Abstract
The visual systems of aquatic animals are able to adapt to the ambient light environment (Lythgoe 1972, Bowmaker 1994, Yokoyama and Tada 2000). In fact, many studies have focused on the spectral adaptation between different fish species (Hunt et al 1996, 2001; Hope et al 1997). However, little work has been done on evolutionary adaptation between separated populations. In studying this, our objective is to illuminate the constraints under which the evolutionary tuning of visual pigments works (tempo, available molecular routes, signal/noise trade-offs). We measured the rod absorbance spectra (lmax) of four sand goby populations (Baltic, Swedish, English, and Adriatic) by microspectrophotometry (MSP). After MSP, the rod opsins of individual fishes were sequenced to reveal potential amino acid substitutions underlying the spectral differences. We detected differences in lmax values between populations (Jokela et al. 2003). The greatest lmax shift was between the Baltic (508.3 nm) and the Adriatic (503.0 nm) populations. The variation of the Baltic lmax values was significant (506–511 nm), but it correlated well with the opsin sequence substitutions. Fishes with 506–507 nm had “normal” F261 while redshifted fishes (510–511 nm) had F261F/Y. Baltic common gobies (515 nm) have also this substitution and it is known to redshift spectra (Merbs and Nathans 1993). Evidently, the selection pressure in the Baltic Sea is unambiguous since the Baltic sand goby population is polymorphous. This is also the first time that spectral and sequence differences of individual animals other than the Primates are correlated.