Comparative analysis of locomotory organs and tracheal system in Halarachne halichoeri: implications for marine parasite habits

Abstract The nasal mite Halarachne halichoeri thrives in a unique niche: the respiratory tracts of deep-diving seals, where it faces high hydrostatic pressures, hypoxia, and osmotic stress. Using synchrotron X-ray microtomography, 3D reconstruction, scanning electron microscopy, and confocal laser scanning microscopy, we reveal the mite’s adaptations to these extreme conditions. Its specialized attachment system, comprising two claws and a dynamic pad (arolium), enables secure anchorage to the soft mucosal surfaces. When anchoring to rough surfaces, the arolium actively folds inward with the help of a sclerite-tendon mechanism, allowing the claws to firmly embed into and interlock with the substrate. In contrast, on smooth surfaces, leg and claw angles are actively adjusted by tendons to retract the claws proximally, enabling the unfolded and extended arolium to contact and adhere to the substrate. The mite’s leg musculature is strongly developed, with powerful flexors and depressors ensuring stable attachment, while protractors and levators facilitate movement. The respiratory system features a highly reduced tracheal volume (only 0.04% of body volume) suggesting a predominant reliance on cuticular respiration. Thickened, taenidia-reinforced tracheae provide mechanical stability against collapse during the intense pressure changes encountered during host dives. These findings highlight the exceptional morphological and physiological strategies enabling H. halichoeri to survive in extreme environments.