SOME ASPECTS OF PHOTORECEPTION AND VISION IN FISHES
177
chiefly Sqwclua and Mustelus, observed no difference in the disposition
of chorioidal pigment of dark-adapted and light-adapted dogfish.
Obviously, there is some doubt about the occlusible properties of
the elasmobranch tapetum, and its presence in some species still awaits
confirmation. After long neglect, interest in this structure recently haa
been renewed. It has been found that in some benthic selachians from
the neritic zone, the tapetum is fixed, i.e. non-occlusible ; examples are
Scyliorhinua and Raia. The eyes of these animals always exhibit a
bright fundus, and a black ventral field below the optic stalk. In the
latter region reflecting material is absent and there ie s layer of blwk
chorioidal pigment outside the retina. Pupillary movement ie rapid and
extensive. In some pelagic sharks, on the other hand, the whole
interior of the dark-adapted eye shines brightly, but soon beoomes very
dark when exposed to light, owing to migration of black pigment which
obscures the tapetum lucidum. Examples are Mustelus and Squalus
(Nicol, 1961a, c). In these animals pupillary movement is slight or
absent. Gilbert (1961) observed that the tapetum of the sharp-toothed
shark Negaprion became occluded in less than an hour in bright light,
but required 4 hr or more to become uncovered (when the shiny surfam
is exposed).
The chorioidal tapetum of selachians contains guanin, to which it
owes its reflecting properties. Guanin was discovered in the ohorioid of
Sqwzlus, Mugtelua and Scyliorhinua by Kuhne and Sewall (1880), who
found that extracts gave a positive reaction to a purine test, involving
the successive use of dilute nitric acid followed by sodium hydroxide.
Extracts treated with sulphurous acid gave rise to crystals of gypsum,
and it has been suggested that the guanin is compounded with calcium
in vivo. The presence of guanin in the chorioid of Squalus acunthiae
was confirmed by Pirie and Simpson (1946), who movered guanin
hydrochloride from extracts. Also, a green fluorescent substance is
present, which appeara to be a pterin, xanthopferin.
Many intriguing problems, presented by the tapetum luoidum of
elasmobranchs, still await clarification and resolution. Among them
are the functional significance of the tapeturn in terms of visual sensitivity and acuity ; the mode of regulating the movement of chorioidel
pigment to and fro over the reflecting plates ; the significance of the
orientation of the plates in diverse regions of the eye ; and the efficienqg
of the system.
It is generally believed that elssmobranchs find food ohidy by
smell, and it haa been uncertain to what extent they make use of vieion.
Benthic species (5cyliorhinus, R a h , Rhinu) hunt by soent, generally
without making use of their eyes, but simple observation shows that
177
chiefly Sqwclua and Mustelus, observed no difference in the disposition
of chorioidal pigment of dark-adapted and light-adapted dogfish.
Obviously, there is some doubt about the occlusible properties of
the elasmobranch tapetum, and its presence in some species still awaits
confirmation. After long neglect, interest in this structure recently haa
been renewed. It has been found that in some benthic selachians from
the neritic zone, the tapetum is fixed, i.e. non-occlusible ; examples are
Scyliorhinua and Raia. The eyes of these animals always exhibit a
bright fundus, and a black ventral field below the optic stalk. In the
latter region reflecting material is absent and there ie s layer of blwk
chorioidal pigment outside the retina. Pupillary movement ie rapid and
extensive. In some pelagic sharks, on the other hand, the whole
interior of the dark-adapted eye shines brightly, but soon beoomes very
dark when exposed to light, owing to migration of black pigment which
obscures the tapetum lucidum. Examples are Mustelus and Squalus
(Nicol, 1961a, c). In these animals pupillary movement is slight or
absent. Gilbert (1961) observed that the tapetum of the sharp-toothed
shark Negaprion became occluded in less than an hour in bright light,
but required 4 hr or more to become uncovered (when the shiny surfam
is exposed).
The chorioidal tapetum of selachians contains guanin, to which it
owes its reflecting properties. Guanin was discovered in the ohorioid of
Sqwzlus, Mugtelua and Scyliorhinua by Kuhne and Sewall (1880), who
found that extracts gave a positive reaction to a purine test, involving
the successive use of dilute nitric acid followed by sodium hydroxide.
Extracts treated with sulphurous acid gave rise to crystals of gypsum,
and it has been suggested that the guanin is compounded with calcium
in vivo. The presence of guanin in the chorioid of Squalus acunthiae
was confirmed by Pirie and Simpson (1946), who movered guanin
hydrochloride from extracts. Also, a green fluorescent substance is
present, which appeara to be a pterin, xanthopferin.
Many intriguing problems, presented by the tapetum luoidum of
elasmobranchs, still await clarification and resolution. Among them
are the functional significance of the tapeturn in terms of visual sensitivity and acuity ; the mode of regulating the movement of chorioidel
pigment to and fro over the reflecting plates ; the significance of the
orientation of the plates in diverse regions of the eye ; and the efficienqg
of the system.
It is generally believed that elssmobranchs find food ohidy by
smell, and it haa been uncertain to what extent they make use of vieion.
Benthic species (5cyliorhinus, R a h , Rhinu) hunt by soent, generally
without making use of their eyes, but simple observation shows that