Sensory Biology of Sea Turtles
95
and inner ear regions of sea turtles need to be reexamined using the latest
laboratory techniques. Furthermore, behavioral responses by multiple life history
stages of sea turtles to sound stimuli, in the form of behavioral audiograms, need
to be pursued in future research studies.
3.4 CHEMORECEPTION
3.4.1 ANATOMY OF THE NASAL STRUCTURES
The structure of the sea turtle nose is relatively simple: it opens to the outside world
through external nares and into the palate through the internal nares on the posterior
end. The external nares are connected to the nasal cavity by a tubelike vestibulum,
and the nasal cavity is connected to the palate by a long nasopharyngeal duct (Scott,
1979). The nasal cavity is divided into two regions: the intermediate region and
the olfactory region (Figure 3.7). The intermediate region lies ventrally and is
attached to both the vestibulum and the nasopharyngeal duct. The intermediate
region is large, occupies 3 / 4 of the nasal cavity, and has two pockets of sensory
epithelium called the Jacobson’s organs. The functional significance of the Jacobson’s organ is unknown, and although it appears to be capable of chemoreception,
it has been assumed that this region is nonolfactory in the anatomy literature. In
the sea turtle, these Jacobson’s organs receive information in the same manner as
olfactory epithelium. However, the information from this sensory epithelium is sent
to the accessory olfactory bulb and the trigeminal nerve system. Posterodorsally in
the nasal cavity lies the olfactory region, which is small compared to the intermediate region. The olfactory region is lined with a second type of sensory epithelium,
Bowman’s glands, which send information directly to the main part of the olfactory
bulb. The olfactory nerve arises from these two types of sensory epithelium of the
nose and forms two groups of trunks that lead to distinct portions of the olfactory
bulb and accessory bulb. In the sea turtle, both the olfactory and accessory bulbs
are notably large for a vertebrate (Parsons, 1959; 1971; Scott, 1979).
Tucker (1971) discussed the nonolfactory response within the nasal cavity and
argued that the intermediate region received chemical stimulation in a similar manner
to the olfactory region. However, because the intermediate region is ventrally located
within the nasal cavity, it is almost continually bathed with water. The olfactory region,
on the other hand, could contain an air bubble because of its dorsal location and thus
remain dry as the turtle draws water into the nasal cavity. Tucker (1971) also made
the assumption that an air-breathing animal cannot smell underwater. Thus, only the
region called the olfactory region, and not the intermediate region, could be responsible
for olfactory, chemosensory reception. The intermediate region was assumed to be
involved with nonolfactory chemoreception (Parsons, 1971; Tucker, 1971). These
assumptions, based on anatomical descriptions, have been debunked by several behavioral studies, and in fact sea turtles have been shown to “smell” underwater (see Section
3.4.2.2). In addition, recent research on fishes (Walker et al., 1997) has found that the
receptor organs for geomagnetic orientation are located in the olfactory epithelium
and are innervated by the trigeminal system. Sea turtles have been shown to have an
elegant geomagnetic sense (Lohmann and Lohmann, 1994; Lohmann et al., 1997).
Could the Jacobson’s organ be the location of geomagnetic receptors in sea turtles?
1123 book.book Page 95 Monday, November 11, 2002 11:11 AM
95
and inner ear regions of sea turtles need to be reexamined using the latest
laboratory techniques. Furthermore, behavioral responses by multiple life history
stages of sea turtles to sound stimuli, in the form of behavioral audiograms, need
to be pursued in future research studies.
3.4 CHEMORECEPTION
3.4.1 ANATOMY OF THE NASAL STRUCTURES
The structure of the sea turtle nose is relatively simple: it opens to the outside world
through external nares and into the palate through the internal nares on the posterior
end. The external nares are connected to the nasal cavity by a tubelike vestibulum,
and the nasal cavity is connected to the palate by a long nasopharyngeal duct (Scott,
1979). The nasal cavity is divided into two regions: the intermediate region and
the olfactory region (Figure 3.7). The intermediate region lies ventrally and is
attached to both the vestibulum and the nasopharyngeal duct. The intermediate
region is large, occupies 3 / 4 of the nasal cavity, and has two pockets of sensory
epithelium called the Jacobson’s organs. The functional significance of the Jacobson’s organ is unknown, and although it appears to be capable of chemoreception,
it has been assumed that this region is nonolfactory in the anatomy literature. In
the sea turtle, these Jacobson’s organs receive information in the same manner as
olfactory epithelium. However, the information from this sensory epithelium is sent
to the accessory olfactory bulb and the trigeminal nerve system. Posterodorsally in
the nasal cavity lies the olfactory region, which is small compared to the intermediate region. The olfactory region is lined with a second type of sensory epithelium,
Bowman’s glands, which send information directly to the main part of the olfactory
bulb. The olfactory nerve arises from these two types of sensory epithelium of the
nose and forms two groups of trunks that lead to distinct portions of the olfactory
bulb and accessory bulb. In the sea turtle, both the olfactory and accessory bulbs
are notably large for a vertebrate (Parsons, 1959; 1971; Scott, 1979).
Tucker (1971) discussed the nonolfactory response within the nasal cavity and
argued that the intermediate region received chemical stimulation in a similar manner
to the olfactory region. However, because the intermediate region is ventrally located
within the nasal cavity, it is almost continually bathed with water. The olfactory region,
on the other hand, could contain an air bubble because of its dorsal location and thus
remain dry as the turtle draws water into the nasal cavity. Tucker (1971) also made
the assumption that an air-breathing animal cannot smell underwater. Thus, only the
region called the olfactory region, and not the intermediate region, could be responsible
for olfactory, chemosensory reception. The intermediate region was assumed to be
involved with nonolfactory chemoreception (Parsons, 1971; Tucker, 1971). These
assumptions, based on anatomical descriptions, have been debunked by several behavioral studies, and in fact sea turtles have been shown to “smell” underwater (see Section
3.4.2.2). In addition, recent research on fishes (Walker et al., 1997) has found that the
receptor organs for geomagnetic orientation are located in the olfactory epithelium
and are innervated by the trigeminal system. Sea turtles have been shown to have an
elegant geomagnetic sense (Lohmann and Lohmann, 1994; Lohmann et al., 1997).
Could the Jacobson’s organ be the location of geomagnetic receptors in sea turtles?
1123 book.book Page 95 Monday, November 11, 2002 11:11 AM