Metazoans have relatively conserved mitochondrial gene content and structure. These organellar genomes in animals usually contain a set of 13 protein-coding genes, two ribosomal RNA genes, and approximately 22 tRNA genes. They typically occur in the form of a circular molecule approximately 15–20 kb in size (Boore 1999; Gissi et al. 2008). Nevertheless, some notable exceptions exist, such as a lack of the atp8 gene in many nematodes (Hu and Gasser 2006). Exceptions to overall structure are seen in the rotifer Brachionus plicatilis, whose mitochondrial genome consists of two separate circular DNA molecules (Suga et al. 2008), whereas those of many cnidarians consist of one or more linear molecules, which have also lost most of their tRNA genes (Kayal et al. 2012).