Cetaceans can be divided into two large groups—the mysticetes and the odontocetes. The mysticetes are the baleen whales—such as the Blue whale, Humpback, Bowhead whales, California Gray whale, Southern right whale, and so on. The odontocetes include the Sperm whale, Orca (inappropriately named as the killer whale) and most dolphins. As we understand more about behavior of these creatures, sensory adaptations become more important and interesting. Aquatic vision presents significant problems that terrestrial animals do not face. The water to cornea interface removes the potential for refraction by the cornea thus requiring a bulkier lens for focusing, and making accommodation more challenging. Water also restricts light by absorption and reflection. Vision is not possible over long distance because of turbulence and clouding. Hence, while vision is the principal sensory modality for most terrestrial animals, that is not necessarily so for all lineages of aquatic ones. The marine mammals present a lineage that must live in two worlds to some extent, at least. Certainly, they must live in an aquatic environment, but also must face an air-cornea interface when they are out of the water. Depending on the species, this happens to varying degrees. Still, vision plays some role in the behavior of most marine mammals. While these eyes resemble those of other vertebrates, there are key differences and at least one mysterious anatomical finding.
Odontocetes are perhaps deeper and more consistent diving animals, but most of the mysticetes dive to considerable depths as well. Staggeringly deep when compared to most fish, and vision is likely to be of little use at these depths. In many cases, the great whales will dive to depths that are beyond the quit point which is the maximum penetration of light, and in the absolute clearest of waters is about 1000 meters.. Generally, light penetrates to no more than about 200-300 meters because of clouding. Vision is probably of limited use beyond the quit point unless the target is bioluminescent, and the eyes must very large to collect the low light level emitted by this mechanism. Even at the shallower depths of 200-300 meters, vision is probably only a secondary sense compared to olfaction or audition. Nevertheless, vision must play an important role, at least in the shallower depths where light still penetrates because the eye remains a robust organ in these lineages. Understanding the anatomy and physiology of these eyes will help us understand the role vision plays in these amazing giants.
All vertebrate eyes, including those of humans, have an outer tough coat called the sclera. In the human eye and most other vertebrate eyes, this outer tunic is white, and protects the vital contents within the eye. Those contents include the choroid and the retina. The choroid is the vascular supply to the interior of the eye, and the retina is the neurologic machinery that, in combination with the brain, converts light rays into vision. It is the scleral anatomy that poses an unanswered question and mystery.
The eyes of the great whales are large although not disproportionately so. If one compares the eye size to body size, the eyes would seem to be relatively small, although absolutely large, when compared to many terrestrial species. Yet, these eyes are robust and contain the necessary visual elements to create a retinal image.
Sclera extends from the cornea to surround the eye 360º, similar to the rind of an orange. The anterior sclera immediately adjacent to the cornea, and the cornea are an expected thickness for such an eye, but the posterior sclera is thickened several times beyond what it should be for such an eye, and some whales have enormously thick posterior sclera. All great whales have thickened sclera to differing degrees.
Why should this be?
Previous observers and anatomists have attributed this thickened sclera as protection for the intraocular contents from the pressure of deep diving. But, this doesn’t seem to be true for the following reasons. The anterior sclera that would be bathed in sea water is of normal thickness for such an eye. It is only the posterior sclera or back of the eye that is thickened, and the eye can produce fluid internally to match the pressure of the water above it. The eye of a human diver will do this to equalize pressure—otherwise humans could not dive so deeply without distorting the eye, at least temporarily.
The Great whales with thickened sclera all vocalize. The vocalizations have been documented to be (at least) 178 decibels at 10 meters away from the head where it is generated. The decibel level at the site of posterior sclera must be very high—perhaps over, if not well over 200 decibels.
This noise level could well damage sensitive cells such as photoreceptors or create detached retinas or do other intraocular damage.
If the whale is going to communicate with other whales over great distances or use the songs or other vocalizations to find prey or fool predators, then the sound must be intense at the site of initiation. But, such sound levels may well be traumatic and harmful to the eyes, and perhaps other internal structures.
Other deep diving marine mammals, such as elephant seals, do not have such thickened sclera. And, these seals do not vocalize under water although they do when on land. Seals and other deep diving marine mammals that dive deeply do not have thickened sclera if they don’t vocalize. The elephant seal and other seals do not vocalize under water, and have normal thickness sclera for such eyes, But, the southern right whale is a coastal whale for much of its life and does not dive so deeply yet has very very thick sclera. The same whale has loud and often subsonic vocalizations. In a beautiful paper by Buono et al(1) these investigators carefully review the morphology of this whale’s eye over many specimens and illustrate the markedly thickened sclera. All whale eyes are not so carefully analyzed, though.
The vocalizations, then, are likely associated, if not directly responsible for the evolution of, the thickened sclera. This is just another example of ocular evolutionary development to fit a particular niche.
(1) Buono MR, Fernandez MS, and Herrera Y: The Morphology of the eye of the Southern Right Whale (Eubalaena australis)