PHOTORECEPTOR SPECTRAL ABSORBENCY IN LARVAL AND ADULT WINTER FLOUNDER(PSEUDOPLEURONECTES-AMERICANUS)

The habitat occupied by larval winter flounder (Pseudopleuronectes ame ricanus) differs considerably in light regime from that of the adult. To understand how the visual system has adapted to such changes, photo receptor spectral absorbance was measured microspectrophotometrically in premetamorphic and postmetamorphic specimens of winter flounder. Be fore metamorphosis, larval flounder retinas contain only one kind of p hotoreceptor which is morphologically cone-like with peak absorbance a t 519 nm. After metamorphosis, the adult retina has three types of pho toreceptors: single cones, double cones, and rods. The visual pigment in single cones has a peak absorbance at lambda(max) = 457 nm, the dou ble cones at lambda(max) = 531 and 547 nm, and the rod photoreceptors at lambda(max) = 506 nm. Double cones were morphologically identical, but the two members contained either different (531/547 nm) or identic al pigments (531/531 nm). The latter type were found only in the dorsa l retina. The measured spectral half-bandwidths (HBW) were typical of visual pigments with chromophores derived from vitamin A1 with the pos sible exception of the long-wavelength absorbing pigment in double con es which appeared slightly broader. Because the premetamorphic pigment absorbance has a different lambda(max) than those of the postmetamorp hic pigments, different opsin genes must be expressed before and after metamorphosis.