1. VISUAL PIGMENTS
5
pure-rod retinae of several genera of deep-sea fishes: Platytroctes and
Bathytroctes (Brauer, 1908; the latter may be Sear&, according to Munk,
1966), Bathylugus (Vilter, 1954a,b; Munk, 1966) , Scopelosaurus and
Searsia ( Marshall, 1966), and Platytroctegen ( Munk, 1966). Because
several families are represented, it seems probable that pure-rod foveae
have evolved more than once. As in shallow-water teleosts, a prominent
aphakic space is correlated with these temporal foveae (Marshall, 1966).
One other pure-rod fovea has been reported in the rhynchocephalian,
Sphenodon (Walls, 1942), but Vilter (1951) has shown that its visual
cells are cones. Therefore, the pure-rod foveae of deep-sea fishes are
unique.
B. Image Formation and Accommodation
In all groups of fishes the corneal index of refraction is about the
same as that of water (1.33) and the ocular humors. Refraction and image
formation, therefore, depend almost entirely upon the lens. The lens is
spherical, with a very high effective index of refraction (about 1.67).
Since the maximum index for any transparent material of biological origin
is about 1.53 (Pumphrey, 1961), the fish lens cannot be homogeneous.
That its refractive index is highest at the center (1.53) and gradually
decreases (to 1.33) toward the outside was confirmed by Pumphrey
( 1961), who showed that the lens has no spherical aberration. This permits unaberrated image formation without stopping down the lens
(f/0.8 in teleosts). Stopping down would be disadvantageous because
the lack of corneal refraction means that the lens must protrude through
the iris to achieve a wide visual field. This same gradation of refractive
index from 1.53 to 1.33 occurs in the small lenses of young fish and large
lenses of older fish. The lens substance shows concentric discontinuities,
suggestive of growth increments; but the way in which the index of refraction of the inner and outer parts could be altered continuously and
differentially during growth is not understood. The lens lacks any appreciable chromatic aberration, but the means by which this is achieved is
also unknown (Pumphrey, 1961). An almost constant feature of fish eyes
is the distance from the center of the lens to the retina divided by the
radius of the lens. This ratio is about 2.55 ( Matthiessen’s ratio).
Accommodation results from changing the distance between the lens
and the retina rather than altering lenticular shape. Lampreys have a
unique corneal muscle that inserts on the spectacle covering the eye.
When it contracts, the cornea is flattened, pushing the lens closer to the
5
pure-rod retinae of several genera of deep-sea fishes: Platytroctes and
Bathytroctes (Brauer, 1908; the latter may be Sear&, according to Munk,
1966), Bathylugus (Vilter, 1954a,b; Munk, 1966) , Scopelosaurus and
Searsia ( Marshall, 1966), and Platytroctegen ( Munk, 1966). Because
several families are represented, it seems probable that pure-rod foveae
have evolved more than once. As in shallow-water teleosts, a prominent
aphakic space is correlated with these temporal foveae (Marshall, 1966).
One other pure-rod fovea has been reported in the rhynchocephalian,
Sphenodon (Walls, 1942), but Vilter (1951) has shown that its visual
cells are cones. Therefore, the pure-rod foveae of deep-sea fishes are
unique.
B. Image Formation and Accommodation
In all groups of fishes the corneal index of refraction is about the
same as that of water (1.33) and the ocular humors. Refraction and image
formation, therefore, depend almost entirely upon the lens. The lens is
spherical, with a very high effective index of refraction (about 1.67).
Since the maximum index for any transparent material of biological origin
is about 1.53 (Pumphrey, 1961), the fish lens cannot be homogeneous.
That its refractive index is highest at the center (1.53) and gradually
decreases (to 1.33) toward the outside was confirmed by Pumphrey
( 1961), who showed that the lens has no spherical aberration. This permits unaberrated image formation without stopping down the lens
(f/0.8 in teleosts). Stopping down would be disadvantageous because
the lack of corneal refraction means that the lens must protrude through
the iris to achieve a wide visual field. This same gradation of refractive
index from 1.53 to 1.33 occurs in the small lenses of young fish and large
lenses of older fish. The lens substance shows concentric discontinuities,
suggestive of growth increments; but the way in which the index of refraction of the inner and outer parts could be altered continuously and
differentially during growth is not understood. The lens lacks any appreciable chromatic aberration, but the means by which this is achieved is
also unknown (Pumphrey, 1961). An almost constant feature of fish eyes
is the distance from the center of the lens to the retina divided by the
radius of the lens. This ratio is about 2.55 ( Matthiessen’s ratio).
Accommodation results from changing the distance between the lens
and the retina rather than altering lenticular shape. Lampreys have a
unique corneal muscle that inserts on the spectacle covering the eye.
When it contracts, the cornea is flattened, pushing the lens closer to the