Towards a mitogenomic phylogeny of mud dragons (Kinorhyncha): two new mitogenomes from the Pycnophyidae family.

Mud dragons (Kinorhyncha) are an understudied phylum of microscopic marine invertebrates inhabiting a wide range of marine sediments, from shallow coastal habitats to abyssal depths, and occurring from tropical to polar regions. Despite their broad distribution, their minute size often makes them difficult to study. Consequently, molecular resources for the group remain extremely limited and only three complete mitogenomes have been published from this phylum to date. To help fill this substantial gap in genetic data, we sequenced and annotated two new complete mitogenomes from the family Pycnophyidae, Pycnophyes greenlandicus Higgins & Kristensen, 1988 and Cristaphyes cryopygus (Higgins & Kristensen, 1988). Obtained mtDNA sequences were compared with available transcriptomic data from other Kinorhyncha species, providing a more comprehensive basis for the phylogenetic analysis of the phylum. The results showcased unexpected rearrangements in the gene order across all examined taxa, an unusual cox1-tRNA-Glu genes overlap and an unstable phylogenetic position within the Pycnophyidae family. These new sequences not only significantly expand the mitogenomic data available for Kinorhyncha, but also provide an important step toward a better understanding of phylogenetic relationships within the phylum.