Meet the Metazoans! A look into the far corners of the animal kingdom
Figure 1 The diversity of the animal kingdom that we are most familiar with.
Animals, also called “metazoans ”, are extremely diverse and fascinating. Let us start this article with a short exercise! Think of an animal right now. There are over 1.5 million described animal species on earth, making it impossible for me to know which animal you thought of. However, it is highly probable that the animal you envisioned had a face, a body, limbs, maybe wings or fins, and could move around. I had used this exercise previously when I used to teach. I taught introductory biology courses for five years, and before beginning a new semester, I would always ask the students to tell me their favorite animal. The most common answers were dog, cat, shark, lion, and eagle. The goal of that question, and the above exercise, was to showcase some of the biases we have regarding our understanding of animals. I would spend the next multiple weeks, introducing the students to the diverse animal kingdom!
In this article, I hope to do the same - impress on you the sheer diversity of the animal kingdom by delving deep into the fringes and depths of the animal phylogenetic tree, and hopefully, change your perception of what being an animal means. So, lets meet the Metazoans!
The microscopic and parasitic
Some animals are so small that we need a microscope to see them! In 2016, in the beautiful state of Montana, USA, nearly 180 miles of the beautiful Yellowstone River had to be shut down, stopping all activities like fishing, boating, and swimming . This happened because the fish populations were decimated by a parasite – Myxobolus cerebralis (Figure 2A). This parasite was microscopic and causing the fish to die from proliferative kidney disease. It belongs to a group called the myxozoans, which spend most of their lives as microscopic organisms made up of just a few cells in number (Figure 2A-C). They have an intriguing life-cycle where they alternate between two hosts- fish, bryozoans, annelids, and even mammals (Figure 2D).
It turns out that these tiny parasites are animals! Essentially, microscopic animals are parasitizing other animals. To get a sense of how small these animals are, check out the image of the spores of a myxozoan in a flounder (Figure 2E). There are over 2,000 described species of myxozoans, and scientists believe that are many more left to be identified. They are close relatives of animals we are more familiar with- the cnidarians, a group containing marine animals like jellyfishes, corals, sea-anemones, and more. Additionally, they have some of the smallest nuclear and mitochondrial genomes in animals, along with their significantly reduced morphological features .
Figure 2 Diversity of the myxozoans- A] Myxoboluscerebralis (the animal that shutdown the Yellowstone River), B) spores of the animal Kudoaseptempunctata, c] Another myxozoan- Kudoaiwatai, D] the life-cycle of myxozoans, and E] Kudoa septempunctata in olive flounder muscles
The ancient and immobile
When we think of animals, we imagine them running, walking, swimming, or flying! Essentially, we expect animals to move about, unlike plants. But members of one of the most diverse and ancient groups of animals seem to be quite content staying put in one place. Members of the phylum Porifera are called sponges and are some of the oldest animals in this world, sometimes referred to (incorrectly) as living fossils (Figure 3). Some of the oldest sponge fossils date back to 600 million years ! The giant barrel sponges in the Caribbean have been estimated to be over 2,300 years old .
There are nearly 10,000 species of sponges globally, of which just 150 live in freshwater. Indeed, they were thought of as plants for quite some time because they are attached to a substrate and do not move. It is important to note that while adult sponges are sessile, juvenile sponges move freely or drift in the water. Much like not moving, sponges display some more oddities. Sponges feed by filter-feeding (however, some sponges are now known to be carnivorous! (Figure 3B)). Most of them are hermaphrodites- they can act as male or female. While it may sound intuitively wrong, animals also can reproduce asexually. In sponges, asexual reproduction can happen by budding or fragmentation .
Figure 3 Diversity of phylum Porifera- A] Aplysina fistularis, B] Chondrocladia lampadiglobus (the carnivorous sponge), C] Euplectella aspergillum (the glass sponge).
The immortal and intriguing
Animals do not come with a reset switch. Animals are born, age, and die. There is one animal that seems to do something rather peculiar. A jellyfish, the size of our pinky nail, the Turritopsis dohrnii, is fondly called the “immortal jellyfish” (Figure 4). Like other jellyfish, it starts as a larva and swims and settles on the seafloor. It now grows into a colony of immobile polyps. The polyps then give rise to the free-living medusa stage, the stage we are most familiar with as jellyfishes (Figure 5).
Figure 4Turritopsis dohrnii medusa- the immortal jellyfish
When faced with physical damage or starvation, the medusae pull off a fantastic trick. They revert to the polyp stage via an intermediate stage- the cyst. The process behind it- transdifferentiation, has critical medical applications and can help improve our understanding of cellular processes like aging. It is important to note that despite these fantastic abilities to revert to the earlier stage, these jellyfish do die in the wild from predation or disease without undergoing this change. Thus, while it is not immortal in the truest sense of the word, it’s notable capacity to transdifferentiate and revert to an earlier stage sets it apart from every other animal on this planet.
