An investigation of retinal specializations was carried out
in larval and juvenile dhufish, Glaucosoma hebraicum
(Glaucosomidae, Teleostei). The development of photoreceptors
and formation of the retinal mosaic was followed by light and
electron microscopy. At hatching the eye was undifferentiated.
Cone photoreceptors were present by day 3 posthatch (dph), when
exogenous feeding began. Single and multiple cones were present
in a row arrangement from 3 dph to 20 dph, when the first rod
nuclei were observed. Between 20 dph and approximately 3 months
posthatch (mph), the row arrangement was replaced by a square
mosaic of four double cones surrounding a single cone, and the
cones increased in size, with the outer segments reaching up
to 30 μm in length. During the period of spatial rearrangement,
triple cones were often observed. From their first appearance,
rod photoreceptors were added rapidly. Investigation of ganglion
cell topography in 3-mph fish that had attained the adult-like
square photoreceptor mosaic was carried out using retinal
wholemounts. The highest densities of neurones in the ganglion
cell layer were in temporal retina but no well-defined area
centralis was observed. Microspectrophotometric measurements
of the visual pigments within the outer segments of the
photoreceptors of 3-mph fish revealed double cones with identical
absorption spectra in each member of the outer segment, and
the wavelength of maximum absorption (λmax) located
at 522 nm. Single cones were found to possess a visual pigment
with λmax at 460 nm and rods with a
λmax of 498 nm. The results imply that the larvae
and juveniles are adapted for survival in coastal waters and
may be active in relatively low light levels from early stages
of development.