Placozoa

Diversity

Phylum Placozoa contains just two species of very small (2 to 3 mm in diameter and only 15 to 20 µm in width), simply organized, non-bilaterian metazoan organisms that superficially resemble amoebas. Treptoplax reptans has not been seen since it was first described from the waters of Naples, Italy in 1896, but Trichoplax adhaerens , first discovered on the walls of a marine aquarium in Australia in 1883, is found in tropical and subtropical marine waters around the world. These organisms are composed of differentiated dorsal and ventral epithelial cell layers, which enclose a mesenchymal syncytial net. Placozoans move via gliding, aided by the ciliated cells of the basal epithelial layer, and feed by engulfing particles of organic detritus. They are able to reproduce asexually via fission, but are also known to reproduce sexually.

Geographic Range

Placozoans are found globally in tropical and subtropical marine waters.

• Biogeographic Regions
• nearctic nearctic
  • native native
• palearctic palearctic
  • native native
• oriental oriental
  • native native
• ethiopian ethiopian
  • native native
• neotropical neotropical
  • native native
• australian australian
  • native native
• mediterranean sea
  • native native

• Other Geographic Terms
• holarctic holarctic

Habitat

Placozoans were first identified from the walls of a marine aquarium. While they are considered benthic organisms, they are also found in the water column. They are most commonly found near shore, in the littoral zone, in warm waters.

• Habitat Regions
• temperate temperate
• tropical tropical
• saltwater or marine saltwater or marine

• Aquatic Biomes
• pelagic pelagic
• benthic benthic
• reef reef
• coastal coastal

• Other Habitat Features
• intertidal or littoral intertidal or littoral

Systematic and Taxonomic History

Trichoplax adhaerens was first described by the German zoologist Franz Eilhard Schulze in 1883, with an English-language report of his findings published in the United States in the same year by Charles S. Minot. Treptoplax reptans , the only other named member of phylum Placozoa , was described by the Italian biologist Francesco Monticelli in 1893. However, this species has not been observed since, and many question whether it ever existed as a valid taxon.

At its first discovery, it was obvious to many scientists that Trichoplax adhaerens presents significant differences from other known groups of organisms. However, a report classified it as the larval form of the cnidarian species Eleutheria krohi , which became the accepted hypothesis for the next several decades. In the 1970s, scientists, particularly the German protozoologist Karl Gottlieb Grell, renewed examinations of this species and demonstrated that the individuals examined were, in fact, adults. Trichoplax adhaerens has since been widely accepted as the sole representative of the phylum Placozoa , a name coined by Grell in 1971. Several recent morphological and molecular phylogenetic analyses have indicated that many additional species likely exist within this phylum, including several higher order taxonomic groups. To date, however, none of these have been formally described or named.

Our understanding of the relationships of placozoans to other animal phyla remains in a state of flux. Early molecular phylogenetic analyses generally recovered them as the sister group to cnidarians or ctenophores . Subsequent studies, using much higher numbers of independent genetic markers, indicated that they occupy a basal position within metazoans as the sister group to the Eumetozoa , being placed between sponges (phylum Porifera ) and all other multicellular animals. In contrast, results from analyses of mitochondrial genomes and total evidence approaches suggested that animals form two clades based on the organization of their tissues, with Placozoa representing the basal lineage in the dibloblast clade, which also includes sponges, cnidarians, and ctenophores. However, the most recent molecular phylogenetic analyses indicate that there is no strong support for any of these hypotheses, leaving the true phylogenetic position of placozoans as a matter of intense, widespread scientific interest and debate.

Physical Description

Placozoans are very small animals, measuring just 2 to 3 mm in diameter and typically 15 to 20 µm thick. They have traditionally been described as being composed only four types of cells: cover (squamous), columnar, glandular, and fiber. They are asymmetrical (although smaller animals tend to be circular), and their bodies lack anterior or posterior ends. They do, however, have distinct ventral and dorsal sides, and are essentially made up of three layers: dorsal epithelia, mesenchyme, and ventral epithelia. Dorsal epithelial cells are cover, or squamous, cells; they are flattened, contain lipid droplets, and each has a single cilia. Ventral epithelial cells are more columnar, lack lipid droplets, but are also mainly monociliate. The ventral layer also has unciliated glandular cells. Between these two layers, forming the interior of the animal, is a layer of mesenchyme, composed of star-shaped fiber cells. The points of the “stars” are connected, creating a network. There appears to be no basement membrane between the epithelial layers and mesenchyme.

