Henneguya zschokkei is found in fish as an ovoid spore with two anterior polar capsules and two long caudal appendages.[6] Individuals are very small (about 10 micrometers in diameter),[7] but are found aggregated into cysts 3–6 mm in diameter at any place in the animal's musculature.[8]
H. zschokkei is ultimately a highly derived cnidarian and is distantly related to jellyfish, sea anemone and corals. However, this obligate internal parasite so little resembles cnidarians (let alone other multicellular animals) that it, along with many other species in the class Myxosporea, were initially categorized as protozoa. It is nevertheless most closely related to jellyfish. This species, like most myxosporeans, lacks many of the diagnostic criteria that identify cnidarians. Indeed, it is without nervous, epithelial, gutormuscle cells of any kind.[11]
This parasite has not only lost its mitochondria and the mitochondrial DNA residing in them, but also the nuclear genes that code for mitochondrial reproduction. What genetic instructions for these functions that remain lie in useless pseudogenes.[4]
The origin and cause of H. zschokkei's highly reduced genome are not yet known. While eukaryotes are known for aerobic respiration, a few unicellular lineages native to hypoxic environments have also lost this capacity. In the absence of oxygen these single-celled organisms lose the portions of their genome that anticipate and govern aerobic respiration. These unusual eukaryotes have developed mitochondria-related organelles (MROs) that fulfill many of the functions of conventional mitochondria. However there is no evidence of such an adaptation in the multicellular H. zschokkei.[4]
One theory put forward to explain the highly unusual habitofH. zschokkei and its fellow myxosporeans invokes the cancers of cnidarians. On this explanation, animals such as H. zschokkei were originally cancerous growths in free-swimming jellyfish that escaped their parent organism, thereafter becoming a separate species that parasitized other animals. Such an origin is referred to as a SCANDAL, a loose acronym of the phrase speciated by cancer development in animals.[12]
^ ab"Henneguya salminicola". fishpathogens.net. Oregon State University. Oregon Department of Fish and Wildlife. Archived from the original on 2020-02-28. Retrieved 2020-02-28.
Fish, Frederic F. (1939). "Observations on Henneguya salminicola Ward, a Myxosporidian Parasitic in Pacific Salmon". The Journal of Parasitology. 25 (2): 169–172. doi:10.2307/3272359. JSTOR3272359.
Clouthier, Sharon C; Gunning, Derek J; Olafson, Robert W; Kay, William W (December 1997). "Antigenic characterization of Henneguya salminicola". Molecular and Biochemical Parasitology. 90 (2): 543–548. doi:10.1016/s0166-6851(97)00200-4. PMID9476801.
