Structure and spectral sensitivity of photoreceptors of two anchovy species: Engraulis japonicus and Engraulis encrasicolus.

The morphology, fine structure and spectral sensitivity of retinal photoreceptors of two anchovy species were investigated using light and electron microscopy and microspectrophotometry. Distinct regional specialisation of cones was observed. Long and short (bilobed) cones were observed in the horizontal retinal belt, including the nasal and temporal retinal zones. Only triple cones with two long lateral components, one small central component were observed in the dorsal and ventro-nasal retinal regions. The long cones presented various lamellar organisation patterns: (1) in parallel along the cell axis in the central retina, (2) oriented transversely at the base of the outer segment, and (3) tilted longitudinally while extending to the tip of the cone in the retinal periphery. In the short cones, the lamellae were always oriented along the cell axis, and their planes were perpendicular to the lamellae in the long cones, providing a structural basis for the detection of polarisation of incident light. The lamellae in all the outer segments of the triple cones are arranged perpendicular to the long cell axis. In both species, the long and short cones from the ventro-temporal retina were slender and more densely packed, and the outer segments of the long cones lay far more sclerad compared with the outer segments of the bifid cones. Microspectrophotometry revealed that in both species the lateral components of the triple cones displayed a maximum absorbance wavelength (λ(max)) of approximately 502nm, while the short central components were more shortwave sensitive (λ(max)=475nm). The λ(max) of all long and short cones in the ventro-temporal zone was 492nm, compared to 502nm in other retinal regions. Anchovies are unique among vertebrates in that they contain clear structural basis for both colour and polarisation vision in the same retina.