Modern Interpretation and Modification of the Placula Hypothesis of Metazoan Origin
Here, a nonsymmetric and axis-lacking bauplan (placula) transforms into a typical symmetric metazoan bauplan with a defined oral–aboral or anterior– posterior body axis. In the placula transformation, a primitive disk consisting of an upper and a lower epithelium (lower row), which can be derived from a flattened multicellular protist, forms an external feeding cavity between its lower epithelium and the substrate (second row from bottom). The latter is achieved by the placula lifting up the center of its body, as this is naturally seen in feeding Trichoplax (i.e., the two Trichoplax images derive from a nonfeeding (first row) and feeding (second row) individual. If this process is continued, the external feeding cavity increases (cross section, third row) while at the same time the outer body shape changes from irregular to more circular (see oral views). Eventually, the process results in a bauplan in which the formerly upper epithelium of the placula remains outside (and forms the ectoderm) and the formerly lower epithelium becomes ‘‘inside’’ (and forms the entoderm; upper row). This is the basic bauplan of Cnidaria and Porifera. Three of the four transformation stages have living counterparts in the form of resting Trichoplax, feeding Trichoplax, and cnidarian polyps and medusae (right column).
The above-outlined transformation of a placula into a cnidarian bauplan involves the development of a main body axis and a head region, which allows the invention of new structures and organs for feeding. From a developmental genetics point of view, a single regulatory gene would be required to control separation between the lower and upper epithelium (three lower rows). If the above scenario were correct, the following empirical data would be congruent with it. In the form of the putative ProtoHox/ParaHox gene, Trox-2, in Trichoplax, we find a single regulatory gene, marks the differentiation of an as yet undescribed cell type at the lower–upper epithelium boundary in Trichoplax. More than one regulatory gene would be required to organize new head structures originating from the ectoderm–entoderm boundary of the oral pole (upper row). Quite noteworthy, two putative descendents of the Trox-2 gene, Cnox-1 and Cnox-3, show these hypothesized expression patterns (Diplox expression upper row; for simplicity, only the ring for Cnox-1 expression is shown). Cnox-1 and Cnox-3 expression both mark the ectoderm-entoderm boundary at the oral pole in the hydrozoan Eleutheria dichotoma. Both genes are expressed in parallel in a ring-shaped manner at the tip of the manubrium, with Cnox-3 being expressed more ectodermally and Cnox-1 being expressed more entodermally (unpublished data).
The most primitive metazoan animal phylum. At present, this phylum harbors a single named species, the enigmatic Trichoplax adhaerens.
Description. In 1883, the German zoologist Franz Eilhard Schulze discovered this microscopic marine animal on the glass walls of a seawater aquarium at the University of Graz in Austria. The animal, measuring less than 5 mm (0.2 in.) in diameter and 10– 15 micrometers (0.0004–0.0006 in.) in height, looked like an irregular and thin hairy plate sticking to the glass surface. Schulze named this animal Trichoplax adhaerens (Greek for “sticky hairy plate”).
In contrast to typical multicellular animals (metazoans), Trichoplax does not have a head or tail, nor does the animal possess any organs, nerve or muscle cells, basal lamina, or extracellular matrix. Trichoplax lacks an axis and any type of symmetry. It almost looks like a giant ameba (a protozoan) when it crawls over the substrate.
The immediate and defining characteristic between placozoans and protozoans is the number of somatic (non–germ line) cell types. In contrast to Protozoa, which consist of either a single cell or several cells of the same somatic cell type. Placozoa have at least six defined somatic cell types: lower epithelial cells, upper epithelial cells, gland cells, fiber cells, crystal cells and lipophil cells. These cells are arranged in a sandwich-like manner, with the lower epithelial, lipophil and gland cells at the bottom, the upper epithelial cells at the top, and the fiber cells in between while crystal cells can be found near the rim. Cells of the lower epithelium attach the animal to a solid substrate, enable the animal to crawl (with the aid of cilia), and allow feeding. During feeding, the animal lifts up the center region of its body to form an external digestive cavity between the substrate and lower epithelium. Interestingly, the upper epithelium is also capable of feeding. Algae and other food particles are trapped in a slime layer coating the upper epithelium and are subsequently taken up (phagocytized) by the inner fiber cells; this unique mode of feeding is called transepithelial cytophagy. Placozoa harbor endosymbiontic bacteria in the endoplasmic reticulum of the fiber cells. A possible role for these endosymbionts in feeding is not yet understood.
In general, very little is known about the biology of Placozoa, and almost all current knowledge derives from laboratory observations. Field data are limited to records of finding placozoans on hard substrate surfaces from tropical and subtropical marine waters around the world.
Reproduction and development. The complete life cycle of Placozoa remains unknown, although three different modes of reproduction are known: (1) fission (normal type of vegetative reproduction in Placozoa), (2) swarmer formation (occasional type of vegetative reproduction), and (3) bisexual reproduction (the standard mode of reproduction in Metazoa). In fission, animals grow to a certain size and divide into two approximately equally sized daughter individuals; then each of these regrows to about the former size of the parent before it divides again. This process can go on ad infinitum and leads very efficiently to a rapid increase in population density. The second mode of vegetative reproduction occurs when environmental conditions become unfavorable (for example, food shortage) and transfer to a new place becomes advantageous. Under these conditions, placozoans may develop small spherical swarmers which are planktonic (free-floating) and thus are taken by water currents to new habitats. The transition from a swarmer into an adult placozoan, however, has not been observed. Similarly, the development of sexually produced embryos (third mode of reproduction) into an adult animal also remains unknown. Scientists have seen embryos only up to a 128-cell stage where epithelial formation takes place, but have not seen any development into a known form, another generation, or a larval form. To what degree Placozoa reproduce sexually in the field is unknown.
Genetic analysis. Genetic analysis of placozoan specimens from different coral reefs around the world as well as from the subtropical Mediterranean revealed the presence of cryptic species, that is, species which are basically morphologically undistinguishable. While the phylum Placozoa presently harbors the single named species, T. adhaerens, the real biodiversity is estimated to include several dozen genetically and ecologically distinguishable species.
After its original description in 1883, Trichoplax attracted particular attention because it possibly mirrored the basic and ancestral state of metazoan organization. The German zoologist Karl Grell further highlighted this view in the 1970s and provided sufficient support for placing T. adhaerens in a new phylum, the Placozoa. The new phylum was named after one of several models of early metazoan evolution, the placula hypothesis, which sees a placozoan-like animal as the urmetazoon, that is, the very first metazoan bauplan (body plan). A variety of new molecular data support the traditional view that Placozoa are close to the very root of metazoan origin.