1. VISUAL PIGMENTS
13
authors have speculated on their functional significance (see review by
Marshall, 1954). Certain of these fragile animals can be caught at night
at the ocean surface, but their vision has scarcely been studied. Anatomical investigations, however, have continued ( Munk, 1959, 1963, 1964a,b,
1965a,b, 1966; Pearcy et al., 1965).
Many ocular features of deep-sea fishes must increase visual sensitivity. In deep-sea elasmobranchs and teleosts the retina typically contains only rods, which may be extremely numerous and have very long
outer segments. The one well-established exception is Omosudis, which
has an almost pure-cone retina (Munk, 1965b). Some teleosts have their
rod outer segments arranged in several distinct layers (Vilter, 1954a;
Munk, 1963, 1966; Pearcy et al., 1965). Presumably this arrangement
increases sensitivity, perhaps without decreasing resolving power; other
suggestions were also discussed by Munk (1963, 1966). Black pigment
is often absent from the back of the eye, and a tapetum may be well
developed in elasmobranchs (Denton and Nicol, 1964). Weale ( 1955)
pointed out that binocular vision may approximately double the monocular sensitivity, the increase being as much or more than that from a
tapetum. Binocular vision is well developed in a number of teleost groups,
most extravagantly in those with tubular eyes (Brauer, 1908; Walls, 1942;
Munk, 1966). The eye is elongate and the lens very large; the iris is
absent and the mechanisms of accommodation are rudimentary or absent
(Fig. 5A). The main retina is at the bottom of the tube (Matthiessen’s
ratio still holds true, however); in effect, the tubular eye is equivalent to
the axial part of a much larger conventionally shaped eye. Lightgathering power is increased at the expense of narrowing the visual field.
This is usually compensated to some extent by an accessory retina located
along the inner wall of the eye where it touches the lens. Presumably,
it detects light and motion in a larger visual field. Tubular eyes are often
directed upward (e.g., Argyropelecus and Opisthoproctus), but in some
species are turned forward (Giganturu and Winteria) . This orientation
is probably related to the animal’s method of prey-capture or other behavior ( Clarke, 1963).
Degeneration of the eyes in fishes from great depths has been examined by Munk (1964a, 1965a). The most interesting of these forms
is Zpnops. This benthic animal has a spatulate snout, with transparent
bony plates covering the orbital area. Beneath these bones are odd
flattened organs that have been described either as photophores (Walls,
1942) or as modified eyes. Although they lack cornea and lens, Munk
(1959) has shown unequivocally that they are eyes, with rod cells and
optic nerve.
Another very peculiar fish ( Bathylychnops) has eyes (Pearcy et al.,
13
authors have speculated on their functional significance (see review by
Marshall, 1954). Certain of these fragile animals can be caught at night
at the ocean surface, but their vision has scarcely been studied. Anatomical investigations, however, have continued ( Munk, 1959, 1963, 1964a,b,
1965a,b, 1966; Pearcy et al., 1965).
Many ocular features of deep-sea fishes must increase visual sensitivity. In deep-sea elasmobranchs and teleosts the retina typically contains only rods, which may be extremely numerous and have very long
outer segments. The one well-established exception is Omosudis, which
has an almost pure-cone retina (Munk, 1965b). Some teleosts have their
rod outer segments arranged in several distinct layers (Vilter, 1954a;
Munk, 1963, 1966; Pearcy et al., 1965). Presumably this arrangement
increases sensitivity, perhaps without decreasing resolving power; other
suggestions were also discussed by Munk (1963, 1966). Black pigment
is often absent from the back of the eye, and a tapetum may be well
developed in elasmobranchs (Denton and Nicol, 1964). Weale ( 1955)
pointed out that binocular vision may approximately double the monocular sensitivity, the increase being as much or more than that from a
tapetum. Binocular vision is well developed in a number of teleost groups,
most extravagantly in those with tubular eyes (Brauer, 1908; Walls, 1942;
Munk, 1966). The eye is elongate and the lens very large; the iris is
absent and the mechanisms of accommodation are rudimentary or absent
(Fig. 5A). The main retina is at the bottom of the tube (Matthiessen’s
ratio still holds true, however); in effect, the tubular eye is equivalent to
the axial part of a much larger conventionally shaped eye. Lightgathering power is increased at the expense of narrowing the visual field.
This is usually compensated to some extent by an accessory retina located
along the inner wall of the eye where it touches the lens. Presumably,
it detects light and motion in a larger visual field. Tubular eyes are often
directed upward (e.g., Argyropelecus and Opisthoproctus), but in some
species are turned forward (Giganturu and Winteria) . This orientation
is probably related to the animal’s method of prey-capture or other behavior ( Clarke, 1963).
Degeneration of the eyes in fishes from great depths has been examined by Munk (1964a, 1965a). The most interesting of these forms
is Zpnops. This benthic animal has a spatulate snout, with transparent
bony plates covering the orbital area. Beneath these bones are odd
flattened organs that have been described either as photophores (Walls,
1942) or as modified eyes. Although they lack cornea and lens, Munk
(1959) has shown unequivocally that they are eyes, with rod cells and
optic nerve.
Another very peculiar fish ( Bathylychnops) has eyes (Pearcy et al.,