In zebrafish, the first class of cone photoreceptor to become morpholo
gically distinct is the ultraviolet-sensitive short single cone, at 4
days postfertilization, whereas the last class, the red- and green-sen
sitive double cone, becomes distinct at 10 days postfertilization. We
have examined the time course of visual pigment gene expression in zeb
rafish using whole-mount in situ hybridization. Within the retina, ops
ins may be detected as early as 40 h postfertilization with the ultrav
iolet and rod visual pigments being expressed before the blue- (48 h)
and red- (60 h) sensitive pigments. In the pineal, red-sensitive opsin
is expressed at 48 h postfertilization.