VISUAL PIGMENTS AND THE PHOTIC ENVIRONMENT - THE COTTOID FISH OF LAKEBAIKAL

The endemic cottoid fish of Lake Baikal in Eastern Siberia offer a sin gular opportunity for examining within a number of closely related spe cies, the relationships of visual pigments, photoreceptor complements and depth within a deep freshwater environment. The lake, the deepest (1600 m) and one of the largest and most ancient in the world, is uniq ue in that the oxygen levels at the bottom are only reduced to about 8 0% of the surface levels. We have studied, by light microscopy, micros pectrophotometry and visual pigment extraction, the retinas from 17 sp ecies of Baikal cottoids that live at different depths within the lake . Generally the retinas contain, in addition to rods, large green-sens itive double cones and small blue-sensitive single cones: surprisingly for freshwater fish, the visual pigments are based on Vitamin A(1). T he lambda(max) of both rods and cones are displaced to shorter wavelen gths with increasing depth. Surface species have cones with lambda(max ) at about 546, 525 and 450 nm and rods at 523 nm, deeper living speci es retain cones, but with lambda(max) shifting towards 500 and 425 nm and with rods at 480 nm, whereas the deepest living fish possess only rods (lambda(max) 480-500 nm). These data clearly show a correlation b etween photoreceptor complement, visual pigment lambda(max) and depth, but question the hypothesis that there is a correlation of pigment la mbda(max) with water colour since, in contrast to oceanic waters, the maximum transmission of Baikal water is between 550 and 600 nm.