all warm oceans and are distributed globally in tropical and sub-tropical waters. They reproduce by binary (sometimes trinary) fission, by budding off small swarmers or by sexual reproduction.

The placozoan Trichoplax adhaerens is more simply organized than any other living metazoan. This tiny marine animal, with a size of up to 2mm, looks like an irregular “hairy plate” (“tricho plax”) whose unique bauplan is based on a simple, irregular sandwich organization. An upper and a lower epithelium surround a loose network (not an epithelium) of so-called fiber cells (Fig. 4). Traditionally only four cell types have been described in Trichoplax,upper and lower epithelial cells, gland cells within the lower, feeding epithelium, and fiber cells sandwiched between the epithelia [6-10]. No organs or specialized nerve or muscular cells are present. A basal lamina and extracellular matrix are likewise lacking. All these simple bauplan characteristics make placozoans more similar to protozoans than to any other existing metazoans. Body shape is irregular and changes constantly. No symmetry of any kind is seen, and nothing like an oral-aboral or even a dorso-ventral polarity exists. All of the above justified the construction of an own phylum, Placozoa, for Trichoplax adhaerens [11].

After its original description by F.E. Schulze 1883 [12], Trichoplax attracted particular attention as a potential candidate representing the basic and ancestral state of metazoan organization. The simplest of all metazoan morphologies suggested a basal position for T. adhaerens. Presently the phylogenetic position of Placozoa is subject of hottest debates (e.g. [13-16]).

For details and references on placozoan biology and the history of placozoan research see [17, 18].


Fig. 4: Placozoan morphology. Cross section of Trichoplax adhaerens; from Syed and Schierwater [19] modified after Grell & Ruthmann [20]. Please note that this traditional view is soon to be revised [21].

Sampling, conserving, studies

Animal sampling

To sample placozoans we use two different methods. In the first ‘rock sampling’ method, stones and other hard substrates, such as coral parts and mussel shells are collected at a depth of up to 1m and placed in plastic bottles with seawater from the sampling site. As a second method, standard microscopic glass slides (76 x 26 mm) are placed in plastic microscope slide containers (‘slide samples’), which are cut open at the top and the bottom to enable the flow-through of seawater (Maruyama 2004; Eitel & Schierwater, in prep.).

DNA/RNA isolation

To fix animals for DNA preparation, FTA Elute cards micro (Whatman) are used (see Eitel & Schierwater (in prep.), Signorovitch et al. 2005). Animals are dropped onto the cards with as little seawater as possible. After drying the DNA can be stored on the card for several months to years. Alternatively, animals can be put directly in 80-98% ethanol and stored for DNA preparation at 4°C for several months. To preserve RNA, animals can be fixed in RNAlater (Quiagen) and processed when returning from the field to the laboratory.

Experimental studies

Placozoa are easily amenable to experimental studies, and almost the complete spectrum of biological studies has been applied to this group (see Schierwater et al. 2009a for refs.).

New samples, new species

For the genetic identification of placozoan samples please send samples to the Schierwater lab for free genetic analysis (please contact Bernd Schierwater). If new species are identified a joint effort taxonomic circle approach for valid species descriptions is encouraged (please contact Bernd Schierwater).

Editor

Bernd Schierwater

Associate Editors

Michael Eitel
Rob DeSalle

References

  1. Behrendt, G., Ruthmann, A. (1986). The cytoskeleton of the fiber cells of Trichoplax adhaerens (Placozoa). Zoomorphology 106, 123-130.
  2. Buchholz, K., Ruthmann, A. (1995). The mesenchyme-like layer of the fibre cells of Trichoplax adhaerens: A syncytium. Z Naturforsch [C] 50c, 282-285.
  3. DeSalle, R., and Schierwater, B. (2008). An even "newer" animal phylogeny. Bioessays 30, 1043-1047.
  4. Dunn, C.W., Hejnol, A., Matus, D.Q., Pang, K., Browne, W.E., Smith, S.A., Seaver, E., Rouse, G.W., Obst, M., Edgecombe, G.D., et al. (2008). Broad phylogenomic sampling improves resolution of the animal tree of life. Nature 452, 745-749.
  5. Eitel M, Schierwater B. The phylogeography of the Placozoa suggests a taxon rich phylum in tropical and subtropical waters Mol Ecol (in prep.)
  6. Eitel, M. et al. (in prep.).
  7. Grell, K.G., Ruthmann, A. (1991). Placozoa. In Microscopic Anatomy of Invertebrates, Placozoa, Porifera, Cnidaria, and Ctenophora, Volume Vol. 2, F.W. Harrison, Westfall, J.A., ed. (New York: Wiley-Liss), pp. 13-28.
  8. Grell, K.G. (1971). Trichoplax adhaerens F.E. Schulze und die Entstehung der Metazoan. Naturw. Rdsch. 24, 160-161.
  9. Hejnol, A et al. (2009) Assessing the root of bilaterian animals with scalable phylogenomic methods. Proc R Soc B (in press).
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  12. Monticelli, F.S. (1893). Treptoplax reptans n.g., n.sp. Atti dell´ Academia dei Lincei, Rendiconti (5)II, 39-40.
  13. Philippe, H., Derelle, R., Lopez, P., Pick, K., Borchiellini, C., Boury-Esnault, N., Vacelet, J., Renard, E., Houliston, E., Queinnec, E., et al. (2009). Phylogenomics revives traditional views on deep animal relationships. Curr Biol 19, 706-712.
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  17. Schierwater, B. (2005). My favorite animal, Trichoplax adhaerens. BioEssays.
  18. Schierwater, B., de Jong, D., and Desalle, R. (2008).Placozoa and the evolution of Metazoa and intrasomatic cell differentiation. Int J Biochem Cell Biol.
  19. Schierwater B et al. (2009a) Placozoa, and the evolution of Metazoa and intrasomatic cell differentiation. Int J Biochem Cell Biol 41: 370–379.
  20. Schierwater B et al. (2009b) Concatenated Molecular and Morphological Analysis Sheds Light on Early Metazoan Evolution and Fuels a Modern “Urmetazoon” Hypothesis. PlosBiology 7: e1000020.
  21. Sidall ME (2009) Cladistics (in press).
  22. Signorovitch AY et al. (2005) Molecular signatures for sex in the Placozoa. Proc Natl Acad Sci USA 102: 15518-22.
  23. Signorovitch, A.Y., Dellaporta, S.L., and Buss, L.W. (2006). Caribbean placozoan phylogeography. Biol Bull 211, 149-156.
  24. Srivastava M et al. (2008). The Trichoplax genome and the nature of placozoans. Nature 454: 955-60.
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  26. Syed, T., Schierwater, B. (2002). The Evolution of the Placozoa: A new morphological model. Senckenbergiana lethaea 82, 315-324.
  27. Tiemann, K. et al. (in prep.).
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