Intraretinal variability and specialization of cones in Japanese anchovy (Engraulis japonicus, Engraulidae).

The retina of anchovies is characterized by an unusual arrangement and ultrastructure of cones. In the retina of Japanese anchovies, Engraulis japonicus, three types of cones are distributed into rows. The nasal, central, temporal, and ventro-temporal regions of the retina were occupied exclusively by the long and short cones. Triple cones, made up of two lateral components and one smaller central component, were found only in the dorsal and ventro-nasal retinal regions. In the outer segments of all short and long cones from the ventro-temporal region, the lamellae were oriented along the cell axis and were perpendicular to the lamellae in the long cones, providing a morphological basis for the detection of polarization. This lamellar orientation is unique to all vertebrates. The cones were examined with respect to regional differentiation in their size and spectral properties via light microscopy, transmission electron microscopy, and microspectrophotometry. Various dimensions of cones were measured in preparations of isolated cells. The cones from the ventro-temporal region had different dimensions than cones of the same type located in other retinal regions. Triple cones from the dorsal region were significantly larger than triple cones from the ventro-nasal region. The spectral absorbance of the lateral components of triple cones in the ventro-nasal retina was identical to the absorbance of all long and short cones from the ventro-temporal region. These are shifted to shorter wavelengths relative to the absorbance of the lateral components of the triple cones located in the dorsal retina. Thus, the retina of the Japanese anchovy shows some features of regional specialization common in other fishes that improves spatial resolution for the upwards and forwards visual axis and provides spectral tuning in downwelling light environment. That results from the differentiation of cone types by size and by different spectral sensitivity of various retinal areas.

The retinomotor reaction of the retina of young dog salmon Oncorhynchus keta on adaptation to light and the field of a permanent magnet.

Studies of Pacific Ocean dog salmon fry of the species Oncorhynchus keta addressed two retinomotor reactions: after transfer of fish from the dark to the light and after placing fish in the field of a permanent magnet (magnetic induction, 80 G) in complete darkness. The reactions of photoreceptors and the pigment epithelium (PE) to the magnetic field were analogous to those of the retinal reaction on adaptation to light. The morphological characteristics of photoreceptors and retinal PE cells lead to the view that the effect of the field of a permanent magnet on the retina of young dog salmon is comparable to that of light.

Morphological changes in the retina in Pacific ocean salmon Oncorhynchus masou fry in response to neutralization of the geomagnetic field in conditions of normal illumination.

The studies reported here provide the first demonstration that retinal responses in both the fry of the migratory salmon trout Oncorhynchus masou and the dwarf form of this species changed in conditions of experimental neutralization of the geomagnetic field (GMF); migratory salmon trout fry and dwarves showed different changes. The responses of different types of retinal photoreceptor in migratory salmon trout fry to neutralization of the GMF differed: while rods and double cones perceived neutralization of the GMF as the onset of darkness (the scotopic reaction), single (generally blue-sensitive) cones responded to neutralization of the GMF both as presentation of blue light or (very rarely) ultraviolet irradiation. The retina of dwarf male salmon trout responded to neutralization of the GMF with a double response: rods showed a light (photopic) response, while double (red/green-sensitive) cones produced dark (scotopic) responses. Single (blue-sensitive) cones responded to neutralization of the GMF as bright blue light. Thus, the morphological picture of the retina in dwarf male salmon trout in these experimental conditions corresponds to the perception of blue light. The initial conditions were different--normal diffuse daylight with a brightness of about 7.5 Lx. It is likely that neutralization of the magnetic field had no effect on rods, while double, red-green, cones responded as to darkness, i.e., the fish did not perceive red or green light in the visible spectrum, but perceived only blue and, possibly, ultraviolet light by means of central blue-sensitive and accessory cones. Thus, these experiments demonstrated that in conditions of normal daylight illumination, retinal photoreceptors in salmon fry respond to changes in the earth's magnetic field, i.e., objectively function as magnetoreceptors.

Morphological characteristics of the retinomotor response in salmon trout (oncorhynchus masou) fry in a magnetic field and red light.

The retinomotor response was studied in fry of the salmon trout Oncorhynchus masou during experimental exposures to a constant magnetic field and red light alone and together. The responses of photoreceptors and the pigmented epithelium to red light were mesopic in nature. The mesopic state of the retina after exposure of fish to a magnetic field in the dark differed from the pigment epithelium response after exposure to red light. On exposure to the magnetic field after red light, the effects of these two treatments were additive. Rods adapted to low-light conditions, while cones adapted to light. The simultaneous operation of these two mechanisms of perception is thus possible, although it never occurs in normal conditions. On exposure to red light after the magnetic field, the retinomotor response reflected a physiological dysfunction in which neither rods nor cones were operative. The pigmented epithelium is actively involved in responses to changes in the magnetic field. It is suggested that cells of the pigmented epithelium can function as light-sensitive magnetoreceptors.