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

Abstract

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.

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

“…Small pieces were excised from certain regions of the same retina and subjected to morphological and microspectrophotometric investigations. The locations from which the material was resected corresponded to the retinal map of the Japanese anchovy (Miyazaki and Kobayashi, ) that is similar to retinal maps of other species of the same genus Engraulis (Heß et al, ; Koch and Heß, ; Novales Flamarique, ).…”

“…That agrees with the calculated cone ratios for the ventro‐temporal and central retina in northern and European anchovies (1: 3.5 and 1: 4, respectively) (O'Connell, ; Heß, ; Koch and Heß, ). The detectable distance of a 1 mm diameter prey estimated using the formula provided by Miyazaki and Kobayashi () is approximately 12–25 cm for the Japanese anchovy. Hence, this species may use both particle feeding in which fish catch large‐sized zooplankton as well as filter feeding on phytoplankton and smaller zooplankton as is typical for several anchovy species (Konchina, ; Louw et al, Tanaka et al, ).…”

“…As a result, a single cone layer, common for other retinal regions, is transformed into two layers of cones. Ventro‐temporal retina includes the region of the area temporalis in which the cone density and thickness of the retina are increased (Koch et al, ; Miyazaki and Kobayashi, ). A simple calculation shows that the distance between long axes of adjacent long and short cones is reduced 1.85 times versus the distance measured in other retinal regions (4.7 µm vs. 8.7 µm, dimension 5, see Table ).…”

Retinal ganglion cell topography and spatial resolution in the smelt Hypomesus japonicus (Brevoort, 1856)

“…Hypomesus japonicus and H. nipponensis are similar in the GC topography to a large number of fishes, in particular, pelagic zooplankton feeders (Koch et al, 2010;Koch & Hess, 2011;Miyazaki & Kobayashi, 2015). In et al, 2021).…”