Adaptations of seal louse nits to underwater life: morphology, respiration and attachment.

Marine mammals host a variety of parasites, engaged in continuous evolutionary competition. Our understanding of parasitic insects associated with these mammals, specifically their physiological adaptations to the marine environment, remains limited. Among thirteen insect species capable of enduring extended underwater periods in open oceans is the seal louse, Echinophthirius horridus, which infests true seals. This species relies on adaptations to survive extreme conditions such as hypoxia, temperature variations, hydrostatic pressure, and drag forces during host dives. These conditions affect both adult lice and louse embryos within their nits. Questions arise about how these early stages survive underwater, respire inside the eggs during dives, and what limits their survival. Using CLSM, SEM, and synchrotron X-ray microtomography, we identified morphological structures consistent with a potentially specialized respiratory system in the nits, a multi-layered eggshell and structural features that may help limit water entry. Adhesive force tests showed that the nit sheath attaching eggs to seal hair is not the primary vulnerability during host dives; rather, prolonged submersion likely imposes oxygen limitation over time. This research enhances our understanding of ectoparasite-host dynamics in marine mammals and demonstrates remarkable adaptability of insects to challenging aquatic niches.