The photoactivation energy of the visual pigment in two spectrally different populations of Mysis relicta (Crustacea, Mysida).

We report the first study of the relation between the wavelength of maximum absorbance (lambdamax) and the photoactivation energy (Ea) in invertebrate visual pigments. Two populations of the opossum shrimp Mysis relicta were compared. The two have been separated for 9,000 years and have adapted to different spectral environments ("Sea" and "Lake") with porphyropsins peaking at lambdamax=529 nm and 554 nm, respectively. The estimation of Ea was based on measurement of temperature effects on the spectral sensitivity of the eye. In accordance with theory (Stiles in Transactions of the optical convention of the worshipful company of spectacle makers. Spectacle Makers' Co., London, 1948), relative sensitivity to long wavelengths increased with rising temperature. The estimates calculated from this effect are Ea,529=47.8+/-1.8 kcal/mol and Ea,554=41.5+/-0.7 kcal/mol (different at P<0.01). Thus the red-shift of lambdamax in the "Lake" population, correlating with the long-wavelength dominated light environment, is achieved by changes in the opsin that decrease the energy gap between the ground state and the first excited state of the chromophore. We propose that this will carry a cost in terms of increased thermal noise, and that evolutionary adaptation of the visual pigment to the light environment is directed towards maximizing the signal-to-noise ratio rather than the quantum catch.