Morphological Characterization and Complete Mitochondrial Genome of a New Epizoic Hexamerous Brittle Star Gymnolophus sexradia sp. Nov

The brittle star of genus Gymnolophus Brock, 1888 (Ophiuroidea: Ophiotrichidae Ljungman, 1867) is commonly found in epizoic relationships with crinoid hosts, such as Comaster schlegelii (Crinoidea: Comatulidae Fleming, 1828), benefiting from food and protection. Here we describe a new hexamerous epizoic species, Gymnolophus sexradia sp. nov., from the South China Sea. We provide a detailed description of the ossicle structure, based on micro‐CT scanning, and sequences of 1464 nuclear exons and the complete mitogenome of the new species. The specimen features a hexamerous body plan and six arms which may indicate fissiparity. The shape and articulation of the arm vertebrae, along with the lateral arm plates and overlapping split dorsal arm plates, enhance arm flexibility in the horizontal direction, facilitating its grasping behavior toward the crinoid host. Phylogenetic analysis of nuclear exons clusters the new species with Ophiomaza cacaotica (Lyman, 1871) and Gymnolophus obscura (Ljungman, 1867). The complete mitogenome consists of 15,669 bp and 37 genes with a conserved gene order identical to that of Ophiactis savignyi and other studied mitogenomes in Ophiotrichidae. Notably, 14 residues in four mitochondrial genes were identified as positively selected, which may reflect adaptations linked to the epizoic lifestyle with crinoid hosts.