Morphological characteristics of visual organs in the killer whale Orcinus orca

The eyeball and Harderian gland of the killer whale Orcinus orca were investigated at macroscopic and microscopic levels. The eyeball was covered with multiple ocular muscles, and consist of three layers including the fibrous tunic of the eye (cornea and sclera), vascular tunic of the eye (ciliary body, iris and choroid), and internal tunic of the eye (retina). The rods in the outer nuclear layer of the retina were present in much larger numbers than cones, and the structure of the retina was similar to that of nocturnal animals. A comparison of the size of the eyeball of the killer whale with other cetacean species in previous works revealed a similar morphology with the delphinid species. The Harderian gland encircled the eyeball similar to a belt form on mainly the corneal side, and the multiple secretory ducts of the gland opened into the conjunctival fornix and palpebral conjunctiva. Histochemically, the secretions were mainly glycosaminoglycans and contained no lipids. These results indicated that the eyes of the killer whale have as efficient light-gathering and eye protection functions as known in other cetacean species.