Hydrus Granulatus
Schneider 1799
Chersydrus granulatus - Schneider
180x
Acrochordus fasciatus Shaw 1802
Pelamis granulatus - Daudin 1803
Hydrus granulatus - Raffles 1822
Acrochordus granulatus - Cantor 1847
Chersydrus annulatus Gray 1849
Chersydrus fasciatus — Dumeril and
Bibron, 1854: 41
Distribution. A wide ranging species known from the coastal areas McDowell (1979) describes its distribution as b
oth coasts of the
Indian Peninsula, Sri Lanka, and Indo-Chinese and Malayan regions; at least
Luzon, Cebu, and Bantayan in Philippine Islands; the Greater Sunda Islands,
Sulawesi, and at least Flores and Timor in Lesser Sundas; coasts of New Guinea
and at least Lorentz, Sepik, and Fly-Strickland river systems; Biak and Japen
Islands, Geelvink Bay; Kiriwina Island, Trobriands; at least Mussau Island and
the Willaumez Peninsula of New Britain in the Bismarck Archipelago;
Bougainville Island and at least Morgusaia (Shortland Group), Santa Isabel, Malaita, and Guadalcanal
Islands in the Solomon Islands.
External Morphology
Males reach 70 cm, and have slender bodies compared to females. Females reach 76 cm and have heavier, bulky bodies compared to males. Skin is covered with juxtaposed, granular scales in about 100 rows. They are gray- brown with black transverse bands.
The following is adapted from McDowell (1979)
Nostrils directed upward at about
45 degrees, set behind tip of snout, each surrounded by a nasal scute forming a
narrow rim; nasals separated from each other by a granular scale at their
closest approximation; nasal to eye scales 5-7; eye to eye scales 10-15; eye to
lip scales 5-7; scales on roof of muzzle distinctly larger than those between
eyes and on parietal region; usually with scale behind nasal enlarged and about
twice size of adjacent scales; supralabials
distinctly enlarged anteriorly, 13-22; lower labials 12-18; corner of mouth
concealed from lateral view by rictal fold, thus defining a scale-lined pocket
at corner of mouth; rostral and mental scutes barely visible externally, but
extending within mouth as distinct cornifications;
middle of lower lip produced forward as a median lobe fitting closely into
median notch of upper lip; no mental groove defined by modified scales or
median depression. Scales juxtaposed, with little or no skin exposed between
them; along midline of belly, on a well-defined median fold, 3 or 4 rows of
scales with long median cusp, but other scales with cusps reduced to
short pointed tubercles, the median
always longer than flanking cusps (latter often obscure); scales of back about
twice size of lower flank scales; scales at mid-body 122-159 other workers,
perhaps using a minimum path count, report 85-110 scales at mid-body. No anal
scute, the cloacal region U-shaped, the arms of the U directed backward as a
pair of bilobate pockets, the paired anal gland opening into the medial lobe of
each pocket and lying dorsal to cloaca without backward extension into tail,
while hemipenis continues lateral lobe of each pocket into tail; lining of
cloaca with granular scales near cloacal lips, but no specimens seen with
cloaca everted as tube; tail compressed, its vertical diameter twice its transverse
diameter, about 6-14 percent of total length (accurate measurement of my
material is impossible, and it is difficult to determine in all cases whether
tail is complete). Maxillary teeth 14-18, showing significant geographic
variation in region under discussion; for New Guinea: range 14-16; for
Solomons, including Bougainville: range 14-18.
Juveniles grey with blackish rings
around the body, which may alternate vertebrally and are frequently joined to a
black vertebral stripe; midline of belly usually pale, but midline of underside
of tail and extreme rear of trunk usually black, so that pale bands of tail are
isolated. On southern coast of New Guinea (and rivers and lakes with southern
drainage), the banded pattern evident in adults, although pale bands are
darkened to reduce contrast; on northern coast (including Sepik River) and in
Solomons, large adults with pattern totally obscured by darkening; head of
young with white spots, obscured in adults; dark bands on trunk in specimens
from northern coast and drainage of New Guinea, from Bismarck Archipelago, and from
Solomons 35-47 (mean=41.8, s=3.41, Sx = 0.78, N=19); dark bands on trunk in specimens
from Western and Gulf Provinces, Papua New Guinea, 50-65 (mean = 58.9, s =
4.59, Sx = 1.27, N = 13); this increase in number of bands in southern shore
specimens partially the result of splitting of bands, which tend to alternate
on left and right sides, by the pale band from one side extending across the
belly to divide the dark band of the opposite side (the pattern thus reticulate
near belly); tail bands apparently showing a similar trend (8-15 in northern
New Guinea, Bismarck Archipelago, and Solomons; 11-15 in Western and Gulf Provinces)
but counts less reliable because of specimens with damaged tail and because
tail darkens before trunk, obscuring pattern. (MCZ 139516, juvenile female,
Daru I., uniformly pale yellowish tan, an apparent albino.)
Diet and hunting strategies are sexually dimorphic the males actively hunt prey whereas the females sit and wait as ambush predators (Shine, 1991). They are found mainly in small ponds, swamps and estuaries, but have also been found at sea.
