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In This Issue

Aerobic respiration is a hallmark of eukaryotes but has been lost in several single-celled eukaryotic lineages adapted to low-oxygen environments. Whether certain animals also rely exclusively on anaerobic metabolism and have mitochondrionrelated organelles (MROs) has been debated. Through deep sequencing and microscopy, Dorothée Huchon et al. (pp. 5358–5363) found that the parasitic cnidarian Henneguya salminicola, which can live in low-oxygen environments in hosts, has lost its mitochondrial genome, the capacity for aerobic respiration, and nearly all nuclear genes involved in transcription and replication of the mitochondrial genome. In addition, the anaerobic MROs of this species are unusual due to the presence of cristae, or folds in the mitochondrial inner membrane. The findings show that aerobic respiration, a crucial metabolic pathway, is absent in at least one multicellular animal, not just in certain single-celled eukaryotes. The findings could shed light on the evolutionary transition from aerobic metabolism to exclusively anaerobic metabolism. Moreover, the results might lead to strategies to combat the substantial negative economic impact of parasitic cnidarians on fisheries and aquaculture, according to the authors. — J.W.