Evolution is the selective process by which a species survives in a changing environment. Convergent evolution is the tendency of distinct animals to evolve similar structural or physiological characteristics under similar environmental conditions. River dolphins live in a very different environment from that of other cetaceans, inhabiting dark and cloudy rivers, rather than the clear water of the ocean surface. One of the most striking examples of convergent evolution and environmental adaptation is seen in the ocular retrogression of various members of family Platanistoidea.
Lipotes vexillifer, the Yangtze River Dolphin, while having the least amount of eye regression of the fresh water species, possesses some of the most fascinating optical adaptations to its environment. The water of the Yangtze is not nearly so dark as that of the Ganges or Indus Rivers, yet still shows marked turbidity. In response to these conditions, Lipotes exhibits the remarkable adaptation of having its vision permanently directed forwards and upwards. This positioning has presumably come about because objects deep in the water are too dark to see, so Lipotes has no need to look down. At the same time, objects near the surface receive more light and can be more easily distinguished. Upon dissection it is observed that the trochlear nerve, which functions to lower the eyeball, is absent, and the abducens, which innervates the muscles that center the eyeball (prevents crossing of the eyes), is greatly reduced. The position of the eye in the head is also unusual. Compared to other dolphins, the eyes are rostral and lie high up in the head. The eyes are still visible when viewed from above the animal, allowing light to enter from in front and above. Evidence for this unique eye positioning being a secondary environmental adaptation is found in the examination of a Lipotes fetus. Here the eye is much more lateral and lower than in the adult; the bulb migrates during development (Zhou, Pilleri, and Li, 1979).
Pontoporia Blainvillei is a Platanistoid living in the La Plata River and off the Atlantic coast of Uruguay. Here the water is shallow and relatively clear so that light is able to penetrate fairly far beyond the water surface. Thus having good vision is environmentally useful to this species, and indeed they exhibit better eyesight and a more complete eye anatomy than do many of the other river dolphins. The eyeball is a slightly flattened globe which measures 16mm in horizontal diameter. This is the largest bulb size of the species discussed here. The ratio of eyeball diameter to cornea diameter is 1:1.6, and the lens is well-developed and slightly flattened. The lens is supported by a long ciliary process and ciliary muscles. The presence of the lens with its strong supportive bodies suggests that this animal not only takes in light but also has some ability to focus and distinguish shapes. Moreover, the retina is of a type seen in nocturnal animals, with rods and cones present (Pilleri, 1977). The eye slit in a Pontoporia fetus ranges from two to four times the length of the eye slit in a Platanista fetus of the same size. Also, the position of the eye slit in the skull is more dorsal in the La Plata River Dolphin than in the Indian river dolphins in which it is more ventral (Pilleri and Gihr, 1976). Though Pontoporia has good vision relative to other river dolphins, there is still some anatomical evidence for optic retrogression. For instance, the trochlear nerve is lacking in this species. Also, in her dissertation on Pontoporia, Pinedo (1991) observed that the length of the lacrimal bone and the supraorbital width varied greatly among individuals, probably indicating a loss of eye functionality.
The Ganges and Indus Rivers are extremely dark and cloudy, with minimal light penetration. Accordingly, the Ganges River Dolphin (Platanista gangetica) and the Indus River Dolphin (Platanista indi), display the largest amount of ocular degeneration of all the river dolphins. Virtually blind, the Indian river dolphins exhibit extreme microophthalmia with a pear shaped eyeball only 5mm in horizontal diameter. The ratio of eyeball diameter to cornea diameter is 1:1, with the periphery of the retina receiving more light than in any other species. Because there is so little light penetration through the water, there is little need to focus light, and accordingly, these dolphins have no lens at all. That this is a secondary environmental adaptation is supported by the fact that young animals exhibit “scanty remains of the capsular epithelium as evidence of a rudimentary lens” (Pilleri, 1977). The extreme microophthalmia of this species coincides with a decrease in optic muscles and nerves, and an increase in the retrobulbar fat body. The eye is sunken into a large fatty body, the retina has a reduced number of neurons, and the optic nerve is very thin, capable only of perceiving light and the direction from which it comes (Pilleri and Pilleri, 1987). Also the retina is nocturnal in nature with very few receptors. Since there is no lens to support, the ciliary processes and muscles are almost non-existent. As with the Yangtze River Dolphin, there is no need to look down through the dark water, and thus the eyeball is completely immobile.
As we can see, there is a strong correlation between water turbidity and eye regression in river dolphins. In the Yangtze and La Plata Rivers, where there is still fairly good light penetration, Lipotes and Pontoporia retain comparatively good vision while still exhibiting signs of ocular atrophy. In the Indus and Ganges Rivers, however, Platanista indi and gangetica live in virtual darkness, and, having little use for their eyes, they are well on their way to blindness.
Pilleri, G. and Gihr, M. (1976). On the Embryology of the La Plata Dolphin, Pontoporia blainvillei. Investigations on Cetacea ed. G. Pilleri, 7: 65-89.
Pilleri, G. and Pilleri, O. (1987). Indus and Ganges River Dolphins, Platanista indi and Platanista gangetica. Investigations on Cetacea ed. G. Pilleri, 20: 2-35.
Pinedo, Maria Cristina (1991). Development and Variation of the Franciscana, Pontoporia Blainvillei. University of California at Santa Cruz 406pp.
Zhou, Kaiya, Pilleri, G., and Li, Yuemin (1979). Observations on the Baiji (Lipotes vexillifer) and the Finless Porpoise (Neophocaena asiaeorientalis) in the Changjiang (Yangtze) River between Nanjing and Taiyangzhou, with Remarks on Some Physiological Adaptations of the Baiji to its Environment. Investigations on Cetacea ed. G. Pilleri, 10: 109-120.