A Simple Animal?

Trichoplax adhaerens. An animal. This particular specimen is about 0.5 mm wide. Image from Eitel et al. (2013).

See that seemingly shapeless blob of cells in the image above? That is the world’s simplest animal. It is Trichoplax, a placozoan.

Now, it might seem strange that I called Trichoplax a simple animal. After all, we are so familiar with “simple” and “complex” being associated with evolution and development. The basal creatures are the simple stuff and they eventually evolve into the more derived and complex things. However, I am not making that claim at all. I am simply stating that, in order for a living organism to be considered an animal, it must meet certain requirements. Trichoplax is the simplest design known that fulfills all of the requirements of being an animal.

What are these characteristics of animals? Here is a list of five general characteristics of animals. These come from a textbook that I use in one of the classes that I teach.[1] 

  • Animals typically have the power of movement or locomotion by means of muscle fibers.
  • Animals are multicellular; most have specialized cells that form tissues and organs.
  • Animals typically have a life cycle where the adult is diploid.
  • Animals usually reproduce sexually and produce an embryo that undergoes developmental stages.
  • Animals are heterotrophic, typically by ingestion.

You may have noticed that several of these characteristics have the word “typically” or “usually” in them. This is because there is enough variation among animals that there are always going to be a few exceptions to the rules. To give you an idea how diverse animals are, there are over 30 named animal phyla. A phylum is a broad group of animals. Think of a phylum as defining a way to “be alive.” For example, many animals have an internal skeleton composed of bone. These all belong to a phylum called Chordata. Some animals have a ridged exoskeleton with jointed appendages. These all belong to a phylum called Arthropoda. Some animals have a body that consists of a mantle, visceral mass, and a muscular foot. These animals all belong to the phylum Mollusca. Notice how each phylum describes a different way animals can be “put together.” Such a diversity means that something in invariably going to be a little different than expected.

Another thing you may have noticed about the list of animal characteristics is that it is all Greek. Okay, it is given in English, but many of the words and concepts are probably unfamiliar to most people. Thus, I will go through each characteristic one at a time and describe it briefly.

  • Animals typically have the power of movement or locomotion by means of muscle fibers.

The important thing here is that animals are typically mobile. Unlike plants, which are rooted in place, or fungi, which have to grow towards a food source, animals have the power of movement. Typically, this is accomplished by muscles fibers or muscle-like structures. Those animals that are sessile (attached to a surface) will still be able to freely move parts of their body.

  • Animals are multicellular; most have specialized cells that form tissues and organs.

All animals have multiple cells. Moreover, these multiple cells have special shapes and special roles. For example, some cells may be specialized to secrete products and form glands. Some cells will be specialized for carrying oxygen and will form blood. Some cells will be specialized for carrying impulses and be part of the nervous system. The point is, an animal’s body is not made up of generic cells. Rather, the cells perform specific, specialized functions.

As a side note, since animals are all multicellular, then there are, by definition, no single-celled animals. There are plenty of single-celled organisms, but those single-celled organisms that resemble animals are protozoans and belong to different kingdoms: they are not animals (this includes things like Amoeba and Paramecium).

  • Animals typically have a life cycle where the adult is diploid.

The word “diploid” means that the cells have pairs of chromosomes. That means, for every gene in the genome, there is a second copy of that gene. Now, these two copies may be slightly different: that is where a lot of genetic diversity comes from. Nevertheless, there are essentially two copies of every gene in every cells in animal’s body. This is in contrast to organisms that are haploid, which means there is one copy of each chromosome. Most fungi are haploid and plants go through a lifecycle that is diploid half the time and haploid half the time.

  • Animals usually reproduce sexually and produce an embryo that undergoes developmental stages.

Sexual reproduction is a way to take the two copies of every gene, shuffle them up, and then combine them with the shuffled genes of another individual to produce an offspring. Basically, it is why you inherited half of your genes from from dad and half of your genes from your mom. Sexual reproduction helps insure that each new generation is the same, yet at the same time different, from its parent generation.

While most animals can reproduce sexually, a number of them can reproduce asexually, too. Asexual reproduction is essentially making clones of oneself: some cells from the animal’s body begin to form an offspring. This offspring is genetically identical to the parent.

https://upload.wikimedia.org/wikipedia/commons/0/01/The_animans_and_man%3B_an_elementary_textbook_of_zoology_and_human_physiology_%281911%29_%2814598161549%29.jpg
In case you want to see an example of asexual reproduction, here is a hydra producing an offspring. A hydra is a tiny relative of jellyfish that lives in freshwater. See the little, miniature hydra growing off of the larger one? That will eventually break off an become a new individual. That all happens without sexual reproduction.
  • Animals are heterotrophic, typically by ingestion.

“Heterotrophic” refers to how an organism acquires energy and materials. It specifically means that the creature gets its energy and materials from prefabricated organic material. Basically, it has to eat something. This is in contrast to autotrophs which make their own food. Plants, for example, are autotrophs.

To say that animals are heterotrophic by ingestion, it means that not only to animals eat other things, they also take the food into their bodies and digest it internally. This is in contrast to fungi, which are heterotrophic like animals, but have not digestive system. Instead, they secrete digestive enzymes around themselves, the enzymes digest food around them, and then they absorb the digested products.

