1837 Coluber (Homalopsis) prevostianus
Eydoux & Gervais, Magasin de Zoologie, 3:5, pl 15. Holotype: Location unknown. Type locality:
"Manila." Collector: unknown.
1849 Gerarda bicolor Gray, Catalogue of Specimens of Snakes in the
Collection of the British Museum, p. 77.
Type locality: "West Indies," from Mr. Launa's
collection. This type locality is in
error. Holotype: BMNH 1946.1.2.33.
1854 Campylodon prevostianum - Duméril,
Bibron and Duméril, Erpètologie
Gènèrale...Reptiles, 7:946.
1884 Helipophis flavescens Müller, Verhandlungen der
Naturforschenden Gesellschaft, 7:286. Type locality: "East Indies." Holotype: NMB 1771. Collectors: G. and F.
Müller.
1890 Gerardia prevostiana - Boulenger, Fauna of British India...Reptilia and
Batrachia, p. 379.
1930 Gerarda prevostiana - Smith, Bulletin of the Raffles Museum, 3:62.
Etymology
Wall (1921) stated that this species
was named in honor of Monsieur Prevost, Superintendent of the Paris Museum at the
time specimen was named.
Distribution
Gerarda
prevostiana is distributed in coastal areas from the vicinity of Bombay, India
westward to the Philippines, this distribution may be discontinuous. While it
seems probable that it is present in Indonesia I have seen no specimens or
literature references to specific specimens that suggest its presence there. It
also seems absent from tropical coastal China. Manthey and Grossmann (1997)
have a map that suggests it is restricted to the west coast of Myanmar,
Thailand, and the Malayan peninsula; however Taylor’s (1965) specimens (a
series of 15) were collected in Chon Buri Province along the coast of the Gulf
of Thailand. Iskandar and Colijn (2001)
consider it present in Indonesia but list no specific localities or specimens and
they list the Philippine type locality as an error (probably following Gyi’s
comments). However, Auth et al. (1990) verified that it is indeed present in
the Philippines. The verification of its presence in the Philippines took 153
years (Murphy and Voris, 2002). The distribution
may be disjunct but this snake should be expected wherever mangrove forest is
found. Due to its secretive habitats it has been considered rare by many
authors, but because of its habits it cannot be easily deleted from a faunal
list from any area with mangrove forest between Bombay and the Philippines. However,
the apparent total lack of specimens from Indonesia is puzzling. Its distribution overlaps with several other
widespread coastal homalopsids, including its sister species Fordonia leucobalia and members of the
genus Cerberus, except that Gerarda is absent from the Sahul Shelf
(Australia and New Guinea).
Diagnosis
This is a small homalopsid with a
dorsum that is uniform gray-brown or black. The nasal scales are separated by
an internasal scale, and it has 17 rows of dorsal scales at midbody, the lowest
count of any homalopsid. Its scales lack keels or striations, but may be pitted
with irregular erosions of the scale surface. Its close relative, Fordonia leucobalia has 23 - 27 rows
of dorsal scales at midbody, but it also has smooth scales and the nasal scales
separated by the internasal. Cantoria
violacea and Cantoria annulata
also have smooth dorsal scales (C.
annulata females and juveniles have weakly keeled scales on rows 1 - 4 in
the area of the vent) and the internasal separating the nasals. However these
two snakes have 21 and 19 rows of scales respectively at midbody. Heurina has its nasal scales separated
by the internasal and 27 scale rows at midbody; but it has a completely divided
nasal scale. And, Bitia has its nasal
scales separated by the internasal but it has 37 - 43 scale rows at midbody
that are triangular in shape and have exposed skin between them.
Size
Boulenger
(1896) gave its maximum length as 520 mm. Twelve specimens were measured for
this study; the largest was a male with a total length of 514 mm, and a 65 mm
tail. The largest female had a total length of 472 mm with a 57 mm tail. The
smallest specimen examined had a total length of 230 mm with a 29 mm tail. An
examination of 12 museum specimens from the eastern portion of the range (India
to Thailand, five males and seven females) suggest that the tail/SVL ratio
tends to be higher in males (14.4 - 17.6), than in females (13.7 - 15.8). This
was also supported by data collected on the Singapore population by Karns et
al. (2002).
External
Morphology
The head is slightly distinct from
the neck and slightly depressed. The body is cylindrical to slightly depressed.
The eyes are dorsolateral, but more lateral than dorsal.
On the head the rostral is a
constricted rectangle, about twice as broad as high, and is barely visible from
above. The nasals are entire and separated completely by a large internasal
that is equal or greater than the nasals. The internasal is not in contact with
the loreal. The prefrontals are about equal to the nasals in size. The frontal
is relatively short, its length being less than the interorbital distance, and
the eye-nostril distance. The parietals are long, about 1.2 times the length of
the frontal. The loreal is about as long as high, and in contact with the first
three labials. The supraocular is close to being rectangular. The preocular is
single and taller than long. There are two postocular scales (rarely three),
the upper one is smaller, and the lower scale partially underlies the orbit.
There are no subocular scales. The temporal formula can be 2 + 2 or 1 + 3. The
upper labials number seven or eight, the fourth labial enters the orbit. The
head shields on these snakes are imbricate and are frequently pitted or eroded.
On the chin the lower labials number
7 - 9, the first four (rarely five) are in contact with the anterior pair of
chin shields. Gulars number three to five, usually five.