Jellyfish belong to a group of an exceptionally diverse group of animals- the phylum Cnidaria. This phylum includes animals like corals, sea anemones, and the myxozoans we spoke about earlier. At this point, you may wonder, why am I talking about these marine invertebrates so much? This is because all animals belong to one of five major clades- Porifera, Cnidaria, Ctenophora, Placozoa, and Bilateria. Bilateria, a group that includes humans, is probably the most well-studied group of animals among them all. This happens because humans and all the best-studied model organisms are within Bilateria, and most research tends to be human-centric. This results in a lot of unexplored biology and diversity in the remaining four groups of animals collectively referred to as the “non-bilaterian” animals. Our human-centric view of biodiversity leads to a wealth of “hidden biology” in such animals, which we tend to underestimate. If we want to understand the scope of animal diversity and evolution, we need to include all animals, not just the bilaterians .
Figure 5 The life-cycle of the immortal jellyfish, compared to other jellyfish. Unlike the typical Hydrozoa life cycle (blue line), the Turritopsis dohrnii life cycle (in red) allows it to revert to a younger life stage under unfavorable circumstances. Image taken from .
The ones with the strange feeding habits
At this point, I will remain with the same phylum. And it is time for another exercise! I want you to reflect on this for a while- How do you eat, and how do animals eat? The answer may vary a little depending on which animal you think of. But the basic concept would probably not. Animals have a mouth, through which the food enters when the animal opens the mouth. There is a group of animals that takes this step to the extreme. The point of this example is to demonstrate that even in something as essential as eating, animals can be utterly and ridiculously diverse.
Hydra are freshwater cnidarians best known for their regeneration abilities after their tissue is torn apart (Figure 6). Like other animals, they cannot prepare their food and need to eat. But they do not have a permanent mouth! Every time the animal eats, it needs to tear itself a new mouth. The skin cells separate from each other to form the mouth opening, and then after eating, they close. Sometimes the mouth opening is bigger than the body!  Check out the amazing video here!
And it is not just mouths that can be transient. In another group of animals- the ctenophores, the anus is transient. Phylum Ctenophora includes some of the most beautiful marine invertebrates on this planet. One of them is Mnemiopsis leidyi (Figure 7). In this amazing animal, scientists found that the anus appears only when the animal wants to excrete! 
Figure 6 Hydra attached to a substrate.
Figure 7 The ctenophore Mnemiopsis leidyi
The ones with fascinating reproductive strategies
Like feeding, excretion, mobility, and aging, reproduction too is quite diverse in the animal kingdom. Animals reproduce both sexually and asexually, as we saw in sponges. Asexual reproduction can happen via budding, fragmentation, etc. Like we saw before, some animals do not move! While asexual reproduction is not an issue, sessile animals evolve interesting strategies for sexual mating. For instance, in the coral reef, the male and female corals cannot move towards each other to mate as they are sessile. In asexual reproduction, new polyps can bud off from parent polyps to form a new coral. Additionally, several coral species release massive numbers of eggs and sperm into the water, which give rise to new larvae/planulae . Another sessile species, the barnacle, an arthropod, overcomes this immobility barrier by having the highest male reproductive organ to body size ratio in the animal kingdom (nearly eight times their body length) .
Figure 8 The barnacle
This article's goal was to showcase some fantastic examples of animal diversity and drive home the point that animals are much more than what we see on television. Even channels with good intentions, like National Geographic, struggle to balance viewer interest and the scope of metazoan diversity. Today, we met all sorts of animals:
Animal we need a microscope to see.
Animals who are parasites of other animals.
Animals who cannot move.
Animals who are theoretically immortal.
Animals with transient feeding and excretion organs.
Animals who cannot move but can mate sexually.
The reason I focused on marine animals is that I have been working on them throughout my life. Such outstanding examples can be found scattered across the animal phylogenetic tree.
All we must do is look!
 The Atlantic, https://www.theatlantic.com/science/archive/2016/08/the-parasite-that-just-shut-down-a-montana-river-has-an-unbelievable-origin/496817/
 Chang, E. Sally, et al. "Genomic insights into the evolutionary origin of Myxozoa within Cnidaria." Proceedings of the National Academy of Sciences 112.48 (2015): 14912-14917.
 Sponge grade body fossil with cellular resolution dating 60 Myr before the Cambrian, PNAS, Zongjun Yin, DOI: 10.1073/pnas.1414577112
 McMurray, S.E., Blum, J.E. & Pawlik, J.R. Redwood of the reef: growth and age of the giant barrel sponge Xestospongia muta in the Florida Keys. Mar Biol 155, 159–171 (2008).
 The Tree of Life (http://tolweb.org/treehouses/?treehouse_id=4291)
 Matsumoto, Yui, Stefano Piraino, and Maria Pia Miglietta. "Transcriptome characterization of reverse development in Turritopsis dohrnii (Hydrozoa, Cnidaria)." G3: Genes, Genomes, Genetics 9.12 (2019): 4127-4138.
 Dunn, Casey W., Sally P. Leys, and Steven HD Haddock. "The hidden biology of sponges and ctenophores." Trends in ecology & evolution 30.5 (2015): 282-291.
 Carter, Jason A., et al. "Dynamics of mouth opening in Hydra." Biophysical journal 110.5 (2016): 1191-1201.
 Tamm, Sidney L. "Defecation by the ctenophore Mnemiopsis leidyi occurs with an ultradian rhythm through a single transient anal pore." Invertebrate Biology 138.1 (2019): 3-16.
 American Association for the Advancement of Science. "Rethinking Barnacle Reproduction." (2013): 258-258.