Vision

Image-forming vision is undoubtedly the most important of the cephalopod senses. In some sepiolids (bobtail squids) the eyes, together, can account for half of the

Figure 9.5

Diagram summarizing and illustrating the five classes of information that octopuses can extract from their visual world. (Reprinted from J. B. Messenger, The eyes and skin of Octopus. Endeavour 3: 92-98. Copyright 1979, with permission from Elsevier.)

Figure 9.5

Diagram summarizing and illustrating the five classes of information that octopuses can extract from their visual world. (Reprinted from J. B. Messenger, The eyes and skin of Octopus. Endeavour 3: 92-98. Copyright 1979, with permission from Elsevier.)

animal's weight, and in some squids the volume of the optic lobes can be more than four times that of the rest of the brain (Messenger, 1981). Learning and memory experiments have revealed that octopuses can make fine discriminations between pairs of objects differing in brightness, size, orientation, form, or plane of polarization (figure 9.5) (Messenger, 1991).

General Morphology of the Eye Superficially, cephalopod eyes are astoundingly similar to those of marine vertebrates (Pumphrey, 1961; Levine, 1980; Messenger, 1981; for detailed reviews, see Messenger, 1981, 1991; Budelmann et al., 1997). They are fluid filled, with well-defined retina, lens, and pupil. Their transparent lens crys-tallins are derived by recruiting detoxification stress proteins such as glutathione S-transferase (Tomarev et al., 1991). In cephalopods, the lens is suspended by ciliary muscles and interrupted by a connective that partitions the eye into anterior and posterior chambers. There is almost no spherical aberration in the lens, and its very short focal length conforms to the Matthiessen ratio of 2.5 times the lens radius, as it does in fishes. This allows a depth of focus from a few centimeters to infinity (Pumphrey, 1961; Muntz, 1977a; Sivak, 1991; Sivak et al., 1994).

Visual stimuli can be resolved at up to 100 Hz at high stimulus intensities, which is comparable to the retina of diurnal vertebrates (Hamasaki, 1968a), and the rate of dark adaptation is about the same as that of the nocturnal owl monkey (Hamasaki, 1968a). The rod-only eye of the cartilaginous (sharklike) fish provides the closest parallel to the cephalopod eye (Packard, 1972).

The octopus pupil (figure 9.1d) is a horizontal rectangular aperture in low light, and contracts to a horizontal slit in response to bright light (Muntz, 1977a). The movements of the pupil appear to be correlated with the degree of adaptation of the retina. In dim light, after exposure to a bright flash of light, the pupil first expands and then contracts as the retina regains its sensitivity and the screening pigment is retracted to expose more photoreceptive membrane (Muntz, 1977a; Gleadall et al., 1993).

The photoreceptor outer segments are typically much longer in cephalopod than in vertebrate eyes: some 200-400 mm in octopuses, and as long as 600 mm in the firefly squid. Combined with a high density of visual pigment, which produces a surprisingly broad spectral sensitivity (Hamasaki, 1968b), the cephalopod retina is a superb photon-capturing apparatus. This is particularly the case in the firefly squid.

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