H. zschokkei is notable for its absence of mitochondria, mitochondrial DNA, aerobic respiration and its reliance on an as of yet undiscovered energy metabolism.

H. zschokkei is notable for its absence of mitochondria, mitochondrial DNA, aerobic respiration and its reliance on an as of yet undiscovered energy metabolism. [...] it is the only known multicellular animal that does not rely on the aerobic respiration of oxygen. [...] animals such as H. zschokkei were originally cancerous growths in free-swimming jellyfish that escaped their parent organism and thereafter became a separate species parasitizing other animals. Absolutely wild, thought-provoking biology!