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The Biology of Sea Turtles, Vol. II
of the photoreceptor cells. The outer nuclear layer houses the photoreceptor cell
nuclei and is generally only one cell wide. The outer plexiform layer is homogenous, but in Bartol and Musick’s preparations, the synaptic connections between
the nuclear layers could not be identified. The inner nuclear layer is composed of
the nuclei of bipolar, amacrine, and horizontal cells, although these cells were not
differentiated in this study. The inner plexiform layer is similar to the outer
plexiform layer and is composed of synaptic connections between the inner nuclear
layer and ganglion layer. Finally, the innermost layer, the ganglion cell layer, is
relatively thick (23% of the overall width of the retina) and is composed solely
of the ganglion cells and their axons (Bartol and Musick, 2001).
3.2.2 S ENSITIVITY TO C OLOR
3.2.2.1 Photopigments and Oil Droplets
The spectral sensitivity of sea turtles has been investigated using morphological,
electrophysiological, and behavioral methods. Liebman and Granda (1971) examined
the visual pigments associated with photoreceptor cells of the red-eared freshwater
turtle ( Pseudemys scripta elegans ) and green turtle ( C. mydas ). Microspectrophotometric measurements were performed on preparations of these cells to determine the
absorption spectra of these light-absorbing visual pigments. Both species have a
duplex retina containing both rod and cone photoreceptor cells. For the green turtle,
the rod photosensitive pigments absorbed light maximally at 500–505 nm. This retinal
pigment was indistinguishable from the rhodopsin identified in frog preparations.
Three photopigments were found associated with cone photoreceptors for C. mydas .
The most common pigment, identified as iodopsin, absorbed light maximally at 562
nm. The two other cone visual pigments identified absorbed light maximally at 440
and 502 nm (Figure 3.2). Note that one cone photoreceptor visual pigment was
identical to that of the rod visual pigment. The authors hypothesized that the cone
that absorbs at 502 nm is actually the accessory cone of a double cone pair. The
double cones of C. mydas have been found to have a principal receptor (full-sized
cone with oil droplet) and a secondary receptor (the non-oil droplet member) (Walls,
1942; Liebman and Granda, 1971). Liebman and Granda (1971) suggest that the
accessory cone actually contains the rhodopsin pigment of the rod photoreceptor. The
freshwater turtle ( P. scripta elegans ) examined in this study contained visual pigments
that absorb longer wavelengths than those found in C. mydas ; rods absorbed maximally at 518 nm and cones contained photopigments that absorbed 450, 518, and
620 nm maximally (Figure 3.2). The authors concluded that the light-absorbing visual
pigments in both the freshwater and marine turtle were suitable for the environments
in which the animals reside (seawater transmits shorter wavelengths than freshwater)
(Liebman and Granda, 1971; Granda, 1979).
3.2.2.2 Electrophysiology
The spectral sensitivity of C. mydas has also been investigated through the collection
of electroretinograms (ERGs) from dark-adapted eyes (Granda and O’Shea, 1972).
1123 book.book Page 82 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
of the photoreceptor cells. The outer nuclear layer houses the photoreceptor cell
nuclei and is generally only one cell wide. The outer plexiform layer is homogenous, but in Bartol and Musick’s preparations, the synaptic connections between
the nuclear layers could not be identified. The inner nuclear layer is composed of
the nuclei of bipolar, amacrine, and horizontal cells, although these cells were not
differentiated in this study. The inner plexiform layer is similar to the outer
plexiform layer and is composed of synaptic connections between the inner nuclear
layer and ganglion layer. Finally, the innermost layer, the ganglion cell layer, is
relatively thick (23% of the overall width of the retina) and is composed solely
of the ganglion cells and their axons (Bartol and Musick, 2001).
3.2.2 S ENSITIVITY TO C OLOR
3.2.2.1 Photopigments and Oil Droplets
The spectral sensitivity of sea turtles has been investigated using morphological,
electrophysiological, and behavioral methods. Liebman and Granda (1971) examined
the visual pigments associated with photoreceptor cells of the red-eared freshwater
turtle ( Pseudemys scripta elegans ) and green turtle ( C. mydas ). Microspectrophotometric measurements were performed on preparations of these cells to determine the
absorption spectra of these light-absorbing visual pigments. Both species have a
duplex retina containing both rod and cone photoreceptor cells. For the green turtle,
the rod photosensitive pigments absorbed light maximally at 500–505 nm. This retinal
pigment was indistinguishable from the rhodopsin identified in frog preparations.
Three photopigments were found associated with cone photoreceptors for C. mydas .
The most common pigment, identified as iodopsin, absorbed light maximally at 562
nm. The two other cone visual pigments identified absorbed light maximally at 440
and 502 nm (Figure 3.2). Note that one cone photoreceptor visual pigment was
identical to that of the rod visual pigment. The authors hypothesized that the cone
that absorbs at 502 nm is actually the accessory cone of a double cone pair. The
double cones of C. mydas have been found to have a principal receptor (full-sized
cone with oil droplet) and a secondary receptor (the non-oil droplet member) (Walls,
1942; Liebman and Granda, 1971). Liebman and Granda (1971) suggest that the
accessory cone actually contains the rhodopsin pigment of the rod photoreceptor. The
freshwater turtle ( P. scripta elegans ) examined in this study contained visual pigments
that absorb longer wavelengths than those found in C. mydas ; rods absorbed maximally at 518 nm and cones contained photopigments that absorbed 450, 518, and
620 nm maximally (Figure 3.2). The authors concluded that the light-absorbing visual
pigments in both the freshwater and marine turtle were suitable for the environments
in which the animals reside (seawater transmits shorter wavelengths than freshwater)
(Liebman and Granda, 1971; Granda, 1979).
3.2.2.2 Electrophysiology
The spectral sensitivity of C. mydas has also been investigated through the collection
of electroretinograms (ERGs) from dark-adapted eyes (Granda and O’Shea, 1972).
1123 book.book Page 82 Monday, November 11, 2002 11:11 AM