Abstract
Razor clams, belonging to the Pharidae and Solenidae families, are ecologically and economically important; however, very little research has been conducted on the Pharidae family. The genus Novaculina is a marine-derived freshwater lineage, and Novaculina chinensis is a rare freshwater species of the Pharidae family. In order to understand the phylogenetic relationships of N. chinensis, we sequenced the mitochondrial genome of the genus Novaculina, which is 16,262 bp in length and consists of 12 protein-coding genes (PCGs), 22 transfer RNA genes (tRNAs), and 2 ribosomal RNA genes (rRNAs). The phylogenetic relationships of 69 Imparidentian mitochondrial genomes (mitogenomes) indicated that N. chineisis is closely related to Sinonovacula constricta of the order Adapedonta. Our study also found that the Ka/Ks ratios of 12 protein-coding genes in the Pharidae family are lower than one, indicating the occurrence of negative purification selection. Morphological observations of the siphons of N. chinensis, Novaculina myanmarensis, and Novaculina gangetica indicate that N. chinensis may be the ancestral clade of the genus Novaculina, which has not been proposed in previous studies. Our study provides useful molecular information on the phylogenetic and evolutionary relationships of Pharidae and also contributes to the conservation and management of the germplasm resources of N. chinensis.