90
The Biology of Sea Turtles, Vol. II
from 560 (green-yellow), 580 (yellow), and 600 (yellow-orange) nm wavelengths
when the choice was color vs. a darkened window, but green sea turtles did not.
These results indicate that loggerhead sea turtles are capable of seeing at least
from 360 to 700 nm, whereas green sea turtles see wavelengths from 360 to 500
nm. Furthermore, loggerheads appear to be xanthophobic (averse to yelloworange light) (Witherington and Bjorndal, 1991).
3.2.5 C ONCLUDING R EMARKS
Researchers are just beginning to develop a complete picture of the visual niche of
sea turtles. The mechanisms by which sea turtles, as both hatchlings and adult
females, return to the sea after hatching or nesting on land involve visual cues to
find the ocean, though these cues seem to be restricted to diffuse images, and
brightness levels and/or contrasts. This information has been invaluable in both
defining the ecology of sea turtles on land and providing guidelines for the protection
of these animals from anthropogenic light sources. The role of visual stimuli underwater for sea turtles also has been recently elucidated. From morphological studies,
the roles of visual photoreceptor cells are being defined for both color vision and
visual acuity. Retinal morphology studies may reveal the maximum capability of a
visual system; certain cells and structures must be present for the retina of a typical
vertebrate eye to process visual stimulation. Consequently, predictions have been
made from identifying cell characteristics, describing pathways from one cell layer
to the next, and mapping regions within the retina of high- and low-density cell
counts. Electrophysiological studies on both color vision and visual acuity have
supported the morphological work. Sea turtles have color vision, primarily in the
shorter wavelengths (450–620 nm), and have the visual acuity to discern relatively
small objects within the marine environment. Behavior studies further support these
conclusions.
3.3 HEARING
3.3.1 M ORPHOLOGY AND A NATOMY OF THE E AR
3.3.1.1 Main Structures of the Middle and Inner Ear
Sea turtles do not have an external ear; in fact, the tympanum is simply a continuation
of the facial tissue. The tympanum is posterior to the midline of the skull and is
distinguishable only by palpation of the area. Beneath the tympanum is a thick layer
of subtympanal fat, a feature that distinguishes sea turtles from both terrestrial and
semiaquatic turtles. The middle ear cavity lies posterior to the tympanum; the
eustachian tube connects the middle ear with the throat near the posteroventral edge
of the middle ear cavity (Lenhardt et al., 1985; Wever, 1978) (Figure 3.5).
The ossicular mechanism of the sea turtle ear consists of two elements, the
columella and the extracolumella. The extracolumella is a cartilaginous, mushroomshaped disk under the tympanic membrane, which is attached by its posterior end
firmly to the columella. The columella, a long rod with the majority of the mass
concentrated at each end, travels through a bone channel, and expands within the
1123 book.book Page 90 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
from 560 (green-yellow), 580 (yellow), and 600 (yellow-orange) nm wavelengths
when the choice was color vs. a darkened window, but green sea turtles did not.
These results indicate that loggerhead sea turtles are capable of seeing at least
from 360 to 700 nm, whereas green sea turtles see wavelengths from 360 to 500
nm. Furthermore, loggerheads appear to be xanthophobic (averse to yelloworange light) (Witherington and Bjorndal, 1991).
3.2.5 C ONCLUDING R EMARKS
Researchers are just beginning to develop a complete picture of the visual niche of
sea turtles. The mechanisms by which sea turtles, as both hatchlings and adult
females, return to the sea after hatching or nesting on land involve visual cues to
find the ocean, though these cues seem to be restricted to diffuse images, and
brightness levels and/or contrasts. This information has been invaluable in both
defining the ecology of sea turtles on land and providing guidelines for the protection
of these animals from anthropogenic light sources. The role of visual stimuli underwater for sea turtles also has been recently elucidated. From morphological studies,
the roles of visual photoreceptor cells are being defined for both color vision and
visual acuity. Retinal morphology studies may reveal the maximum capability of a
visual system; certain cells and structures must be present for the retina of a typical
vertebrate eye to process visual stimulation. Consequently, predictions have been
made from identifying cell characteristics, describing pathways from one cell layer
to the next, and mapping regions within the retina of high- and low-density cell
counts. Electrophysiological studies on both color vision and visual acuity have
supported the morphological work. Sea turtles have color vision, primarily in the
shorter wavelengths (450–620 nm), and have the visual acuity to discern relatively
small objects within the marine environment. Behavior studies further support these
conclusions.
3.3 HEARING
3.3.1 M ORPHOLOGY AND A NATOMY OF THE E AR
3.3.1.1 Main Structures of the Middle and Inner Ear
Sea turtles do not have an external ear; in fact, the tympanum is simply a continuation
of the facial tissue. The tympanum is posterior to the midline of the skull and is
distinguishable only by palpation of the area. Beneath the tympanum is a thick layer
of subtympanal fat, a feature that distinguishes sea turtles from both terrestrial and
semiaquatic turtles. The middle ear cavity lies posterior to the tympanum; the
eustachian tube connects the middle ear with the throat near the posteroventral edge
of the middle ear cavity (Lenhardt et al., 1985; Wever, 1978) (Figure 3.5).
The ossicular mechanism of the sea turtle ear consists of two elements, the
columella and the extracolumella. The extracolumella is a cartilaginous, mushroomshaped disk under the tympanic membrane, which is attached by its posterior end
firmly to the columella. The columella, a long rod with the majority of the mass
concentrated at each end, travels through a bone channel, and expands within the
1123 book.book Page 90 Monday, November 11, 2002 11:11 AM