With two animal phyla covered it’s time to move on to the third, so let’s dig into the phylum
Placozoa. The name Placozoa comes from the Greek word “plax” or “plakos” meaning plate, and “zōion”
meaning animal. So, a possible common name for them could be “plate animals,” which makes sense,
as they do sort of look like tiny dinner plates. As of 2021, there are only three species in phylum
Placozoa, the first of which was discovered in 1883, and the last of which was discovered
in 2019. It’s possible that since the making of this tutorial, more species of Placozoans
have been discovered, or that the phylum has been reclassified with new genera, new families, and
even new orders. The evolutionary relationships between the Placozoans and the rest of kingdom
Animalia is still debated. The placozoans have very few distinguishing features and no regular
body shape. They are lacking any kind of symmetry, and at first glance they can be mistaken for large
amoebas. In fact, some of the largest amoebas known, like the multinucleated unicellular
Xenophyophorea are actually twenty times as large, at around 20 centimeters in length, as the largest
placozoans, named Polyplacotoma, which are only 10 millimeters across, and 200 to 400 times larger
than the most common placozoans, Trichoplax, which are one millimeter across or less.
Their evolutionary relationship to other animals is further complicated
by the lack of any fossil record. Traditionally, placozoans were considered to be a possible link
between the sponges and unicellular eukaryotes. According to this hypothesis, placozoans are the
most basal extant animals. This hypothesis was supported in 2006 by an analysis of a placozoan
genome and later by a series of phylogenetic analyses. However, some recent analyses,
including a series of analyses performed in 2019 that compared the placozoans to genomic samplings
of most of the other known animal phyla, put the placozoans as a sister group to the cnidarians.
Placozoans have a number of unique characteristics that continue to fascinate and sometimes confound
researchers. They have no symmetry, no organs, and no muscular or nervous systems.
Like the cnidarians and ctenophores, they have two cellular layers, so they are diploblastic animals.
Since the animals are flat we can split those layers into a top and a bottom layer.
There is of course no such thing as “dorsal” and “ventral” in diploblastic animals, since
dorso-ventral is a characteristic of bilateral symmetric animals. The upper epithelioid layer
is made of ciliated “cover cells” and shiny spheres that function in anti-predator defense,
though their precise mechanism of action is still unknown. Their bottom layer is made up of cylinder
cells that possess cilia used in locomotion, and gland cells that lack cilia and are used in
digestion. Between these layers is an intermediate layer that contains fluid and fiber cells.
Most placozoans are colorless or grey, and they are so thin that they are almost invisible to
the naked eye. These entirely marine animals are rarely observed in their natural habitats,
though they are often found crawling over substrates in marine aquariums.
A single species, Trichoplax adhaerens, has been collected in bodies of water from the
Mediterranean to the Caribbean to the Great Barrier Reef and the ocean surrounding Japan.
They are most common in coastal tidal zones and subtropical areas.
In terms of locomotion, placozoans generally use their cilia to glide across
surfaces in a single direction or swim weakly in the water. When they change direction,
they do so by changing their shape, much like an amoeba does. Remember that these creatures
are asymmetrical, they don’t have an anterior end or a posterior end like a bilateral animal.
So, instead of “turning around” like you and I would, they simply change shape. Since they
don’t have muscles or nerves, this movement can sometimes look very sporadic. They’ve even been
recorded splitting in two when different parts of the body start moving in opposite directions.
In addition, the direction they move in is entirely random. That is, there is a linear
relationship between elapsed time and mean square distance between their arbitrary starting point
and current location, just like the Brownian motion of molecules.
However, the speed of their movement, unlike the direction, is nonrandom. When in areas of very low
nutrient density they move more slowly than when in areas of increased nutrient density. When they
are very close to their food source, such as green algae, blue-green bacteria, or detritus,
they slow their movements again. When their bodies are over a food source they form small folds on
their bottom side, and excrete digestive enzymes into the fold, creating a temporary external
pocket, almost like a stomach. The enclosed nutrients are then absorbed. This particular
mode of feeding mirrors the so-called Placula Hypothesis for the origin of metazoan animals.
Not all ingested microorganisms are consumed, sometimes bacterial organelles have been
discovered maintaining endosymbiosis within the endoplasmic reticulum of living placozoans.
In terms of reproduction, placozoans often reproduce asexually, usually by dividing
in two or three approximately equal parts. An interesting thing to note is that since
a single placozoan can divide without limit, their life span is potentially infinite.
They can also bud off spherules of cells from their upper surface, and like all animals,
can reproduce sexually under conditions of excessive population density.
Placozoans are also capable of extreme regeneration from just a small collection
of cells and can reconstitute their bodies from individual cells. However, this process,
along with their phylogeny, sexual reproduction, and endosysmbiosis, is still being studied.
It is incredible to think that a tiny plate-like organism discovered in an
aquarium in Austria in 1883 is still the subject of such incredible discovery.