engleski

Special senses of the northern elephant seal (Mirounga angustirostris)

Introduction: Northern elephant seals (NES) are one of the deepest diving marine mammals, reaching the extreme depths of the aphotic sea zone. Their special senses (vison, vibrissae and hearing) had to undergo special fine-tuned morphological adaptations in order for them to able to orientate and forage through this extreme environment. The aim of this study was to describe those morphological adaptations. Materials and Methods: The specimens were collected from deceased animals and processed by using standard histological techniques. For stereological counting we used Stereo Investigator 11. Results: NES have comparatively large eyes with completely spherical lens. The iris has prominent constrictor and dilator muscles, whereas the ciliary muscle is underdeveloped. The exceptionally thick tapetum lucidum coats the entire posterior aspect of the globe. Photoreceptor layer is heavily dominated by rods. Retinal ganglion cells are arranged in a single layer and they are reaching their maximum densities (~1, 300 cells per mm2) dorsal to the optic disc. NES have mystacial, supraorbital, rhinal and labial vibrissae innervated by the deep vibrissal nerve. Average number of axons per vibrissa was: 1793 (130) mystacial, 1102 (165) rhinal, 1018 (197) supraorbital and 391(52) labial. NES are lacking the pinna and the external acoustic meatus is fully collapsed. Mucosa of the middle ear is lined by ciliated pseudostratified columnar epithelium and its stroma contains massive cavernous tissue, which fully envelopes the stapes. The middle ear ossicles are exceptionally large in size. Cochlear nerve is averaging 60, 364  55 axons, which are large in diameter. Conclusion: All above mentioned ocular adaptations are consistent with the pronounced scotopic vision, which is an adaptation to dim light conditions. Vibrotactile vibrissal sense is exceptionally well developed in NES, which enables them to follow hydrodynamic trail of fleeing fish. Assessment of the NES auditory capabilities places them right in between cetacean and terrestrial mammals. All husbandry and experimental procedures were approved by the Institutional Animal Care and Use Committee at the Western University of Health Sciences (R09/ IACUC/027).

mirounga angustirostris ; northern elephant seal ; special sensens

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