Barbara I. Evans

Correlation between Histological and Behavioral Measures of Visual Acuity in a Zooplanktivorous Fish, the White Crappie (Pomoxis annularis)

Estimates of visual acuity in a pelagic freshwater zooplanktivorous fish, the white crappie (Pomoxis annularis, Centrarchidae), were made using a behavioral measure, the maximum observed prey pursuit distance (MxPD), and a histological measure, the density of cone cells in the retina. The greatest number of pursuits occurs in the 0-30 degrees wedge of the visual field; 87% of all pursuits occur in the first 40 degrees. The longest pursuits (200 mm) also occur in this area and generally get shorter from 0 to 180 degrees (from forward-directed) in the visual field. Consistent with the behavioral results, the largest number of cone photoreceptors (13,000/mm2) is found in the far temporal retina along the eyes horizontal meridian. Cone cell densities in the corresponding region of the nasal retina are approximately half this value. The number of cones decreases dorsally and ventrally from the horizontal meridian. Although the absolute values of visual acuity calculated from cone cell topography (i.e. MxPDs of 500 mm) are 2-3 times greater than those observed behaviorally (i.e. MxPDs of 200 mm), the trends in visual acuity across the visual field obtained from both measures are consistent. We suggest that overestimates of visual acuity obtained from cone cell counts alone result from this measures not accounting for, among other properties of the nervous system, cone cell convergence onto ganglion cells and higher brain centers. Behavioral measures of visual acuity are, therefore, likely to yield a more accurate estimate of an animals visual abilities.

Retinal transformation at metamorphosis in the winter flounder (Pseudopleuronectes americanus).

Winter flounder (Pseudopleuronectes americanus) are hatched as bilaterally symmetric larvae which live near the ocean surface. At metamorphosis, they become laterally compressed, one eye migrates to the opposite side of the head, and they live the remainder of their lives lying on their blind side on the ocean floor. The present study characterizes and quantifies retinal cell distribution throughout the larval period and contrasts it with the adult retina. Based on light- and electron-microscopic analyses, retinas of larval flounder contain only a single cone-like photoreceptor type, arranged in a hexagonal array. In contrast, after metamorphosis, the adult retina has three types of photoreceptors: rods, single cones, and double cones. Rod photoreceptors are numerous in the ventral retina and decrease in density dorsad. The cone photoreceptor density, in contrast to rods, is higher in the dorsal retina decreasing ventrad. Adult cone photoreceptors are arranged in a square mosaic with four double cones surrounding one single cone. The differences in larval and adult retinal morphology reflect the distinctly different habitat each occupies.

Photoreceptor spectral absorbance in larval and adult winter flounder (Pseudopleuronectes americanus).

The habitat occupied by larval winter flounder (Pseudopleuronectes americanus) differs considerably in light regime from that of the adult. To understand how the visual system has adapted to such changes, photoreceptor spectral absorbance was measured microspectrophotometrically in premetamorphic and postmetamorphic specimens of winter flounder. Before metamorphosis, larval flounder retinas contain only one kind of photoreceptor which is morphologically cone-like with peak absorbance at 519 nm. After metamorphosis, the adult retina has three types of photoreceptors: single cones, double cones, and rods. The visual pigment in single cones has a peak absorbance at lambda max = 457 nm, the double 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 identical pigments (531/531 nm). The latter type were found only in the dorsal retina. The measured spectral half-bandwidths (HBW) were typical of visual pigments with chromophores derived from vitamin A1 with the possible exception of the long-wavelength absorbing pigment in double cones which appeared slightly broader. Because the premetamorphic pigment absorbance has a different lambda max than those of the postmetamorphic pigments, different opsin genes must be expressed before and after metamorphosis.

