The ectoparasitic seal louse, Echinophthirius horridus, relies on a sealed tracheal system and spiracle closing apparatus for underwater respiration.

Marine mammals host a diverse array of parasites engaged in a continuous evolutionary arms race. However, our understanding of the biology of parasitic insects associated with marine mammals, particularly their adaptations to challenging marine environments, remains limited. The seal louse, Echinophthirius horridus, which infests true seals, is one of thirteen insect species capable of enduring prolonged dives in open seas. This ectoparasite has evolved several adaptations to withstand extreme conditions, such as low oxygen levels (hypoxia), temperature fluctuations, hydrostatic pressure, and strong drag forces during dives. To prevent drowning during their host's 20-35 min dives, seal lice have developed specialized respiratory mechanisms that allow them to survive in oxygen-poor waters and at depths up to 600 m. Advanced imaging techniques, including CLSM, SEM, synchrotron X-ray microtomography, and histological sectioning and 3D-reconstruction, have revealed a specialized spiracle closing apparatus for storing oxygen in their tracheal system. Furthermore, our buoyancy experiments showed that the lice consume oxygen under water and, with morphological data, provide what is to our knowledge the first direct evidence against plastron presence. These findings enhance our understanding of the physical adaptations of lice and their survival in extreme ecological conditions, contributing to broader ecological and evolutionary theories.