Le. Stenfors et al., How can the hooded seal dive to a depth of 1000 m without rupturing its tympanic membrane? A morphological and functional study, ACT OTO-LAR, 121(6), 2001, pp. 689-695
Recent studies using a satellite-linked dive recorder have shown that the h
ooded seal (Cystophora cristata), a common Arctic pinniped, can dive to a d
epth of > 1000 m and stay submerged for close to 1 h. At these depths the w
ater 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 we
re sectioned and stained for light microscopy. The thicknesses of the pars
tensa and pars flaccida were found to average 60 and 180 mum, respectively.
The ME cavity hosts a cavernous tissue of thin-walled vessels beneath the
modified respiratory epithelium. The ME and external ear canal (EAC) volume
s can be altered appreciably by filling/emptying the cavernous tissue with
blood. The ossicles were fixed by contracting the tensor tympani and staped
ius 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 i
s 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 lateral
ly. 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, b
road-based exostosis in the floor of the EAC lateral to the TM helps to obs
truct the EAC.