THE EMBRYOGENESIS OF ROD PHOTORECEPTORS IN THE TELEOST FISH RETINA, HAPLOCHROMIS BURTONI

Development of the retina, like that of other tissues, occurs via an orderly sequence of cell division and differentiation, producing the functional retina. In teleost fish, however, cell division and differentiation in the retina continue throughout the life of the animal in two distinct ways. Stem cells in a circumferential germinal zone at the periphery of the retina give rise to all retinal cell types and progenitor cells located throughout the retina in the outer nuclear layer (ONL) produce new rod photoreceptors. These processes in adult retina recapitulate in space the embryonic events responsible for forming the retina. Analysis of these events in an African cichlid fish, Haplochromis burtoni, confirmed that cone photoreceptors differentiate first, followed by rod photoreceptors. Correspondingly, at the margin of the eye, cone photoreceptors differentiate nearer to the margin than do rods. Control of photoreceptor production is not understood. Here we present the time of appearance and distribution pattern of GABA and vimentin which are candidates for the control of retinal cell division and differentiation. Antibody staining reveals that both GABA and vimentin exhibit unique patterns of expression during embryonic retinal development. Vimentin immunoreactivity is evident throughout the retina in a spoke-like pattern between developmental Days 4 and 7, as both cone and rod photoreceptors are being formed. GABA is expressed in horizontal cells between Days 5 and 7, corresponding to the onset of rod differentiation in time and in position within the retina. Moreover, the wave of GABAergic staining in the horizontal cells parallels the wave of rod differentiation across the embryonic retina of H. burtoni. Thus, GABA may play a role in the development of rod photoreceptors.

Remodeling of the Cone Photoreceptor Mosaic during Metamorphosis of Flounder (Pseudopleuronectes americanus)

The retinal cone mosaic of the winter flounder, Pseudopleuronectes americanus, is extensively remodeled during metamorphosis when its visual system shifts from monochromatic to trichromatic. Here we describe the reorganization and re-specification of existing cone subtypes in which larval cones alter their spatial arrangement, morphology, and opsin expression to determine whether mechanisms controlling cell birth, mosaic position, and opsin selection are coordinated or independent. We labeled dividing cells with tritiated (3H) thymidine prior to mosaic remodeling to determine whether existing cone photoreceptors change phenotype. We also used in situ hybridization to identify mosaic type and opsin expression in transitional retinas to understand the sequence of transformation. Our data indicate that in the winter flounder retina the choice of new opsin species and the cellular rearrangement of the mosaic proceed independently. The production of the precise cone mosaic arrangement is not due to a stereotyped series of sequential cellular inductions, but rather might be the product of a set of distinct, flexible processes that rely on plasticity in cell phenotype.

A Fish’s Eye View of Habitat Change

In aquatic ecosystems, visually mediated predator-prey interactions are highly dependent on the environmental light regime; however, the wavelengths of ambient light and thresholds of light intensity vary as a function of water depth and dissolved organic matter. Light is also scattered by water molecules creating polarized light and by silts and clays creating turbid conditions. As a result of these changes in the visual environment, the visual systems of fishes have developed many adaptations, and are finely tuned to the spectrum and intensity of light in the microhabitat.

Asymmetric Hybridization of Pink (Oncorhynchus gorbuscha) and Chinook (O. tshawytscha) Salmon in the St. Marys River, Michigan

ABSTRACT Natural hybridization between pink salmon (Oncorhynchus gorbuscha) and Chinook salmon (O. tshawytscha) has been observed only in the Great Lakes, where both species have been introduced. The direction of hybridization between these species is poorly understood, thus the present study analyzed mitochondrial DNA (mtDNA) to determine whether the maternal parents were pink or Chinook salmon. During annual salmon population surveys on the St. Marys River (1998–2002), fifty putative hybrids of pink salmon and Chinook salmon were identified from meristic, morphometric and color characters. We designed primers to amplify a fragment of the mitochondrial D-loop control region that included both a variable and a control restriction site (BstNI). Polymerase chain reaction, restriction endonuclease digestion and capillary electrophoresis of mtDNA were used in order to identify the maternal parent of each hybrid salmon. In addition, the amplified fragments from the three fish species were sequenced to further verify the results. All hybrid salmon specimens were found to possess Chinook salmon mtDNA, indicating that hybridization between Chinook salmon and pink salmon is asymmetrical and likely unidirectional, occurring only between male pink salmon and female Chinook salmon. Influences contributing to the hybridization of these salmonid species could include limited spatial and temporal segregation of spawners, sexual selection, a limiting number of Chinook males, and/or physiological factors.