Dichromatic Vision in West Indian Manatees

While most terrestrial mammals have dichromatic vision, most marine mammals have monochromatic vision due to mutations that have built up in their short wavelength sensitive (SWS) cone opsin. These mutations result in a pseudogene, which have nonfunctional SWS cone expression. However, due to the unique environment and feeding behavior of the manatee, as well as previous studies of the West Indian manatee eye that revealed the presences of rods and two cone types, Newman and Robinson (2006) attempted to determine if the manatee has dichromatic vision.

They sampled the retinal RNA from the manatee eye, then isolated and cloned it using PCR. Then both the SWS as well as the long wavelength sensitive (LWS) cone opsin were sequenced and inserted into a mammalian expression vector. They found that the manatees LWS gene was 93% identical to the African savanna elephant, and had a ,𝜆-𝑚𝑎𝑥. at 556nm. Compared to other marine mammals this is red-shifted. The SWS cones were found to be 89% identical to the African savanna elephant, and 88% identical to bovine SWS. The ,𝜆-𝑚𝑎𝑥. for the manatee was determined to be 411nm.

The results show that the manatee has two functional opsin, one for short wavelengths and the other for long, suggesting that they do have dichromatic vision. This is similar to most terrestrial diurnal mammals, but unique to all marine mammals studied thus far. The possible reasons given for their dichromatic vision included the fact that they live in shallow waters, unlike most marine mammals. Also, they subsist off of submerged grasses in the shallow water, which not only exposes them to a wide range of spectral light, but may also subject them to longer wavelengths of light due to dissolved particulate near shore. This could have lead to the LWS and rod pigments being red-shifted.

This relates directly to our lecture on light reception. We learned that rods and cones are photoreceptors imbedded in the eye. There is only one type of rod, and it is sensitive to low light. On the other hand, there are multiple types of cones, as in the manatee where there are two types, each being sensitive to a different wavelength. In the case of the manatee they have the long wavelength sensitive, and the short wavelength sensitive cones, and it is the cones that allow for color, or dichromatic, vision.

Newman, L.A., & Robinson P.R. (2006). The visual pigments of the West Indian manatee (Trichechus manatus). Vision Research, 46, 3326-3330.