Henneguya zschokkei

Henneguya zschokkei or Henneguya salminicola is a species of a myxosporean endoparasite. It afflicts several salmon in the genus Oncorhynchus. It causes milky flesh or tapioca disease. H. zschokkei is notable for its absence of mitochondria, mitochondrial DNA, aerobic respiration and its reliance on an exclusive anaerobic metabolism. Henneguya salminicola 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 muscle mass. Henneguya salminicola is the only known multicellular animal that does not rely on the aerobic respiration of oxygen, relying instead on an exclusively anaerobic metabolism. It lacks a mitochondrial genome and therefore mitochondria, making it one of the only known members of the eukaryotic animal kingdom to shun oxygen as the foundation of its metabolism. H. zschokkei is ultimately a highly derived cnidarian. This obligate internal parasite so little resembles other multicellular animals that it, along with many other species in 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. 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 remain are in the form of useless pseudogenes. The origin and cause of its highly reduced genome are not yet known. While eukaryotes are known for aerobic respiration, a few unicellular lineages native to hypoxic environments have lost this capacity. In the absence of oxygen these single-celled organisms have lost portions of their genome governing aerobic respiration.