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Histology of the northern elephant seal (Mirounga angustirostris) eye

INTRODUCTION: Northern elephant seals (NES) are the deep- est diving pinnipeds, reaching depths of 1600 m (Delong et al., 1991: Mar. Mammal. Sci. 7, 369–384), zone of absolute darkness. The purpose of this study was to histologically assess their eyes, determine ganglion cell distribution and to describe adaptations to vision in darkness. METHODS: Tissue samples were collected from the NES and fixed in 10% neutral buffered formalin. Eyes were pro- cessed using standardized histological techniques. Intact eyes were used for preparation of retinal wholemounts. Ganglion cell densities were estimated with well-estab- lished stereological techniques, using the optical fraction- ator module of Stereoinvestigator 10. RESULTS: NES have relatively large, nearly spherical eyes. The iris has well developed pupillary constrictor and dila- tor muscles ; however, the ciliary muscle is poorly devel- oped. The well- developed, cellular tapetum lucidum can be up to 50 cells thick at the eye’s fundus. The pigmented epithelium is almost completely devoid of pigments. The photoreceptor layer is predominantly composed of rods. Often, the photoreceptor and outer nuclear layers are folded. The periphery of retina is sparsely populated with ganglion cells, averaging less than 100 cells per mm2 ; whereas, in the dorsolateral and ventromedial quadrants there are two distinct dense zones of up to 450 cells per mm2. CONCLUSIONS: The vascular tunic in NES carries well-devel- oped dilator and constrictor pupillary muscles, as described in other phocids (Jamieson et al., 1972: In: Funct. Anat. Mar. Mamm. Edited by Harrison RJ: Aca- demic Press, London, 2, 245–261 ; Mass et al., 2007: Anat. Rec. 290, 701–715, Welsch et al., 2001: J. Morphol. 248, 165–174). The thick cellular tapetum lucidum extends throughout the whole vitreous surface of the eyeball, as observed in Weddell seal (Welsch et al., 2001: J. Morphol. 248, 165–174). The retina is characterized by extremely low density of ganglion cells. Other seal species report significantly higher numbers of ganglion cells (Welsch et al., 2001: J. Morphol. 248, 165–174 ; Mass., 2009: Dokl. Biol. Sci. 429, 575–578). Retinal folding was often observed. This folding was also described in Sperm whale (Mann., 1946: Biologica. 4, 23–71), Harbor seal (Jamieson et al., 1971: Can. J. Zool. 49, 19– 23) and Megachiroptean bats (Fejer et al., 2001: Act. Biol. Hung. 52, 17–27). Care- ful anatomical and histological evaluation of the NES eye revealed a set of traits consistent with scotopic vision adapted eye.

Histology ; Anatomy ; Veterinary ; Northern Elephant ; Seals ; Deep Divers ; Retina ; Eye ; Pinniped ; Neuron

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