RESULTS AND DISCUSSION
The mitogenome of Runcina aurata spanned a total length of 14,282 bp. It was found to contain the typical 13 coding genes (CDS), the large and small ribosomal RNA genes, and 22 tRNA genes, all of them with the standard gene order of heterobranchs (Fig. 2). The overall GC% content was 25.7%, one of the lowest values in heterobranchs. When focusing on CDS, rRNA and tRNA this value shifted to 26.3%, 23.6% and 25.5% respectively. Parallelly, Facelina auriculatamitochondrial genome length was 14.171, comprising the ordinary 13 coding genes, 2 ribosomal RNA and 22 tRNA, with the standard gene order of heterobranchs (Fig. 2). The overall GC% content of the mitogenome was 35%, in contraposition to the R. aurata ’s one. The CDS ofF. auriculata mitogenome were composed of 35.5% GC, the rRNA were 32% GC and the tRNA were 35.8% GC.
The phylogenetic analysis indicated, as previously reported (Varney et al., 2021), thatRuncina aurata and therefore Runcinida is sibling taxon to a clade composed by Aplysiida and Cephalaspidea with a BS support value of 100 (Fig. 3). Following, Facelina auriculata is sibling to the other facelinid species Sakuraeolis japonica (Baba, 1937), with a bootstrap support value of 100, being facelinids sibling taxon to the other aeolidiaceans Protaeollidiella atra Baba, 1955 andHermissenda emurai (Baba, 1937) (BS = 100) (Fig. 3) (Carmona et al., 2013). With the R. aurata and F. auriculata mitochondrial reference genomes, the inner systematic relationships can be properly assessed and explored, while providing a valuable tool to design specific primers for mitochondrial targeted markers of these groups, which have been hampered when using the standard Folmer set for cytochrome oxidase I, and Palumbi set for 16S.