On the body the dorsal scale rows on
the neck are in 17 - 19 rows; Thailand specimens have 17 - 19 rows on the neck,
while those from Singapore and a single specimen from Sri Lanka had 17 rows on
the neck. The scales at mid-body are in
17 rows and the dorsal scales near the vent are in 13 - 15 rows. Like the head scales the scales in the dorsal
rows are pitted but there are no keels or striations. The first row is ovate; they become more
lanceolate toward the midline. The dorsal scales at posterior body are ovate
and do not elongate toward the midline. The ventral scale count range was 141 -
157 and they are about three times wider than the height of a nearby dorsal
scale. Males have 144 - 157 ventral scales, and females have 141 - 144. There is
little or no indication of geographic variation in the ventral or subcaudal
scales of this species from our sample which combines specimens from a variety
of localities. The anal plate is divided, occasionally into three parts. It is
about twice as long as the preceding ventral.
On the tail the dorsal scales are
similar to the ones above. The subcaudal scales are divided and number 30 - 35;
males have 31 - 35; females have 30 - 32. At the base of the tail the width is
75% of the height based upon an average of five specimens.
Color and Pattern. The crown of the
head is uniform gray. Each of the upper labials is yellow or has a large yellow
spot. The rostral is gray. The chin is mostly yellow with some scales having
darker edges. The first three scale rows are mostly yellow, the rest are gray.
Each ventral scale has diffuse chromatophores concentrated near the anterior
edge of the scale. The ventral surface of the tail has pigmentation similar to
the ventral scales.
Karyotype. Sing et al. (1970) report a 2n number of 36.
Habitat
This is a snake of the mangrove
forest, although it may occasionally stray into adjacent environments. It is
restricted to coastal areas and may be found in rivers that empty into the
ocean, on mud flats, and along rocky shorelines, but it most likely reaches its
greatest abundance in mangroves. Karns et al. (2002) found G. prevostiana this species used a variety of microhabitats from the
landward edge of the mangal to the mud pools in the lower tidal zone at Pasir
Ris Park in Singapore. It has been excavated from mud lobster (Thalassinia anomala and T. gracilis) mounds (Karns et al., 2002;
Voris and Murphy, 2002) which occur toward the landward edge of the mangal.
However, Karns et al. (2002) found that it was not restricted to the area
around mud lobster mounds. This species was found to be active throughout the
night and more active at spring tides than at other times during the tidal
cycle.
The literature comments on this
species all support its coastal distribution and its aquatic, intertidal
habits. Gharpurey (1944) found it to be, “...entirely aquatic, haunting tidal
rivers and estuaries, and often wanders out along the coasts." Tweedie
(1983) wrote, “It lives along sea coasts and in tidal rivers.” Wall (1905)
stated, "It was lying sunning itself in a shallow pool of water, and made
no movement when I walked over it snipe shooting." He (Wall, 1921) later
wrote, “It is entirely aquatic, haunting tidal rivers and estuaries, and
straying out along the coasts. It is sometimes left on the shore by the
receding tide, and I encountered one so stranded in Cannanore. It was lying in
a shallow pool of water, partly submerged. Nicholson records one which had
wandered into a dusty street in Rangoon."
Mangrove habitats were also reported by Lim Boo Lait (1963) for
Selangor, Malaysia; and by Taylor (1965) for the Thailand population.
Diet and Feeding
Behavior
Literature comments about the food
of this species include Wall (1921) who wrote, “No observations have been made,
but it is fairly certain to subsist upon fishes.” And, Cox (1991) stated,
“Fish, shrimp, and other aquatic animals comprise the bulk of the diet.”
Contrary to the literature none of
the five museum specimens examined with stomach contents contained fish, they
all contained crab remains (Voris and Murphy, 2002). Jayne et al. (2002) later
found that it specializes in feeding on recently molted crabs, and that the
snake will tear larger crabs apart in order to ingest them. This is the only
species of snake known to tear its food into pieces that can be swallowed, all
other snakes consume their prey whole (Fordonia
is also a possible exception but in need of documentation). Tearing the prey
into pieces is accomplished by a “loop and pull” behavior that involves the
snake forming a loop of its body around the prey and pulling the prey with its
mouth through the loop. The advantage of this is that it allows the snake to
swallow a much larger prey, than it would otherwise be able to handle. G. prevostiana is known to feed on the
crabs Dotillopsis sp. and Episesarma versicolor (Voris and Murphy,
2002; Jayne et al., 2002).
Reproduction
Little is known about the
reproduction of this snake, Gyi (1970) reported a specimen to contain five
eggs.
Populations and
Abundance
Wall (1921) wrote, "Seemingly a
very rare snake in Ceylon. It is also a very uncommon snake round the coasts of
India, judging from the dearth of specimens in Museums...it is perhaps singular
that it has not been recorded from the eastern coasts of India.” De Silva
(1994) considered this snake extinct in Sri Lanka because according to him no
specimens had been collected since 1888. However, Porej (2001) reported one
from Muthurajawela Marsh (Sri Lanka). A clue to the habitat used by this snake
was provided by Taylor (1965) when he reported finding them at night in a mangrove
forest. This snake appears rare because it uses mud lobster mounds and other
mangrove crustacean burrows as hiding places, a microhabitat that has been
over-looked by herpetologists. Which is probably why Gharpurey (1944) wrote,
"It is found all along the western coast of India, but not in large
numbers." Karns et al. (2002) found this species by nocturnal visits to
mangrove forests. During two spring tide survey nights (full and new moon) they
collected 16 snakes compared to eight neap tide (half moon) survey nights that
produced only 12 snakes. This species made up about 15.9% of the snakes
collected.