The linear mitochondrial genome of commensal hydroid (, , ).

Here, we present the whole mitochondrial genome of commensal hydroid McCrady 1859 (family Eirinidae); this is the first specimen of the family to have its mitogenome sequenced. The linear mitogenome is 15,315 bp in length and consists of 13 protein-coding genes (PCGs), large and small ribosomal subunits (rRNA), methionine and tryptophan transfer RNA () genes (trnM and trnW), and a partial copy of cytochrome oxidase subunit I () pseudogene, as is typical for the class Hydrozoa. Nucleotide sequences of two genes at two ends of the linear mitogenome form a part of inverted terminal repeat. The overall genomic structure and gene arrangement of 13 PCGs were identical to the reported mitochondrial genomes of hydrozoans, except for the positions of two genes. Phylogenetic analysis of 13 PCGs and other cnidarians recovers a closest relationship with the derived cluster of two hydrozoans, and within Leptothecata.