80
The Biology of Sea Turtles, Vol. II
3.1 INTRODUCTION
The study of sensory biology in sea turtles is still in its infancy. Even the basic
morphology of the eye, ear, and nose of sea turtles has been described in detail in
only one or two species. The same may be said for electrophysiological and behavioral studies of sea turtles’ sensory systems. The ontogenetic and interspecific difference in the sensory biology of sea turtles has been little studied and the sensory
biology of the leatherback ( Dermochelys coriacea ), a species whose ecology is
greatly different from the cheloniids, is virtually unknown. The present chapter will
focus on the current state of knowledge of the sensory biology of vision, hearing,
and olfaction in sea turtles.
3.2 VISION
3.2.1 M ORPHOLOGY AND A NATOMY OF THE E YE
3.2.1.1 Main Structures of the Eye
The anatomy of the sea turtle eye appears to be typical of that found in all vertebrates
(Granda, 1979; Walls, 1942). The eyeball is filled with two ocular fluids, aqueous
and vitreous humors, and is organized into three layers: (1) the outermost layer,
consisting of the sclera and cornea; (2) the middle layer, which includes the choroid,
ciliary body, and iris; and (3) the inner layer, or the retina. The sclera is inelastic and
is responsible for the eyeball’s static shape, whereas the aqueous humor keeps this
fibrous layer distended. The anterior portion of the sclera, the cornea, is transparent
and responsible for much of the refraction of light in air, yet is virtually transparent
in water. The choroid of the middle layer is highly pigmented and vascularized; the
pigmentation deflects stray light from entering the eye and prevents internal reflections. The inner layer of the eyeball, the retina, contains the visual cells (rod and
cone photoreceptor cells) and ganglion cells, and is continuous with the optic nerve
(Walls, 1942; Copenhaver, 1964; Granda, 1979; Ali and Klyne, 1985; Bartol, 1999).
The lens of the green sea turtle ( Chelonia mydas ) is nearly spherical and rigid
(Ehrenfeld and Koch, 1967; Granda, 1979; Walls, 1942), and appears to be quite
different from that of freshwater turtles, which have developed an advanced means of
accommodation through the manipulation of an extremely pliable lens. For sea turtles,
however, ciliary processes do not reach the lens and the ringwulst is weakly developed,
and thus active accommodation does not appear to be possible (Ehrenfeld and Koch,
1967). However, this type of spherical lens is ideal for underwater vision. In the absence
of corneal refraction while underwater, the refractive index of the cornea is nearly
identical to that of seawater, and the lens is the only structure responsible for the
refraction of incoming light. The spherical lens has a high refractive index, which
compensates for the lack of corneal refraction (Sivak, 1985; Fernald, 1990).
3.2.1.2 Cells of the Retina
The vertical organization of the retina has been examined in the juvenile loggerhead
sea turtle ( Caretta caretta; Bartol and Musick, 2001) (Figure 3.1). The layers of the
1123 book.book Page 80 Monday, November 11, 2002 11:11 AM
The Biology of Sea Turtles, Vol. II
3.1 INTRODUCTION
The study of sensory biology in sea turtles is still in its infancy. Even the basic
morphology of the eye, ear, and nose of sea turtles has been described in detail in
only one or two species. The same may be said for electrophysiological and behavioral studies of sea turtles’ sensory systems. The ontogenetic and interspecific difference in the sensory biology of sea turtles has been little studied and the sensory
biology of the leatherback ( Dermochelys coriacea ), a species whose ecology is
greatly different from the cheloniids, is virtually unknown. The present chapter will
focus on the current state of knowledge of the sensory biology of vision, hearing,
and olfaction in sea turtles.
3.2 VISION
3.2.1 M ORPHOLOGY AND A NATOMY OF THE E YE
3.2.1.1 Main Structures of the Eye
The anatomy of the sea turtle eye appears to be typical of that found in all vertebrates
(Granda, 1979; Walls, 1942). The eyeball is filled with two ocular fluids, aqueous
and vitreous humors, and is organized into three layers: (1) the outermost layer,
consisting of the sclera and cornea; (2) the middle layer, which includes the choroid,
ciliary body, and iris; and (3) the inner layer, or the retina. The sclera is inelastic and
is responsible for the eyeball’s static shape, whereas the aqueous humor keeps this
fibrous layer distended. The anterior portion of the sclera, the cornea, is transparent
and responsible for much of the refraction of light in air, yet is virtually transparent
in water. The choroid of the middle layer is highly pigmented and vascularized; the
pigmentation deflects stray light from entering the eye and prevents internal reflections. The inner layer of the eyeball, the retina, contains the visual cells (rod and
cone photoreceptor cells) and ganglion cells, and is continuous with the optic nerve
(Walls, 1942; Copenhaver, 1964; Granda, 1979; Ali and Klyne, 1985; Bartol, 1999).
The lens of the green sea turtle ( Chelonia mydas ) is nearly spherical and rigid
(Ehrenfeld and Koch, 1967; Granda, 1979; Walls, 1942), and appears to be quite
different from that of freshwater turtles, which have developed an advanced means of
accommodation through the manipulation of an extremely pliable lens. For sea turtles,
however, ciliary processes do not reach the lens and the ringwulst is weakly developed,
and thus active accommodation does not appear to be possible (Ehrenfeld and Koch,
1967). However, this type of spherical lens is ideal for underwater vision. In the absence
of corneal refraction while underwater, the refractive index of the cornea is nearly
identical to that of seawater, and the lens is the only structure responsible for the
refraction of incoming light. The spherical lens has a high refractive index, which
compensates for the lack of corneal refraction (Sivak, 1985; Fernald, 1990).
3.2.1.2 Cells of the Retina
The vertical organization of the retina has been examined in the juvenile loggerhead
sea turtle ( Caretta caretta; Bartol and Musick, 2001) (Figure 3.1). The layers of the
1123 book.book Page 80 Monday, November 11, 2002 11:11 AM