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The Phylum Placozoa, once considered monotypic, turns out to be surprisingly diverse despite the morphological simplicity and apparent uniformity of these small and often overlooked animals. A paper by Eitel & Schierwater (2010) in this issue of Molecular Ecology, based on a world-wide collecting program, greatly increases our knowledge of the distribution of specific placozoan haplotypes, more than doubles the number of genetically characterized sites, and reports the first genetic data for isolates from the Eastern Atlantic and Indian Ocean. This paper is also important in that it marks an early attempt to map haplotypes with different kinds of distribution patterns.

Placozoa are minimalist metazoans with a correspondingly minimalist (98 megabase) genome. Resembling an amoeba and consisting of only five cell types (Jakob et al. 2004), placozoans are the simplest metazoans known (Fig. 1A,B), and the Phylum includes a single described species, Trichoplax adhaerens. When large or stressed, placozoans can undergo binary fission or bud to produce numerous swarmers, which are capable of regenerating the amoeboid stage. However, this is not the complete life cycle, as there is also evidence of sexual reproduction (Grell 1984; Signorovitch et al. 2005). The phylogenetic position of placozoans has been debated (often hotly) since their description by Schulze (1883). The new phylogenetic opportunities provided by whole genome sequencing have heightened interest in the group, but surprisingly have not solved the riddle of where Trichoplax fits in the grand scheme of things unequivocally. The emerging consensus seems to be that placozoans are secondarily simple and that their divergence post-dates that of sponges (Srivastava et al. 2008; Burger et al. 2009; Philippe et al. 2009; Sperling et al. 2009), although the alternative ‘Placozoa basal’ scenario cannot yet be excluded and has some strong advocates (Schierwater et al. 2009).

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Figure 1.  (A) Drawing of a small Trichoplax adhaerens from Schulze (1891). The white circles represent shiny spheres, brown bodies are vacuoles containing partially digested algae. (B) Photo of a living placozoan approximately 2 mm in diameter (courtesy Bernd Schierwater).

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The status of placozoan biology has been reviewed on a number of occasions (Miller & Ball 2005, 2008; Schierwater 2005; Blackstone 2009; see also http://www.trichoplax.com) but because placozoans are small and inconspicuous, and lack obvious distinguishing features, there has been relatively little work on their ecology and natural history. Placozoans have been found only in shallow, nearshore, tropical and subtropical marine environments, within which they tend to be absent from areas of high current and from bare sand bottoms (Signorovitch et al. 2006; Pearse & Voigt 2007). They have not been reported either in the ocean depths or in the open ocean. Because they are very sensitive to reductions in salinity (Pearse & Voigt 2007), they will presumably not be found in fresh or brackish water or seasonally in areas that are subjected to heavy monsoonal rains. It has also been suggested that they may be sensitive to UV light since they are found predominantly on the undersides of microscope slides used for field trapping. Little is known about seasonality, but studies in Honshu (Maruyama 2004), Okinawa (Pearse & Voigt 2007), and Hong Kong (Eitel & Schierwater 2010) are all consistent with higher numbers in warmer months. However, they have been collected from sites with winter water temperatures of 11–14 °C and summer temperatures up to 27 °C (Eitel & Schierwater 2010).

Placozoans are sampled by placing out racks of microscope slides in the environment, leaving them there for days or weeks, and then bringing the racks, and the water they enclose, back to the lab for microscopic examination. It is unclear what stage in the life cycle is colonizing the slides, particularly those suspended above the bottom, since amoeboid placozoans sometimes appear in water samples days after isolation, where they were previously absent. Given our present knowledge of the life cycle, it appears most likely that colonization is by swarmers, which are the budded products of large animals. Once placozoans appear, growth is rapid. In one case animals with a mean diameter of 183 ± 45 μm on day 1 grew to 337 ± 58 μm on day 3. By day 5 the mean diameter increased only slightly further but a small size class had reappeared, probably as the result of the budding of large animals (Pearse & Voigt 2007).

