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What are the special features of the animals in phylum Placozoa?

Although animals in the placozoa Phylum are very simple and extremely tiny, they are very important to biologists because they are an important link to the evolutionary history of animals. Yet there is still much to be discovered about these small creatures. Because their bodies are so soft, they have no skeleton to leave behind, so there are no fossils of these animals. At this point, because of their small size and elusive nature, they have yet to be extensively studied in their native habitats; scientists have only been able to study them in aquariums in labs.

The word Placozoa comes from the Greek root Plakos for "flat" and Zoon for "animal" thus Placozoa means "flat animal." Do you remember how many species, or types of animals are included in phylum Placozoa? Once you have your answer, click the Show Me button to see how you did.

Even though Placozoa phylum contains two animals, Trichoplax adhaerens and Treptoplax reptans, everything known about this phylum is the result of studying the former species, Trichoplax adhaerens. In fact, Treptoplax reptans was spotted for the first and only time in the Gulf of Naples in 1893. Since then, no one has observed this species, and scientists today question whether this animal even exists; therefore, Trichoplax adhaerens is the model animal for phylum Placozoa.

The name Trichoplax adhaerens comes from the Greek Tricho for "hairy," plax for "plate," and adhaerens meaning "to stick," thus, this animal is hairy and flat and can stick to the walls of aquariums.

Click each tab in the activity below to read about the special features of placozoans.

Body Plan

By Oliver Voigt (Oliver Voigt) [CC-BY-SA-2.0-de (http://creativecommons.org/licenses/by-sa/2.0/de/deed.en), GFDL (http://www.gnu.org/copyleft/fdl.html), CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0/) or CC-BY-SA-2.5-2.0-1.0 (http://creativecommons.org/licenses/by-sa/2.5-2.0-1.0)], via Wikimedia Commons

Placozoans have an irregularly shaped body--they are asymmetrical. Do you remember what other phylum consists of animals with no symmetry? After you have thought of an answer, click the Show Me button to see how you did.

Placozoans are some of the simplest types of animals, having no organs, no nervous system, and no digestive tract. Although they are asymmetrical, their bodies can be differentiated into a ventral (belly or bottom) side and a dorsal (back or top) side.

Body Size

By Ixnayonthetimmay (Own work) [CC0], via Wikimedia Commons

T. adhaerens' bodies are small flat discs that measure in at about 2-3 mm in diameter and are about 15-20 µm thick (1 µm = 0.001 mm). Take a look at the picture of the coins to get an idea of the size of Trichoplax adhaerens. The tiny gold grain all the way to the far left is about 3 mm, slightly bigger than the average T. adhaerens.

Habitat

Trichoplax adhaerens live in tropical and subtropical marine habitats, like the warm water pictured, although they have never been studied in their native habitat. Scientists have only been able to study Placozoans in salt water aquariums in laboratories.

Eating Habits

Trichoplax adhaerens eats by engulfing its food, a process known as phagocytosis, as illustrated in this image. First, the animal glides to its food, and, as it gets closer, the ventral side of the animal caves in to envelope the food particles. T. adhaerens is able to change form easily, so it moves its body to "hug" the food particles. Once the food particles are surrounded, digestive enzymes are secreted to break down the food so that the animal can ingest the nutrients of the food.

Since scientists are still unable to observe T. adhaerens in the wild, it is unknown what they eat in their native habitat. In the lab these animals eat algae and commercial fish food.

Mobility

By Iceclanl (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

When T. adhaerens was first observed, scientists suspected that it was a type of amoeba because it moves very similarly to amoeba. Pictured is an amoeba moving by means of pseudopodia, which literally means "false feet." T. adhaerens, like amoeba, are able to move by projecting a piece of their body forward and in doing so pull along the rest of their body forward. Because they have no bones to keep them stiff, both amoeba and T. adhaerens have a free flow type of movement and are able to change their body shape to help them get around by gliding.

Reproduction

By Merje Toome (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons

Placozoans are different from most animals in the way they reproduce the majority of the time. How do most animals reproduce? Once you have an answer, click the Show Me button to see if you're correct.

T. adhaerens is able to reproduce asexually through a process called budding, like the yeast in the image above. Budding occurs as an outgrowth from the parent that eventually disconnects and becomes an exact copy of the parent organism, leaving behind scar tissue on the parent.

Biologists suspect T. adhaerens is also able to reproduce sexually when conditions are right (i.e. when population numbers are high), but because these tiny creatures are difficult to study in their native environment, the mechanism of their sexual reproduction is still a mystery.

Genome

By Dcrjsr (Own work) [CC-BY-3.0 (http://creativecommons.org/licenses/by/3.0)], via Wikimedia Commons]

T. adhaerens has the smallest genome (the complete set of genes in an organism) known among all animals, which makes sense due to its simplicity and small size. What macromolecule contains the genes of living things? Once you have your answer, click the Show Me button to see how you did. (Hint: The macromolecule shown in the picture is what contains genes.)

Choose the word or phrase that describes a characteristic of T. adhaerens. Drag the circle from the left side to the circle on the right side to choose your answer.

Placozoan mobility
glides with pseudopodia

moves using muscles and skeleton

remains stationary

That is correct. T. adhaerens changes its body shape to glide on surfaces.
Remember, T. adhaerens uses "false feet" to move around, and it can change its squishy body to glide around.
Placozoan reproduction
sexual only

budding only

sexual and asexual

That is correct. Scientists have observed budding in this phylum but suspect sexual reproduction also occurs.
Remember, scientists have observed T. adhaerens reproducing asexually through budding but also suspect these animals are capable of sexual reproduction.
Placozoan genome
average size animal genome

large size animal genome

smallest animal genome known

That is correct. Placozoans have a small amount of DNA compared to other animals.
Remember, T. adhaerens has a relatively small genome in comparison to other animal genomes.
Placozoan eating habits
uses phagocytosis

has jaws and teeth

takes in food through pores


That is correct. T. adhaerens engulfs its food by changing its body shape to wrap around the food to digest it.
Remember, placozoans can change shape to "hug" their food in order to digest it.
Placozoan body plan
bilaterally symmetrical

radially symmetrical

asymmetrical

That is correct. placozoans are irregularly shaped.
Remember, placozoans have no symmetry.
Placozoan body size
3 meters in diameter

20 mm in diameter

2-3 mm in diameter

That is correct. Placozoans are very small animals, measuring in at 2-3 mm.
Remember, placozoans have a very small body with a diameter of only 2-3 mm.

Complete