The first mitochondrial genomes of stylasterid corals (Hydrozoa; Stylasteridae) and their phylogenetic position.

Background: Mitochondrial genomes among Medusozoa show considerable variation in the class Hydrozoa, and many phylogenetic relationships inside the subphylum remain still unresolved. Stylasteridae, one of the most species-rich families among hydrozoans, characterized by a calcareous skeleton, lacks mitochondrial genomic information, limiting our understanding of their evolution and phylogenetic position within Hydrozoa.

Methods and results: This study present the first mitochondrial genome of two stylasterid species from the genus Errina (E. aspera and E. atlantica). Total genomic DNA was extracted and sequenced using Illumina NovaSeq 6000 and mitochondrial genomes were assembled and annotated using two different methods. Results hypothesized linear genomes of 14,938 bp and 14,969 bp respectively, containing 13 protein-coding genes, 2 transfer RNA genes, and 2 ribosomal RNA genes. The gene order was congruent with other Filifera, but lacking the cox1 duplication, transposition, and inversion common in other hydrozoans. Phylogenetic analyses confirmed the position of stylasterids as a sister group to Hydractiniidae within the Filifera suborder.

Conclusions: This study provides the first information about the stylasterid mitochondrial genome structure. Moreover, the phylogenetic analysis confirms the phylogenetic position of Hydractiniidae as sister family of Stylasteridae. The new findings have highlighted the importance of sequencing additional genomic data to better unravel the evolutionary history of this significant family.