Henneguya zschokkei

Henneguya zschokkei or Henneguya salminicola is a species of a myxosporean endoparasite. It afflicts several salmon in the genera Oncorhynchus and Salmo, where it causes milky flesh or tapioca disease''. H. zschokkei is notable for its reliance on an exclusively anaerobic metabolism as well as its lack of mitochondria and mitochondrial DNA. '' It is the only known multicellular animal that does not require oxygen to survive.

Description
Henneguya zschokkei is found in fish as an ovoid spore with two anterior polar capsules and two long caudal appendages. Individuals are very small (about 10 micrometers in diameter), but are found aggregated into cysts 3–6 mm in diameter at any place in the animal's musculature.

Metabolism
So far as is known, H. zschokkei is unique among multicellular animals because it does not make use of the aerobic respiration of oxygen. The creature relies instead on an exclusively anaerobic metabolism, making it the only known member of the eukaryotic animal kingdom to shun oxygen as the foundation of its metabolism. It also lacks a mitochondrial genome and therefore mitochondria.

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, gut or muscle cells of any kind.

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.

Origins
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.

One theory put forward to explain the highly unusual habit of H. 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.

Hosts
Known hosts of Henneguya zschokkei include:
 * Oncorhynchus gorbuscha (Pink salmon)
 * Oncorhynchus keta (Chum salmon)
 * Oncorhynchus kisutch (Coho salmon)
 * Oncorhynchus nerka (Sockeye salmon)
 * Anadromous forms of Oncorhynchus mykiss (Rainbow trout)
 * Oncorhynchus tshawytscha (Chinook salmon)
 * Salmo salar (Atlantic Salmon)