Anyone who has gone scuba diving knows the difficulties of underwater vision. Visibility can be affected by the turbidity of the water, and the deeper you get the less light penetrates and you slowly lose colours, and then light altogether. Depth perception also changes due to the refraction of light, and objects may closer or further away than they are. Objects may also be blurry due to the scattering of light. Without a mask the salt water and particles irritate your eyes, and at depth the pressure becomes uncomfortable, not to mention a painfully frigid water temperature. The eyes of cetaceans (whales and dolphins) are adapted to all these problems to aid in vision underwater.
Whales and dolphins also need to come to the surface to breathe however, so at the same time their eyes are adapted to be able to see in air, and deal with rapid changes in brightness as animals dive from well-lit surface waters to low light water depths. As cetaceans need to see in a wide range of light levels, iris muscles contract creating species-specific pupil shapes which regulate the amount of light entering the pupil. Unlike humans where a contracted iris in bright light results in a small circular pupil, in some cetaceans this results in a U-shaped slit, while in low light conditions the pupil is large and round or slightly oval. The retina of the eye contains both rods and cones, but is rod dominated to aid vision in low light conditions.
Cetaceans have a highly developed choroid (vascular network) to prevent cooling in the eye due to low underwater temperatures. The thickened sclera (supporting wall of the eye, the white part of the eye) and cornea (transparent front of the eye) also help insulate against low underwater temperatures and mechanical damage caused by high pressure associated with diving deep underwater. There are a large number of particulates suspended in the sea, and while whales and dolphins do not have eye lashes to keep particulates out of their eyes as humans do, they have a Harderian gland which continuously bathes the eye with an oily protein mucous which flushes any irritants which may have settled on the eye (whales do not blink nearly as much as humans do).
Due to the refractive index of light under water the cornea takes very little part in underwater light refraction. The lens of a cetacean eye is almost circular, and is almost solely responsible for light refraction and focusing of an image on the retina. Most cetaceans, however, do not have very high visual acuity; they do not need high resolving power.
Written by: Minke Tolsma
For further reading:
- Griebel, U. & Peichl, L. 2003. Color vision in aquatic mammals – facts and open questions. Aquatic Mammals 29: 18-30.
- Mass, A.M. & Supin, A.Y. 2007. Adaptive features of aquatic mammal’s eye. The Anatomical Record 290: 701–715.
- Mobley, J.R. & Helweg, D.A. 1990 Visual Ecology and Cognition in Cetaceans. In: Thomas, J.A. & Kastelein, R.A. (eds) Sensory Abilities of Cetaceans. NATO ASI Series (Series A: Life Sciences), vol 196. Springer, Boston, MA.