Host-parasite coevolution leads to underwater respiratory adaptations in extreme diving insects, seal lice (Lepidophthirus macrorhini).

Lice are obligate ectoparasites that have co-evolved with their hosts, particularly during the transition of mammals from terrestrial to amphibious habits, as sea lions, seals and walruses, and have undergone parallel adaptations to the extreme conditions of the deep sea. By combining morphological, physiological and genomic analyses, we are shedding light on a key process for surviving prolonged submersion: respiration. Under water, lice immobilise, close their spiracles, reduce their oxygen consumption to a minimum and breathe through their tegument. The presence of haemoglobin genes in their genome also strongly suggests the ability to store oxygen during host dives. Remarkably, seal lice have no anatomical features or physiological capabilities that distinguish them from other insects. This reinforces the idea that the absence of insects in the deep sea is not due to any inherent limitations in their form or function, but rather a result of their evolutionary pathways.