Substantial mitochondrial gene order rearrangements and differential evolution rates within the family Capitellidae (Annelida)

Capitellidae is a family of marine annelids commonly found in coastal to deep-sea sediments. These organisms are characterized by capillary chaetae at the anterior and long-handled hooks at the posterior part. Although mitochondrial genomes (mtgenomes) are widely used in phylogenetic analyses of invertebrates, their application is limited in many marine annelid families, particularly in Capitellidae. In this study, we obtained complete mtgenomes (except control region) through high-throughput sequencing from eight species across five genera of Capitellidae: Barantolla sp., Capitella teleta, Mediomastus sp., Notodasus sp. A, Notodasus sp. B, Notodasus sp. C, and Notomastus sp. A and Notomastus sp. B. Our results indicate that species from genera with fewer capillary chaetae (Barantolla and Mediomastus) exhibit a relatively conserved mitochondrial gene order, while those from other genera show significant gene order rearrangements. Phylogenetic analysis based on the mtgenome data revealed three distinct clades within Capitellidae used in the present study: Clade 1 includes Mediomaastus and Barantolla; Clade 2 consists of Notodasus and Capitella; and Clade 3 comprises Notomastus. Notably, Clade 2 is sister to Clade 3, and both form the sister group to Clade 1. In contrast, a phylogenetic tree constructed from nuclear genes (18S, 28S, and H3) identified Capitella as an early branching clade within Capitellidae. Additionally, the Ka/Ks ratios of 13 mitochondrial protein-coding genes in Mediomaastus and Barantolla were much lower than those in Notodasus or Notomastus. All results together indicated different evolutional strategies in the Capitellidae family that correspond to their gene order rearrangements.