How can the hooded seal dive to a depth of 1000 m without rupturing its tympanic membrane? A morphological and functional study.

Recent studies using a satellite-linked dive recorder have shown that the hooded seal (Cystophora cristata), a common Arctic pinniped, can dive to a depth of > 1000 m and stay submerged for close to 1 h. At these depths the water pressure reaches 100 atm, entailing obvious risk of serious damage to the hearing apparatus, mainly the tympanic membrane (TM) and middle ear (ME). We dissected and photodocumented the temporal bones of five newborn and three adult hooded seals in order to study the temporal bone structure and reveal its protective mechanisms for extreme pressure changes. Specimens were sectioned and stained for light microscopy. The thicknesses of the pars tensa and pars flaccida were found to average 60 and 180 microm, respectively. The ME cavity hosts a cavernous tissue of thin-walled vessels beneath the modified respiratory epithelium. The ME and external ear canal (EAC) volumes can be altered appreciably by filling/emptying the cavernous tissue with blood. The ossicles were fixed by contracting the tensor tympani and stapedius muscles simultaneously with complete occlusion of the EAC. According to Boyle's law, the volume of the gas-filled ME cavity at a depth of 1000 m is only 1% of its volume at the surface of the sea. Ascent from such a depth allows the gas in the ME cavity to expand, causing the TM to bulge laterally. This movement is counteracted by a reduction in the blood volume inside the cavernous sinuses, action in the tensor tympani and stapedius muscles and discharge of gas through the Eustachian tube. The presence of a firm, broad-based exostosis in the floor of the EAC lateral to the TM helps to obstruct the EAC.