In situ hybridization studies of gene expression in the cells of Trichoplax adhaerens have indicated that this organism likely possesses more than just four cell types. Though the newly identified cell types are morphologically indistinguishable, differential gene expression patterns between them and previously characterized cell types strongly suggest that they possess unique, albeit currently unknown, functions.

• Other Physical Features
• ectothermic ectothermic
• heterothermic heterothermic

• Sexual Dimorphism
• sexes alike

Development

New animals may be produced via binary fission or budding. Budding creates multicellular flagellated “swarmers,” each of which becomes a new individual. Sexual reproduction may occur, in which case holoblastic cell division proceeds following fertilization. At the 64 cell stage, cell division ceases, while nuclear DNA multiplication continues until the nucleus bursts.

Reproduction

Reproduction in placozoans is mainly asexual. Sexual reproduction may be observed during times of high population density, high water temperature (23°C and greater), and food depletion. Sexual reproduction occurs only through degeneration of the mother. A single egg/oocyte as well as small, unflagellated cells (assumed to be sperm) develops in the interspace of a degenerating placozoan.

Placozoans may reproduce asexually (via transverse fission or budding) or sexually. Sexual reproduction seems to be triggered by environmental factors including water temperature; this implies that, in some regions, animals may have both sexual and asexual phases that may be dependent on the season.

• Key Reproductive Features
• seasonal breeding seasonal breeding
• year-round breeding year-round breeding
• sexual sexual
• asexual asexual

Parental investment is not known to occur in placozoans.

• Parental Investment
• no parental involvement

Lifespan/Longevity

While strains of Trichoplax adhaerens have been maintained in lab settings for many years, little data is available regarding their lifespans.

Behavior

Placozoans move using ciliary action and changes in body shape; there is evidence that smaller (presumably younger) individuals may swim.

• Key Behaviors
• motile motile
• sedentary sedentary
• solitary solitary

Communication and Perception

Little is known regarding how placozoans may perceive their environments. In laboratory settings, they have been observed to react strongly when exposed to ultraviolet radiation.

• Perception Channels
• ultraviolet

Food Habits

Placozoans feed by phagocytosis, using their ventral surfaces (some of the glandular cells located there produce digestive enzymes). In laboratory settings, they are known to feed on flagellated chromists ( Cryptomonas sp.) and chlorophytes ( Chlorella sp.), other algae, the nauplii of Artemia species, and commercial fish food. It is suspected that they are opportunistic grazers, and they may feed on organic detritus as well.

• Primary Diet
• carnivore carnivore
  • eats non-insect arthropods
• herbivore herbivore

Predation

Potential predators have been observed reacting negatively to placozoans. In one instance, a snail was observed touching a placozoan with its tentacle, then recoiling; in another study, placozoans dropped onto the tentacles of hydroids caused paralysis. Structures known as "shiny spheres," present in the upper epithelium of placozoans may somehow serve as predator deterrents, though the mechanism by which they may do this is completely unknown. The only predators reported for placozoans are snails in genus Rhodope and a small nemertean species.

Ecosystem Roles

Although they are ciliates , nematodes , and other small animals have been observed around or even on placozoans, they do not seem to elicit any response and no parasitic or commensal relationships are known.

Economic Importance for Humans: Positive

Beyond potential scientific interest, there are no positive effects of placozoans on humans.

• Positive Impacts
• research and education

Economic Importance for Humans: Negative

There are no known adverse affects of placozoans on humans.

Conservation Status

There is no concern of either known placozoan species becoming threatened or endangered at this time.

Additional Links

Encyclopedia of Life

Contributors

Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Texas State University.

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