The mitochondrial genome of Palythoa tuberculosa (Esper, 1805) contains a novel ORF with unknown function.

Recent advances in sequencing technology have enabled rapid and cost-effective recovery of mitochondrial genomes (mt-genomes), which used in phylogenetic and evolutionary studies. Among cnidarians, the order Zoantharia possesses the largest mt-genomes on average (20,976 bp). Here, we present the complete mt-genome of Palythoa tuberculosa (Hexacorallia: Zoantharia: Sphenopidae), which was 23,044 bp in length, approximately 2,000 bp larger than the zoantharian average. Synteny analysis revealed that this increase resulted from a novel open reading frame (ORF) located between trnW and cox2. This ORF was also present in the closely related species P. caribaeorum, indicating its origin in their common ancestor. Importantly, this represents the first case of a novel ORF insertion in zoantharians, clearly distinct from previously reported ORF extensions. While mt-genome expansions are occasionally observed in cnidarians, both the insertion sites and inserted elements vary among lineages, highlighting the lineage-specific mechanisms driving mitochondrial genome evolution in this group.