Spectral characteristics of visual pigments in two mullet species (Pisces, Mugilidae)

The spectral properties of photoreceptors in two closely related fish species of the family Mugilidae, differing in their preferred salinity habitat, were subjected to a comparative study using microspectrophotometry and liquid chromatography. The results have shown that both species are tetrachromats and possess two classes of single cones and two classes of double cones, with paired elements containing visual pigments with the same absorption maximum (λ_max). In the flathead grey mullet (Mugil cephalus), a predominantly marine species, the rods and the single (two classes) and double cones (two classes) contain rhodopsins with λ_max = 506, 464, 518, 560 and 574 nm, respectively. In the so-iuy mullet (Planiliza haematocheilus), with a freshwater life-history stage, the photoreceptors contain a mixture of rhodopsin and porphyropsin in different proportions. In different P. haematocheilus individuals, values of λ_max of the rods and the single and double cones vary in the following ranges: 501-522, 462-470, 518-539, 573 and 570-620 nm, respectively. The set of visual pigments indicates the identity of opsins in both species, whereas the long-wave shift in the spectral sensitivity in P. haematocheilus is due to a changed composition of the pigment mixture with a greater proportion of porphyropsin that utilizes the same opsins as in M. cephalus. The conclusion about the consistency of our results with the previously found correlation between the supposed connections of horizontal retinal cells and individual chromatic classes of photoreceptors in mullets is made based on literature data. The present study, in combination with studies on the molecular genetics structure of photopigments, can contribute to further development of views on the evolution of visual pigments.