Pycnogonida, or sea spiders, is a speciose clade with ~1,300 described species that is especially diverse in the Southern Ocean. Evolutionary patterns within sea spiders remain unresolved between and within the ten putative, extant families due to due to shared morphological traits and highly reduced forms. Herein, we find that the order of mitochondrial protein coding genes is consistent between all sampled sea spiders, even though arrangement of tRNA genes, as well as placement and length of the control region, vary. The presence of large noncoding introns present in multiple families indicates that pycnogonid mtDNA have a fast evolutionary rate. A consensus phylogeny of recognized families is still debated, and previous work based on various methodologies has produced contrasting hypotheses of relationships. To better understand phylogenetic relationships between major clades of sea spiders, we constructed a dataset of whole mitochondrial genomes from representatives of all ten currently recognized families. Our results suggest a novel familial-level phylogeny within Pycnogonida, with a monophyletic clade containing Callipallenidae and Nymphonidae placed as the sister to all other recognized families.

EMBOSS palindrome (Rice et al. 2000) was used to search each intron for terminal repeats known to be associated with transposons.