If you retained anything from middle school biology, it’s probably that the mitochondria are the powerhouse of the cell. This is true of every animal in the world, except for one. Scientists have reported that a microscopic parasite known as Henneguya salminicola has no mitochondrial genome. Therefore, it’s the only animal ever discovered with no ability to breathe.
Henneguya salminicola is a microscopic myxosporean parasite of fish that is related to the phylum Cnidaria (jellyfish, sea anemones, etc.). Scientists believe that H. salminicola and other members of its genus may have been more similar to jellies in the past, but their parasitic nature led to the loss of many traits to simplify reproduction. One of the things it lost, apparently, was respiration.
For part of its life, Henneguya salminicola is a free-swimming spore as seen above. The two eye-like spots on each organism are actually stinging cells, one of the few features Henneguya salminicola hasn’t lost over time. It uses those to latch onto hosts. The parasites spend most of their lives buried deep in the flesh of fish hosts where it would have little access to oxygen.
Evolution usually tends toward more complexity and capabilities, but the selective pressure on myxosporean parasites is so narrow that genetic complexity became a disadvantage. Over time, they lost their muscle tissue, nerve cells, and most other body systems. It’s essentially evolving in reverse to become more like a single-cell organism. The race to simplify its genome seems to have even led to the complete loss of its mitochondrial genome, which is home to the genes that control biological respiration.
Henneguya salminicola is usually a benign infection, appearing in fish as a small, white nodule. This is sometimes called tapioca disease.
When animals breathe, that oxygen helps power a process in mitochondria called the electron transport chain. That produces ATP, a molecule that powers most biological activity inside cells. That’s why every other animal scientists have studied has some form of respiration — it would be a huge biological disadvantage to lose that ability. Somehow, that was an advantageous change for H. salminicola.
That raises the question of where H. salminicola gets its ATP. It’s possible it gets by with the very small amount of ATP produced by other processes, or it may be leeching ATP from its host. Scientists will have to study this bizarre organism in more detail to find out.
(Top image credit: Stephen Douglas Atkinson)