Sensory Biology of Sea Turtles
85
Both summation and regional density of photoreceptor cells have been examined in both hatchling and juvenile sea turtles (Oliver et al., 2000; Bartol and
Musick, 2001). Oliver et al. (2000) examined the ganglion cell densities of three
species of sea turtle hatchlings: greens ( C. mydas ), loggerheads ( C. caretta ), and
leatherbacks ( D. coriacea ). From plots of contour maps of ganglion cells, visual
streaks were found for all three species; however, the streaks varied in shape.
Caretta mydas was found to have a narrow and long streak, with a much higher
cell concentration within the streak as opposed to areas outside the streak. Of the
three turtles, C. mydas had the most characteristically horizontal streak. Caretta
caretta had a wider streak dorsoventrally, with lower density counts than the green
sea turtle. The retina of D. coriacea contained a distinct rounded area temporalis
(a site of high cell counts) as well as a horizontal streak. Cell counts were the
highest for the retina within this area temporalis. The authors attribute the differences among species to the environment that these hatchlings occupy. For example,
as hatchlings, C. mydas may be found in clear water, feeding during the day as
omnivores beneath the flat ocean surface, whereas C. caretta is typically found
within sargassum mats, feeding in an environment with a less defined horizon.
This behavior of feeding beneath a defined, flat surface helps explain why green
sea turtles have a stronger horizontal streak than other sea turtles. Dermochelys
coriacea hatchlings feed on gelatinous prey in the open ocean, an environment
where an area temporalis would be more advantageous than a horizontal streak
(Oliver et al., 2000).
Bartol and Musick (2001) examined the vertical organization of the main
features of the retina as well as the spatial variation of the photoreceptor cells of
large juvenile loggerhead sea turtles ( C. caretta ). On the basis of the properties
of the neural layers, the vertical organization of the retina indicated a low degree
of summation. In animals with a low summation level, the inner nuclear layer
(composed of bipolar cells, horizontal cells, and amacrine cells) and the ganglion
layer are thick relative to the rest of the retina, indicating a high number of neurons
corresponding to each photoreceptor cell (Walls, 1942). In juvenile loggerheads,
these two layers (out of the seven overall layers) comprised approximately 37%
of the total retina (Bartol and Musick, 2001; see Figure 3.1). Bartol and Musick
(2001) also examined the topography of the retina by plotting the counts of cone
and rod photoreceptor cells and ganglion cells (Figure 3.3). Both cone photoreceptors and ganglion cells progressed from high to low density in a stair-step
fashion from the back to the front of the eye. Rod photoreceptors, however, were
more likely to maintain a constant density throughout the back half of the eye,
rapidly decreasing in number near the cornea. Dorsal–ventral differences were
also observed when the cell counts were plotted on a three-dimensional sphere.
A horizontal streak of ganglion cells and cone photoreceptor cells in the dorsal
hemisphere of the eye indicated a region of decreased summation and thus
increased acuity. Rods, however, were found in lower numbers and ubiquitously
throughout the two hemispheres, resulting in a constant sensitivity to low light
situations. This regionalization of cells was hypothesized to aid the juvenile
loggerhead in finding benthic slow-moving prey in their shallow water habitat
(Bartol and Musick, 2001).
1123 book.book Page 85 Monday, November 11, 2002 11:11 AM
85
Both summation and regional density of photoreceptor cells have been examined in both hatchling and juvenile sea turtles (Oliver et al., 2000; Bartol and
Musick, 2001). Oliver et al. (2000) examined the ganglion cell densities of three
species of sea turtle hatchlings: greens ( C. mydas ), loggerheads ( C. caretta ), and
leatherbacks ( D. coriacea ). From plots of contour maps of ganglion cells, visual
streaks were found for all three species; however, the streaks varied in shape.
Caretta mydas was found to have a narrow and long streak, with a much higher
cell concentration within the streak as opposed to areas outside the streak. Of the
three turtles, C. mydas had the most characteristically horizontal streak. Caretta
caretta had a wider streak dorsoventrally, with lower density counts than the green
sea turtle. The retina of D. coriacea contained a distinct rounded area temporalis
(a site of high cell counts) as well as a horizontal streak. Cell counts were the
highest for the retina within this area temporalis. The authors attribute the differences among species to the environment that these hatchlings occupy. For example,
as hatchlings, C. mydas may be found in clear water, feeding during the day as
omnivores beneath the flat ocean surface, whereas C. caretta is typically found
within sargassum mats, feeding in an environment with a less defined horizon.
This behavior of feeding beneath a defined, flat surface helps explain why green
sea turtles have a stronger horizontal streak than other sea turtles. Dermochelys
coriacea hatchlings feed on gelatinous prey in the open ocean, an environment
where an area temporalis would be more advantageous than a horizontal streak
(Oliver et al., 2000).
Bartol and Musick (2001) examined the vertical organization of the main
features of the retina as well as the spatial variation of the photoreceptor cells of
large juvenile loggerhead sea turtles ( C. caretta ). On the basis of the properties
of the neural layers, the vertical organization of the retina indicated a low degree
of summation. In animals with a low summation level, the inner nuclear layer
(composed of bipolar cells, horizontal cells, and amacrine cells) and the ganglion
layer are thick relative to the rest of the retina, indicating a high number of neurons
corresponding to each photoreceptor cell (Walls, 1942). In juvenile loggerheads,
these two layers (out of the seven overall layers) comprised approximately 37%
of the total retina (Bartol and Musick, 2001; see Figure 3.1). Bartol and Musick
(2001) also examined the topography of the retina by plotting the counts of cone
and rod photoreceptor cells and ganglion cells (Figure 3.3). Both cone photoreceptors and ganglion cells progressed from high to low density in a stair-step
fashion from the back to the front of the eye. Rod photoreceptors, however, were
more likely to maintain a constant density throughout the back half of the eye,
rapidly decreasing in number near the cornea. Dorsal–ventral differences were
also observed when the cell counts were plotted on a three-dimensional sphere.
A horizontal streak of ganglion cells and cone photoreceptor cells in the dorsal
hemisphere of the eye indicated a region of decreased summation and thus
increased acuity. Rods, however, were found in lower numbers and ubiquitously
throughout the two hemispheres, resulting in a constant sensitivity to low light
situations. This regionalization of cells was hypothesized to aid the juvenile
loggerhead in finding benthic slow-moving prey in their shallow water habitat
(Bartol and Musick, 2001).
1123 book.book Page 85 Monday, November 11, 2002 11:11 AM