Biotic interactions are still rather poorly understood, but there is a characteristic community of organisms with which placozoans are commonly found, consisting of ‘several kinds of sessile ciliates: solitary and colonial vorticellids as well as folliculinids; spirorbid and other serpulid polychaetes; and, in smaller numbers, free-living loxosomatid kamptozoans (entoprocts)’ (Pearse & Voigt 2007). When observed on microscope slides, placozoans appear to be grazing on microalgae, as indicated by their color. Potential predators, such as snails and flatworms, are apparently deterred or paralyzed by a chemical emanating from the ‘shiny spheres’ (Pearse & Voigt 2007; Jackson & Buss 2009).

Due to their small size and simple morphology no one has recognized morphological differences that would form a basis for describing more than the original species, Trichoplax adhaerens. Genetic diversity has nevertheless been established based on both nuclear and mitochondrial rDNA data, but in practice the mitochondrial 16S rDNA has proven the marker of choice for field studies (e.g. Signorovitch et al. 2006; Eitel & Schierwater 2010). The smallest recognizable grouping, based on partial 16S sequence, is referred to as a ‘haplotype’. Voigt et al. (2004) identified eight haplotypes, which they grouped into five clades, and three more haplotypes have been identified since that time (Signorovitch et al. 2006; Pearse & Voigt 2007). In their recent studies Eitel & Schierwater (2010) found seven of 11 of the previously characterized haplotypes and added five new ones. Their study also increased the number of sampling sites from which molecular data has been obtained, from 15 to 37 and added the first genetic information from the Indian Ocean and the Eastern Atlantic. With the caveat that the number of sampled sites is still very small relative to the area sampled, three patterns of distribution are presently apparent. These are (i) cosmopolitan (clades I, III and V); (ii) Caribbean only (clade II); and (iii) restricted to a single site (endemics). However, it is important that the restricted distribution of these ‘endemics’ be verified by more intensive sampling.

The apparently cosmopolitan distribution of many placozoan haplotypes might be expected based on their small size, as Finlay (2002) has suggested that at <1 mm, organisms should be near ubiquitous. Although such models were originally developed for prokaryotes and make assumptions that may not apply to many eukaryotes, undersampling could potentially underlie some apparent distribution patterns. It is difficult to know what an appropriate sampling scale is for a placozoan, but only one study to date approaches that which is probably required (Signorovitch et al. 2006). More intensive worldwide sampling will undoubtedly lead to the discovery of new genotypes/haplotypes and will also enable patterns of distribution to be seen more clearly.

While the work of Eitel & Schierwater (2010), Pearse & Voigt (2007) and others represent important steps in documenting variation within the Placozoa and are of great interest, there are some potentially serious problems that necessitate critical data evaluation, including the sampling issue outlined above. To date, these analyses have been based on a fragment of the (mt) 16S rDNA, and it would be good to see this work followed up by the application of more markers—nuclear as well as mitochondrial. 16S rDNA sequences are often difficult to align, and in many cases (but not, apparently, in Placozoa) the (A+T)-richness of the mitochondrial genome greatly reduces its usefulness in phylogenetics. There is no shortage of SNPs, as the frequency of polymorphisms in the nuclear genome has been estimated at 0.95 ± 0.02% (Srivastava et al. 2008). The mitochondrial genomes of placozoans are atypical in several respects; not only are they large and complex due to the presence of large intergenic regions and potentially mobile introns, but they are also subject to mRNA editing and trans-splicing of group I introns (Burger et al. 2009). When considered in conjunction with phylogenetic analyses based on the mt protein complement (Burger et al. 2009), these features suggest atypically fast evolution of placozoan mt genomes. A further complication arises from the potential alteration of distributions caused by human transport, principally in ballast water, but hopefully more widespread sampling will reveal whether haplotypes with apparently limited or disjunct distributions localize to busy ports. The most serious barriers to a real understanding of placozoan biology are that the complete life cycle remains unknown, as does the means of sexual reproduction and development. In all likelihood, many presently mysterious aspects of placozoan biology will fall into place once these gaps have been filled. These unknowns are most likely to be clarified by someone with a sharp eye and an informed mind, as well as a PCR machine and a sequencing budget.

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E.E.B. and D.J.M. study the genomics, developmental and functional biology of ‘lower’ animals, focusing mainly on the coral Acropora as a model for understanding various aspects of animal evolution.