Sometimes, simplicity is key to success. In our attempts to understand the evolution of animals, from small, single-celled protists to big, multi-cellular animals, we miss out on the story of a fascinating journey of a group of animals which make us wonder “What makes an animal, an animal?”. To meet these animals, let’s travel back to August of 2016, to Montana along the Yellowstone River. Here we witness a scene of carnage, as thousands of whitefish and trout are dying. They have all succumbed to proliferative kidney disease, brought on by tiny parasites. The situation requires strong measures, and in the middle of August, a 183-mile stretch of this river has to be shut down to prevent the spread of this disease. No families or friends can swim, boat, or fish along this wonderful, pristine river. Normally, we associate diseases with bacteria or viruses. But, in this case, the fish are infected with animals – tiny, endoparastic animals!

Image 1. One member of the group of myxozoans, phase-contrast micrograph of Kudoa septempunctata spores. Scale bar = 5 μm, found in olive flounder muscles.

 

Here’s a fun activity, meant to introduce you to the culprits of the Montana fish massacre: the myxozoans. Take a moment to picture an animal in your mind. When you have that image, ask yourself this question: Do I need a microscope to view that animal?  Chances are high that you would not need a microscope to see the animal you thought about. Now, here’s the crazy thing: in most cases, to observe a myxozoan, you most definitely would need a microscope!

Myxozoans, a group of over 2,000 species, are a fascinating group of animals with an intriguing evolutionary journey. In fact, because of their extremely small size (one hundredth to two hundredths of a millimeter), they were initially believed to be part of a group of unicellular eukaryotes: the protists, which include the amoeba, paramecium, and slime mold.  But a few scientists were not content. They noticed something odd about these animals. Myxozoans possess a structure called a “polar capsule” that they use to attach themselves to their hosts.  The polar capsule bears a strong resemblance to nematocysts, which are stinging structures found in jellyfishes! Strong similarities between the nematocysts and the myxozoans suggested that myxozoans could be cnidarians themselves.

To confirm this astounding hypothesis, scientists sequenced and the compared the genomes of myxozoans and cnidarians.  Phylogenomic analyses revealed that these tiny parasitic animals possess genes which are specific to cnidarians, and are, in fact, cnidarians; relatives of animals we know more about-jellyfishes, sea anemones, corals, and hydra.

These animals underwent a remarkable evolutionary journey; from complex to simple – from a free-living cnidarian to a tiny endoparasite. On their way, myxozoans had to make some tough choices. For instance, not only did they undergo a drastic reduction in their body-size and complexity, but they also jettisoned a significant amount of their genome! Indeed, they have nearly one-third fewer genes than their cnidarian relatives. And this again takes us back to the question, “What makes an animal, an animal?”. These tiny endoparasites have lost several genes which are critical in animal development and multicellularity; genes, which make an animal, an animal. Genes retained by these myxozoans, such as nematocyst-specific genes, are important to survive as endoparasites. Like most other animals, myxozoans also possess mitochondria. Mitochondria are energy-producing organelles inside of cells that make up eukaryotes (with multiple other critical functions), and possess their own genome. Some of the mitochondrial genomes possessing the fewest number of genes are from myxozoans. The curious case of the myxozoans is a wonderful testament to the evergreen adage, “sometimes, simplicity is the key to success”.

How do endoparastic myxozoans affect you?  An average person eats half of her/his weight in seafood each year. Some species of myxozoans infect economically important fish, causing a detrimental effect on fisheries and on your dinner. A common group of myxozoans that infects marine fishes, is KudoaKudoa species have a significant detrimental impact on commercial fisheries. While many of these Kudoa species leave the host alive, they lead to visible muscle cysts and softening of the flesh or “soft flesh/jelly flesh” which greatly reduces market value of the fish.

Image 2. Myxobolus cerebralis, the causative agent of Whirling disease, with the polar capsule at the right-hand top corner.

 

Image 3. A fish suffering from Whirling Disease showing skeletal deformities.

Another example of a myxozoan-induced infection is Whirling disease, a serious fish infection that leads to economic and ecological impacts.  Whirling disease appears when another myxozoan, Myxobolus cerebralis, infects the cartilage in the spine of young fish, resulting in skeletal deformities, blackened tail, and nerve damage. As a result of this infection, the fish becomes disoriented, swims erratically in circles “whirling-behavior”, and in many cases is unable to escape from predators. This myxozoan was introduced to the US over 50 years ago! Whirling disease can decimate fish populations and negatively impact the food chain.

All in all, myxozoans help scientists by forcing a debate on what it really means to be an animal. They open an evolutionary conversation on what “evolutionary progress” means, and what defines direction in evolutionary progress.

 

Viraj Muthye is a Ph.D. student in Bioinformatics at Iowa State University, interested in studying the evolution of the mitochondrial proteome, sponges, and science communication.  He has started his own science blog, and his Twitter handle is @awesome_biology, and his Instagram account can be found at awesomebiology.

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