Complete mitochondrial genome of Anadara vellicata (Bivalvia: Arcidae): A unique gene order and large atypical non-coding region

The mitochondrial (mt) genome is a significant tool for investigating the evolutionary history of metazoan animals. The family Arcidae belongs to the superfamily Arcacea in the bivalve order Arcoida, comprising about 260 species. Currently, three complete mitochondrial genomes are available in GenBank, representing 1 subfamily and 2 genera. Here we present the complete mitochondrial genome sequence of Anadara vellicata (Bivalvia: Arcidae), the first report of complete mitogenome from Anadara, Arcidae, and compared its sequence with other available Arcidae mitogenomes. The A. vellicata mitogenome is 34,147 bp in length, including 12 protein-coding genes (PCGs), 25 transfer RNAs (tRNAs), 2 ribosomal RNA (rRNA) genes and non-coding regions (NCR) (20,722 bp). The nucleotide composition of the genome is A + T biased, accounting for 61.03%, with negative AT skew (− 0.12) and positive GC skew (0.41). We report the evidence of alloacceptor tRNA gene recruitment (trnY-trnL2). A conserved 23 bp-long sequence was used as the basis to infer the 3′ terminus of rrnS. Most of the non-coding sequences (16,112 bp) are observed within one segment. In the NCR, the tandem repeat (TR) region is 1143 bp, comprising six tandem repeats with 189 bp to 192 bp in length. In addition, a long thymine-nucleotide stretch (T-stretch) was detected in the NCR of A. vellicata. The gene order and transcriptional polarity of the protein-coding genes is identical to other Arcidae species. tRNA genes are rearranged, making the gene order unique. The results support that mt gene arrangement among Arcidae species is not random, but correlated with their evolutionary relationships.