Henneguya salmonicola and Myxozoans

What if someone told you that an animal exists which cannot perform aerobic respiration? Many people would assume such a creature could not exist. Yet there is indeed an animal that lives without using oxygen in the way most animals do, and the group to which it belongs is so unusual that it challenges many common assumptions about what animals are.

Henneguya salmonicola

This organism belongs to the group Myxozoa within the phylum Cnidaria. It is extremely small, measuring only about 10 micrometers in length—roughly one hundredth of a millimeter. That number may still be difficult to imagine, so consider a comparison: a typical Escherichia coli bacterium is about 2 micrometers long. In other words, Henneguya salmonicola is only about five times longer than a single bacterium.

Henneguya salmonicola is a parasitic animal. It commonly infects salmon, living within the host's tissues. Although each individual parasite is microscopic, large numbers of them may accumulate together inside the fish, forming visible cysts.

White cysts formed in salmon infected with Henneguya salmonicola（圖片來源：Michal Maňas，採用 CC BY 2.5 授權。）

One of the most remarkable features of Henneguya salmonicola is that it lacks mitochondria. Because mitochondria normally carry out aerobic respiration in animal cells, the absence of these organelles means the parasite cannot perform the oxygen-dependent metabolic processes typical of most animals. In addition, it lacks many of the differentiated cell types found in other multicellular organisms. Its body plan is therefore an extreme example of biological simplification.

Myxozoans

The broader group Myxozoa currently includes more than 2,400 known species. Although they are classified within Cnidaria, their appearance and lifestyle are so unusual that their evolutionary identity was debated for a long time. Early researchers once thought they were protists. Later, some scientists proposed that they belonged among bilaterian animals. Only after decades of research and discussion did molecular and morphological evidence firmly place them within the cnidarians.

As illustrated by Henneguya salmonicola, extreme simplification is a central theme in myxozoan biology. All members of this group are parasites, and their bodies are drastically reduced compared with those of other animals. In some species the reduction is so extreme that their entire body contains only a handful of cells. For example, the parasite Myxobolus cerebralis may consist of only six cells at certain stages of its life cycle, and two of those cells are fused together. This extraordinary reduction makes myxozoans the smallest animals known.

Myxobolus cerebralis。圖片來源：Stephen Atkinson fishpathogens.net

Cnidarians are famous for possessing specialized stinging cells called cnidocytes. When these cells are triggered, a coiled thread rapidly shoots outward to pierce prey and inject toxins, immobilizing the target. Myxozoans possess a structure that resembles this weapon: the polar capsule. Inside the polar capsule is a tightly coiled polar filament. When the parasite encounters a host, this filament is explosively discharged and penetrates the host's tissue, anchoring the parasite firmly in place. In Myxobolus cerebralis, two of the six cells present in the organism are polar capsule cells.

Deformed cutthroat trout infected with Myxobolus cerebralis（圖片來源：Dr. Thomas L. Wellborn, Jr. U.S. Fish and Wildlife Service，CC0 1.0 公共領域。）

Hypotheses about their origin

Because myxozoans differ so dramatically from the animals people usually imagine, their evolutionary origins have attracted considerable scientific attention. In 2019, one particularly controversial hypothesis sparked widespread debate. The study suggested that certain characteristics of myxozoans—including the absence of genes that suppress cancer development, the formation of tumor-like cysts within host tissues, and their ability to survive in low-oxygen environments—might point to an unusual evolutionary origin.

According to this proposal, myxozoans could have arisen from transmissible cancer cells that originated in ancestral cnidarians. Over immense evolutionary timescales, these cancer cells might have evolved the ability to spread between hosts and even between species, eventually developing into independent parasitic organisms capable of circulating among different animals.

This idea remains highly controversial and far from universally accepted. For now, it is best viewed as a provocative hypothesis—an intriguing possibility that highlights just how unusual and mysterious these microscopic parasites truly are.

Author: Bai Leng

Reference:

1. Canning, E. U., Okamura, B. (1 January 2003). Biodiversity and Evolution of the Myxozoa. Advances in Parasitology. Vol. 56. Academic Press.

2. Hedrick, R. P. & El–Matbouli, M. (2002). Recent advances with taxonomy, life cycle, and development of Myxobolus cerebralis in the fish and oligochaete hosts. American Fisheries Society Symposium.

3. Panchin, A. Y., Aleoshin, V. V., Panchin, Y. V. (2019-01-23). From tumors to species: a SCANDAL hypothesis. Biology Direct.