Retinal ganglion cell topography in juvenile Pacific bluefin tuna Thunnus orientalis (Temminck and Schlegel)

A lambda-shaped retractor lentis muscle in the yellowfin goby Acanthogobius flavimanus

We identified a morphologically uncommon piscine retractor lentis muscle in the yellowfin goby Acanthogobius flavimanus. This lentis muscle has a shape similar to the Greek small letter lambda (λ). The two legs of the muscle are attached to the retinal periphery at the ventral eyecup, while the tip is connected to the lens surface by a ligament. Scanning electron microscopy showed that the fibers of the lentis muscle run along the length of both the anterior and posterior legs. Immunolabeling with antiacetylated tubulin antibody and neuronal tracing with DiI of the whole lentis muscle revealed that the anterior leg is innervated by one or more nerves. The topographic distribution of ganglion cells in the retina was investigated to identify the visual axis. Three high cell density areas were observed in the dorso-temporal, ventro-nasals and ventro-temporal retina. These findings suggest that the λ-shaped lentis muscle may enable accommodatory movement of the lens toward the temporal as well as the nasal and/or ventral retina.

Vision in two cyprinid fish: implications for collective behavior

Many species of fish rely on their visual systems to interact with conspecifics and these interactions can lead to collective behavior. Individual-based models have been used to predict collective interactions; however, these models generally make simplistic assumptions about the sensory systems that are applied without proper empirical testing to different species. This could limit our ability to predict (and test empirically) collective behavior in species with very different sensory requirements. In this study, we characterized components of the visual system in two species of cyprinid fish known to engage in visually dependent collective interactions (zebrafish Danio rerio and golden shiner Notemigonus crysoleucas) and derived quantitative predictions about the positioning of individuals within schools. We found that both species had relatively narrow binocular and blind fields and wide visual coverage. However, golden shiners had more visual coverage in the vertical plane (binocular field extending behind the head) and higher visual acuity than zebrafish. The centers of acute vision (areae) of both species projected in the fronto-dorsal region of the visual field, but those of the zebrafish projected more dorsally than those of the golden shiner. Based on this visual sensory information, we predicted that: (a) predator detection time could be increased by >1,000% in zebrafish and >100% in golden shiners with an increase in nearest neighbor distance, (b) zebrafish schools would have a higher roughness value (surface area/volume ratio) than those of golden shiners, (c) and that nearest neighbor distance would vary from 8 to 20 cm to visually resolve conspecific striping patterns in both species. Overall, considering between-species differences in the sensory system of species exhibiting collective behavior could change the predictions about the positioning of individuals in the group as well as the shape of the school, which can have implications for group cohesion. We suggest that more effort should be invested in assessing the role of the sensory system in shaping local interactions driving collective behavior.

Morphological characteristics of eyes and retinas of two sardines (Sardinops melanostictus and Etrumeus sadina, Clupeidae) and an anchovy (Engraulis japonicus, Engraulididae)

The morphology of the eyes and distribution of retinal ganglion cells in two sardine species (Sardinops melanostictus and Etrumeus sadina, Clupeidae) and the Japanese anchovy (Engraulis japonicus, Engraulididae) were investigated anatomically and histologically. The eyes of the sardines faced a slightly dorsolateral direction with the visual field extended obliquely upward. In contrast, the eyes in the anchovy were almost laterally directed. It was hypothesized that the sardines may have an advantage in receiving more downward irradiance compared with the anchovy. The lens muscle was larger in these three species than in many other teleosts, and its surface was entirely melanin-pigmented. Also, the lens muscle directly and tightly adhered to the backside surface of the iris. The relative area of the lens muscle to the area of the lens, a referential value of the relative power of visual accommodation were notably larger in the species studied than in other teleost values that have been previously reported. A higher M/L% value of these clupeid fishes could facilitate fast and wide ranging visual accommodation and was considered to be associated with maintaining and/or re-establishing school formations quickly. Analysis of topographical distributions of cells in the ganglion cell layer showed that cell density was highest in the ventrotemporal quadrant of the retina (temporal of the optic cleft) in all three species. Another potentially important role for the black-pigmented lens muscle may be to block the specialized retinal area from intense sunlight that scatters and irradiates upward or laterally in the surface waters that they inhabit. Thus, the sardine and anchovy may take advantage of efficient detection of visual signals in the frontal-upward direction and further improve visibility of the target in this direction.