An Anatomical Study of the Visual Capabilities of the Green Turtle, Chelonia mydas

Several aspects of vision in juvenile and adult Green Turtles (Chelonia mydas) are examined, with special reference to retinal anatomy such as oil droplet topography, transmission electron microscopy of photoreceptors, spectral transmission measurements of the ocular media (cornea, lens, and vitreous humor), and measurements of focal length and optical sensitivity. A detailed study of the distribution of the different color classes of oil droplets shows that all oil droplets are found in high concentrations (>1000 mm⁻²) in the central/temporal parts of the retina. Red oil droplets were the largest, followed by yellow and clear. Oil droplet size varied in different parts of the retina. On average, red oil droplets were found in fewer numbers compared to yellow and clear oil droplets. Two types of clear oil droplets were identified: those that fluoresced under UV illumination and those that did not. We found that the majority (78.5%) of colorless oil droplets fluoresced when viewed under UV light. Spectral transmission measurements of the ocular media show that wavelengths to approximately 325 nm are transmitted. This may suggest ultraviolet (UV) vision in Green Turtles. The optical sensitivity of the Green Turtle eye was relatively low, suggesting an adaptation to high light intensities commonly experienced by this species.

Figure 1

(A) Image of oil droplets of a whole-mounted Green Turtle retina (brightfield illumination).

(B) Topographic map of oil droplet density (number of oil droplets mm⁻²) in different parts of the retina for red, yellow, and clear oil droplets. Each dot represents one of the areas photographed. Density of all oil droplets (indicated by color) increases towards the central/temporal parts of the retina (data, Turtle 5). Scale bar 25 µm. (C) Two images of the same area of retina under brightfield illumination (top) and ultraviolet (UV) illumination (bottom). These images show that while most oil droplets fluoresce under UV light, a small number do not. Scale bar 25 µm.

Figure 2

Mean diameters and standard error means (SEM) of oil droplets in Green Turtles of different ages. Data for post-hatchling turtle were taken from Granda (1970), who unfortunately did not specify SEM values. Means for juvenile and adult turtle were taken across the entire retina. There is a significant difference between the diameters of red, yellow, and clear oil droplets of the juvenile and the adult animal (red: t = 6.21, P < 0.01, n = 30; yellow: t = 2.27, P < 0.05, n = 30; clear: t = 5.24, P < 0.01, n = 30).

Figure 3

(A) Example of a composite image obtained from laser ray tracing of a turtle lens (Turtle 7; scale bar, 5 mm). (B) Transmission electron micrograph of cone photoreceptors of Turtle 5 (Scale bar, 10 µm). pe, pigment epithelium; os, outer segment; is, inner segment; od, oil droplet.

Figure 4

Spectral transmission measurements of the ocular media (cornea, lens, and vitreous humor) of the Green Turtle.

T₅₀ values for lens, 325 nm; cornea, 304 nm. No T₅₀ value can be given for the vitreous humor since it transmits to wavelengths below the limit of the spectrometer. Data are normalized.

Contributor Notes

School of Biomedical Sciences, University of Queensland, Brisbane, QLD 4072, Australia. Present address: (LMM) Marine Biological Laboratory, Marine Resources Center, Woods Hole, Massachusetts 02543. E-mail: (LMM) lmathger@mbl.edu. Send reprint requests to LMM.

Received: 12 Sept 2005

Accepted: 05 Oct 2006