The Placozoa Database
[page updated on 6 May 2011]
This is the smallest of all animal databases yet; it contains a single nominal 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 most likely a fragmented placozoan . With only one species the phylum knows only three taxonomic ranks: Phylum (Placozoa), genus (Trichoplax), and species (T. adhaerens). This situation is soon to change radically. Genetic analyses have suggested that Placozoa harbor at least seven major species clades, which may represent different orders, each with multiple families (see below).
The placozoan species database will be updated immediately when a relevant manuscript becomes accepted. Please send information to Bernd Schierwater.
Placozoa: Biology and History
Placozoa occur in the littoral of all warm oceans and are distributed globally in tropical and sub-tropical waters [3-5]. They reproduce by (i) binary (sometimes trinary) fission, (ii) budding off small swarmers (iii) sexual reproduction [6-9].
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. 4A). 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 [7,10-12]. 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 .
After its original description by F.E. Schulze 1883 , 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 (see below). For details and references on placozoan biology and the history of placozoan research see [14,15].
Placozoa: Relationships 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, including relationships to (i) Cnidaria, (ii) Ctenophora, and (iii) even to Bilateria (reviewed in ). A recent study using a group of genes from the completed nuclear genome placed Trichoplax basal to Eumetazoa with Porifera branching off first . A similar phylogeny was shown in the very recent publication on the first sponge genome . 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. 2; [19,20]). More recent studies show very low or even no support for any of the above hypotheses and illustrate the problems of molecular systematic analyses of large data sets at the base of Metazoa . Thousands of genes or even whole genomes might not be enough to find an 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, development, molecular morphology (secondary structure of various molecules), mitochondrial genome data, and also from whole genome comparisons. At present the only valid support suggests a basal position of Placozoa within the diploblats .
Figure 2. 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 et al. 2009 
Figure 3. Unexpected diversity has been found in the phylum Placozoa, formerly assumed to be monotypic. Shown is a ML phylogram of different placozoans based on fragment of the large mitochondrial ribosomal RNA (16S). 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 .
Figure 4. Schematic cross sections of a Placozoon. The traditional drawing is shown in (A). Here one layer of fiber cells is sandwiched between the upper and the lower epithelium. In a recent study by Guidi et al., 2011 [X] it was shown, however, that fiber cells are arranged in three layers (B,C). Two different modi of shiny sphere production are shown: Shiny spheres are produced either in the interspace and then transferred to the upper epithelium (B) or they are produced in the fiber cell layer directly underneath the upper epithelium (C). From here the shiny spheres are directly integrated into the upper epithelium through extensions of the fiber cells. Images modified from Grell, 1972  and Guidi et al., 2011 .
Placozoa: Biodiversity and Phylogeography
Recent genetic studies have shown a unexpected and very high diversity within the Placozoa. In an initial study by Voigt et al.  identified eight genetic lineages (haplotypes) based on 16S, 18S, 28S and ITS sequence data. These eight lineages were dispersed among five distinct clades (higher entities) possibly representing different families or even orders. Three subsequent studies increased the number of haplotypes to a total of 16 [3,4,22]. The estimated total number of placozoan species is 80 species or more . To resolve the relationships between the discovered lineages and to assign new species and higher taxonomic units a recent study addressed the question of morphological differences. Guidi et al.  studied morphology and ultrastructure of 10 clonal placozoan lineages and scored eight morphological characters at a cellular and in- tracellular level (Table 1). The authors identified five morpho-groups among the different lineages but these groups do not immediately match the suggested genetic phylogeny. As only six out of the known 16 genetic lineages have been studied, further research is urgently needed to erect species and higher taxonomic ranks.
The phylogeography of the Placozoa shows that different clades occupy different ecological niches and identifies several euryoecious haplotypes with a cosmopolitic distribution as well as some stenoecious haplotypes with an endemic distribution (see (Fig. 5; ). Clades differ substantially in their distribution according to latitude indicating ecological species. Although the phylogeography is quite well established for certain regions (Caribbean, Mediterranean) there is still a substantial lack of knowledge in other parts of the world (Southern Atlantic Ocean, Indian Ocean and Western Pacific Ocean). Intense sampling is needed here.
Figure 5. Worldwide distribution of genetically characterized placozoan specimens. Clades are the same as shown in Fig. 3. 1. Oahu, Hawaii (US), 2. Southern California (A.s., US), 3. Caribbean coast of Belize, 4. Caribbean coast of Panama, 5. Pacific coast of Panama, 6. Cubagua Island/Margarita Island (Venezuela), 7. Grenada, 8. Discovery Bay (Jamaica), 9. Bahamas, 10. Bermuda (GB), 11. Tenerife, Canary Islands (Spain), 12. Majorca, Balearic Islands (Spain), 13. Castiglioncello (Italy), 14. Orbetello Lagoon (Italy), 15. San Felice Circeo (Italy), 16. Otranto (Italy), 17. Kateríni and Ormos Panagias (Greece), 18. Bay of Turunç (Turkey), 19. Gulf of Hammamet and near Zarzis (Tunisia), 20. Caesarea (Israel), 21. Elat (Israel), 22. Mombasa (Kenya), 23. Réunion (France), 24. Laem Pakarang (Thailand), 25. ‘Indonesia’ (A.s.), 26. Bali (A.s), 27. ‘Indo-Pacific’ (A.s.), 28. Kota Kinabalu, Sabah (Malaysia), 29. Hong Kong (China), 30. Okinawa, Ryukyu Islands (Japan), 31. Boracay (Philippines), 32. Guam (US), 33. Lizard Island (NE Australia). As one can immediately see there is still a great lack of knowledge on the distribution of the various different clades especially from the coasts of Africa, South America, and Australia as well as from the Indian Ocean. From Eitel & Schierwater, 2010 .
Table 1. Diagnostic morphological characters identified in this study. Eight distinctive ultrastructural characters from the upper epithelium (A1–A5), the lower epithelium (B), and the fiber cells (C1–C2) allow distinguishing between five morphological groups (I–V). Modified from Guidi et al., 2011 .
Placozoa: A revised Morphology
Studying morphological differences among different placozoan lineages revealed that in all lineages fiber cells are always arranged in multiple layers . Trichoplax adhaerens served as a positiv control in this study as the ultrastructure. Even in Trichoplax the fiber cells are, in contrast to earlier observations [10,11], arranged in more than one layer. Fiber cells are thus organized in a 3D meshwork rather than two dimensions only (see Fig. 4B,C). Guidi et al.  propose different subtypes of fiber cells. These new observations help to better understand the nature of the placozoan bauplan.
Sampling, Conserving, StudiesAnimal 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 [3,4,24]. A good, yet underestimated source for collecting placozoans are aquaria. Despite the missing exact geographic assignment of these samples it is obvious, however, that they are a reasonable sources for placozoan specimens that are at least helpful for screening genetic diversity in Placozoa.DNA/RNA isolation
To fix animals for DNA preparation, FTA Elute cards micro (Whatman) are used [3,22]. 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 [14,15] for refs.).New samples, new species
For the genetic identification of placozoan samples please send samples to the Schierwater lab for free genetic analysis. If new species are identified a joint effort taxonomic circle approach for valid species descriptions is encouraged (please contact Bernd Schierwater).
Associate EditorsMichael Eitel
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