Abstract from a Presentation to the Society of Marine Mammalogy in 2002
Morphology of the Bearded Seal (Erignathus barbatus) Muscular-Vibrissal Complex
Christopher D. Marshall, University of Washington, Box 357446, Seattle, WA 98195, USA
Kit Kovacs, Norwegian Polar Institute, 9296 Tromsø, Norway
Christian Lydersen, Norwegian Polar Institute, 9296 Tromsø, Norway
Bearded seals possess an elaborate array of mystacial vibrissae that are assumed to be involved in the identification and manipulation of benthic prey. However, little information is available regarding the anatomy of the vibrissae, facial myology or the innervation of these structures – so there is little evidence available to support or dispute this hypothesis. This study reports on the facial myology, gross innervation, and vibrissal histology of the bearded seal. Specimens were obtained from legal hunts in Barrow, Alaska and Norway. The muzzle is comprised primarily of three enlarged muscular layers, the levator nasolabialis (superficial), maxillonasolabialis (intermediate), and lateralis nasi (deep). These muscles possess bony origins but soft tissue insertions that presumably give the muzzle increased mobility. A complex anastomosing network of CN VII innervates these facial muscles. The disproportionately large infraorbital nerve of CN V courses toward the muzzle and divides into numerous branches that penetrate the external capsule of every vibrissa. Each vibrissal follicle spans the depth of the dermis. Each follicle possesses a blood filled ring sinus, located between the dense external capsule and the inner wall, that encircles the hair shaft. A trabecular cavernous sinus is located apically and basally to the ring sinus. The inner wall is comprised of a ringwulst, mesenchymal sheath, glassy membrane, and outer root sheath. The inner wall joins the external capsule apically to form a pore through which the hair shaft emerges. Nerve fibers penetrating the external capsule course through the basal trabeculae to terminate, presumably as mechanoreceptors, on the inner wall adjacent to the ring sinus. Thus, the anatomical evidence suggests that the vibrissae of bearded seals are highly developed tactile organs that can be mobilized and serve to help locate, identify, and access benthic prey.