Saturday, March 26, 2016

Little File Snake, Acrochordus granulatus, (Schneider 1799)

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 both 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 cuspbut 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.

No comments:

Post a Comment