On habitat, McDowell (1979) wrote,
Acrochordus
granulatus is
not confined to the sea; throughout its range it is known to enter rivers and
in New Guinea extends to Lake Murray in the Fly-Strickland system and to the Ambunti
region in the Sepik system. To judge from records available to me, it does not
enter the Mamberamo-Idenburg system or the Purari system, both of which have
waterfalls near their access to the coast, and it is probably waterfalls,
rather than any physiological inability to deal with freshwater, that limit the
distribution of A. granulatus in rivers and lakes….
The following notes on habitat and
diet are based on Voris and Glodek (1979. Journal of Herpetology 14:108-111).
Habitat. Early reports (Cantor,
1847; Flower, 1899; and Boulenger, 1912) state that A. granulatus is
found in the sea as well as in rivers along the coasts of south Asia. Dunson
and Dunson (1973) give some specific localities and include a range of
salinities where file snakes were found ranging from sea water (520mM Cl) in
the McCluer Gulf (off West lrian) to fresh water (2.3mM Cl) in the lake Laguna
de Bay in Philippines. They also report on 13 snakes collected over a shallow
reef flat exposed at low tide near Townsville, Queensland and one file snake
that was captured floating on the surface in the McCluer Gulf (depth 12 m.). Eighty
six A. granulatus were collected at three localities in the Straits of
Malacca (Fig. 1) in late 1974 and in 1975. The specimens are deposited at the
Field Museum of Natural History. Two male and five female file snakes were
captured at stationary stake nets in the mouth of the Muar River. Small otter
trawls were employed at Sungai Buloh (29 males and 29 females captured) and at
Parit Botak (11 males and 10 females captured). At Muar, seven stake nets are
located 25 to 50 meters from shore over a mud bottom. The water depth ranges
from 4 to 6 meters at these nets. Nets were operating only while the tide was
ebbing. Salinity and turbidity (as determined using a Secchi disk) ranged from
30.5% and 2.0 m respectively at high tide to 6.1% and .25 m at low tide. Water
temperature ranged from 27.5°C to 29.5°C and varied with local rainfall, river
discharge and the tidal cycle. Fishes collected with file snakes at Muar are typically
marine and estuarine. Of 12 families represented, 9 are strictly marine and 3
include estuarine and riverine forms. At Sungai Buloh, situated between the mouth
of the Selangor and the Kelang Rivers, file snakes were captured on or near the
bottom with otter trawls from 2 to 7 km off shore. Salinities taken over
several months ranged from 24 to 28%.
There
are no major coral outcroppings in this area. Sea surface water temperatures
ranged from 28 to 29.5°C and differences between surface and bottom temperature
always seemed negligible (< 1°C). Bottom substrate varied from 95% mud and
5% sand and shell fragments to just the reverse. Fish collections were made
from trawl hauls on three occasions when file snakes were captured. The fish
collected represent eighteen predominantly marine families. The invertebrates
captured in one or more of the hauls included marine shrimp (Penaeus sp.) squid (Sepia sp.), and horseshoe crabs (Umulus sp.). The fishing grounds at Parit Botak are less influenced
by large river discharges than are those at Sungai Buloh. Trawling at Parit
Botak was done 2 to 10 km from shore at depths ranging from 6 to 20 meters.
Salinity was 32% on the two days it was measured. Sea surface temperatures measured
27.5°C on these occasions. Secchi disk readings ranged from 2.30 to 2.95 m.
Some patch reefs of living coral are within the area fished. The bottom substrate
varied from mud to sand and broken shells but there was less fine silt than at
Sungai Buloh. No fish collections were made. The salinity and temperature data
cited tor the above three localities are well within the ranges previously
observed for this region (Soeriaatmadja, 1956; Liong, 1974).
Diet. The literature on A. granulatus frequently states that this
species is a piscivore (Boulenger, 1912; De Rooij, 1917; Neill, 1958). Cogger
(1975) states that "It forages principally in the intertidal zone where it
feeds on small crabs and fishes." However, specific data are few. Flower (1899)
reports on a single file snake caught in the act of swallowing a fish. McDowell
(1979) gives the stomach contents of two snakes to be a goby and mullid
respectively; four other stomachs contained unidentifiable fish remains and two
others contained crustacean and snail fragments. At Sungai Buloh the stomachs
of nine specimens contained identifiable fish remains from four families of the
suborder Gobioidei, which includes all the families of goby-like fishes. Six
stomachs contained a single species of sleeper (Eleotridae) of the genus Eleotris. One of these was entire and
measured 6.5 cm long and had a maximum diameter of 1.2 cm. The snake had a
snout-vent (SV) length of 63.5 cm. Each of the three remaining stomachs
contained a member of the family Gobioididae, a goby-like fish closely related
to the gobies. Two of the gobies were each 5.5 cm in length and came from
snakes of 58.7 and 64.5 cm SV length. The stomach content of one snake collected
at Muar was a distinct species of the family Gobiidae. At Parit Botak two
specimens had eaten the same species of burrowing goby (Trypauchenidae) of the
genus Trypauchen. All of the above fish are small demersal forms, often
living in burrows or under rocks and in crevices. In all the forms studied,
reproduction and care of the young take place in burrows (Breder and Rosen,
1966:522-537). Thus, it is likely that the non-venomous file snake feeds by the
exploration of crevices along the bottom as do many species of true sea snakes (Mccosker,
1975; Voris, 1978). Four of the fish species specified above are also eaten by
the venomous sea snake, Lapemis hardwickii. In fact, Eleotris sp. was
found in the stomachs of 22 L. hardwickii and is a major component of
its diet at Sungai Buloh.