Biology Reference
In-Depth Information
(1960) found that in natricines (now, provisionally natricids) there was a direct cor-
relation between glandular development and increasing occurrence of specialized
dentition, a selective adaptation that was reported to reach its zenith in
Balanophis
(
Rhabdophis
, Wall, 1921)
ceylonensis
.
Balanophis ceylonensis
has grooved, enlarged
posterior maxillary teeth and well-developed Duvernoy's glands (Malnate, 1960). In
the single documented case of a bite from this species, there were only minor local
effects and nonspecific headache (De Silva and Aloysius, 1983;
Table 4.1
). However,
this species should be treated with caution in view of its morphological adaptations
and relationship to
R. tigrinus
and
R. subminiatus
.
The Duvernoy's glands of
R. subminiatus
and
R. tigrinus
are tubuloacinous, well
developed, and comprise at least half of the total supralabial glandular structure
(Mittleman and Goris, 1974). Yoshie et al. (1982) conducted an electron microscopic
study of Duvernoy's glands of
R. tigrinus
. Secretory units of the gland consisted of
columnar secretory and myoepithelial cells. There were well-developed organelles
and homogenous, dense secretory granules accumulated in the apical cytoplasm
for exocytotic release into the lumen (Yoshie et al., 1982). The duct epithelium was
composed of mucous-secreting cells that contained cytoplasmic secretory globules.
The myoepithelial cells encompassed the secretory units and had numerous auto-
nomic innervations via the basement membrane (Yoshie et al., 1982). Histochemical
study indicated that these nerve terminals were enriched with acetylcholinesterase-
reactive and peptide histidine-isoleucine-like immunoreactive fibers (Yoshie et al.,
1988). As many of the neuronal terminals contained small, clear vesicles as well as
dense core granules, were devoid of Schwann cell coverage on the capillary side,
and entered the pericapillary space, the authors hypothesized intravascular release of
neurosecretions (Yoshie et al., 1982, 1988).
4.3.2.2 The Unusual Nuchal Gland of
R. tigrinus
Rhabdophis tigrinus
also possesses a nuchal (sometimes referred to as “cervical”)
gland that produces several novel polyhydroxylated steroid toxins, the bufadienolides
(Azuma et al., 1986; Hutchinson et al., 2007). This can be considered analogous to the
biosynthetic pathway for the batrachotoxins from a few genera of dendrobatid “poi-
son dart” frogs (e.g.,
Dendrobates
and
Phyllobates
), as the toxins present in this gland
are probably derived from dietary sources (i.e., toads; Hutchinson et al., 2007; Mori
and Burghardt, 2000). Some data suggest that the gland serves an antipredator function
(Hutchinson et al., 2007; Mori and Burghardt, 2008). Although this gland and its secre-
tion play no role in snakebite envenomation in humans, there have been documented
cases of ophthalmia (resembling toxic chemosis) resulting from accidental ocular
exposure to the secretion (Kawamoto and Kumada, 1989). The combined presence of
Duvernoy's glands containing toxic venom and the poison-containing nuchal gland
endows this snake with both venomous and poisonous properties.
10
10
As noted earlier, “venomous” implies introduction by biting or stinging (e.g., injection; introduction
into or through the integument of prey or foe) per a specialized apparatus of a toxic mixture utilized for
prey capture or defense. “Poisonous” implies the presence of a toxic substance that exerts its effects after
ingestion or absorption.