Now that we know a little bit about what defines an animal, we can begin to see how familiar creatures meet the definition of an animal. Take a cat, for example. It is clearly a multicellular. While we may not be familiar with its genetics, rest assured that cats are diploid. Cats have muscles that allow them to walk about. We see them eat, so we know that they are heterotrophic by ingestion. Finally, we also know that cats reproduce sexually.

A cat doing what a cat does best: resting. Seriously though, a cat is a complex system. We can only see one organ system externally (the integumentary system), but there are a total of 11 organ systems packed into this cat.

Now, a cat has 11 organ systems that accomplish all of these characteristics. These organ systems are:

  • an integumentary system (the skin),
  • a digestive system,
  • a respiratory system (lungs),
  • a urinary system (bladder and kidneys),
  • a skeletal system,
  • a muscular system,
  • a cardiovascular system (heart and blood vessels),
  • a lymphatic system (lymphatic vessels),
  • a nervous system (brain and nerves),
  • an endocrine system (hormones),
  • and a reproductive system.

That is a lot of systems just to allow a cat to move, eat, and reproduce. Yet, that is the level of complexity with which we are familiar. Now, let us consider Trichoplax, the seemingly shapeless blob of cells with which we began this post.

To begin, Trichoplax belongs to the animal phylum Placozoa. For a long time, it was the only known genus. In fact, for just as long, there was only one known species, Trichoplax adhaerens known for the entire phylum. It turns out that there are more species in the phylum, but the different species all look the same (they all look like shapeless blobs of cells under the microscope). It takes genetic analysis to actually recognize the different species.

At a glance, Trichoplax does not appear to have any order to it. The blob of cells has no front or back. It can move in any direction. It can change its shape at will. It looks like nothing more than a multicullar Amoeba.

Despite the fact that placozoans look like a shapeless blob of cells, it actually is not. It actually has specialized cells: there are six types of cells found in the body of a placozoan. Also, there is a difference between the top and bottom of the animal. The top layer of cells is thin. It seems to do little more than provide a covering of the animal’s body. The bottom layer if much thicker. It is where motion occurs and where digestion takes place. Finally, there is a middle layer of cells that is much looser than the other two. This middle layer of cells connects to all of the other cells within the body of Trichoplax.

Cross-section of Trichoplax. Notice the three layers of cells: a distinct layer at the top, a thick layer at the bottom, and a loose layer of long cells in the middle. Image from Eitel et al. (2013).

The bottom layer of Trichoplax is covered with cilia. These are little hair-like structures. These are used in locomotion. Specifically, it “walks” along these tiny little hairs. In addition, there are cells on the underside of the body that secrete digestive enzymes. When Trichoplax is ready to eat something, it creeps over a source of food, such as algae, then hunkers down on top of it. The bottom layer of cells then release enzymes that digest the food, which is then absorbed. Once it is finished, the creature moves on.

In case you are wondering, placozoans have a global distribution. They are all marine, so they are found in the ocean.

Now that we know a little bit of how Trichoplax lives, let us compare it to our five characgteristics of animals.

  • Animals typically have the power of movement or locomotion by means of muscle fibers.

While there have been no muscle fibers identified, Trichoplax is certainly mobile, so this trait checks out.

  • Animals are multicellular; most have specialized cells that form tissues and organs.

Trichoplax is clearly multicellular, and despite appearances, it actually have multiple cells types. There are only six, but this still fits this trait.

  • Animals typically have a life cycle where the adult is diploid.

Trichoplax has diploid cells. This has been determined genetically.

  • Animals usually reproduce sexually and produce an embryo that undergoes developmental stages.

It is presumed that Trichoplax can reproduce sexually. The genetic diversity found within it matches the expected diversity of a creature using sexual reproduction. Having said that, how sexual reproduction occurs is unknown. It is known, however, that it can reproduce asexually simply by splitting in half.

  • Animals are heterotrophic, typically by ingestion.

Trichoplax is heteroptrophic, since it eats other organisms such as algae. It does not, however, consume prey by ingestion. In that regard, it is an exception among animals.

Notice that, in all of this, Trichoplax is actually a complex creature. There is more than can be said about its structure and behavior. For example, it appears to have cells that function like nerves, it may even have cells that are responsive to light. So it is by no means a truly simple creature. However, compared to other animals, it is pretty simple. It have only six cells types with no real organ systems while more familiar animals have eleven organ systems each with multiple types of cells. In that regard, Trichoplax is indeed a very simple animal.

References:

Michael Eitel, Hans-Jürgen Oslgus, Rob DeSalle, Bernd Schlerwater (2013) “Global Diveristy of the Placozoa” Plos One 8(4): e57131

Kai Kamm, Hans-Jürgen Osigus, Peter F. Stadler, Rob DeSalle, Bernd Schierwater (2018) “Trichoplax genomes reveal profound admixture and suggest stable wild populations without bisexual reproduction” Scientific Reports 8: 11168

Bernd Schierwater (2005) “My favarite animal, Trichoplax adhaerensBioEssays 27(12): 1294-1302

Carolyn L. Smith, Frédérique Varoqueaux, Maike Kittelmann, Rita N. Azzam, Benjamin Cooper, Christine A. Winters, Michael Eitel, Dirk Fasshauer, Thomas S. Reese (2014) “Novel Cells Types, Neurosecretory Cells, and Body Plan of the EArly-Diverging Metazoan Trichoplax adhaerensCurrent Biology 24(14): 1565-1572


[1]

Sylvia Mader and Michael Windelspecht (2014) Inquiry into Life 14th ed., McGraw Hill, New York, New York, pg. 623