File snakes

(Acrochordidae)

Class Reptilia

Order Squamata

Suborder Serpentes

Family Acrochordidae

Thumbnail description
Small to medium-sized aquatic snakes with blunt heads not distinct from the neck, dorsally directed small eyes and nares, flabby and roughened skin bearing small, spinate scales. Body stout and capable of lateral compression for swimming. Sluggish behavior.

Size
20–76 inches (0.5–2 m)

Number of genera, species
1 genus, 3 species

Habitat
Shallow tropical waters associated with coastal mangroves, estuaries, and river systems

Conservation status
Data Deficient, but some populations may be locally threatened

Distribution
Waters of Indo-Australian region, from western India to Southeast Asia, Malaysia, Indonesia, east to Solomon Islands and south to northern Australia

Evolution and systematics

The Acrochordidae, called wart snakes or file snakes, is an unusual family of strictly aquatic snakes consisting of a single genus and three species. The little file snake (Acrochordus granulatus) is the smallest of the three species and has a largely marine distribution. It originally was thought to be venomous and was placed in a separate genus (Chersydrus). It seems best to include this species in the genus Acrochordus, although some systematists have maintained the two separate genera. The Arafura file snake (A. arafurae) and the Java file snake (A. javanicus) are roughly twice the size of the little file snake and are largely freshwater in distribution. A fourth extinct species is known that existed in the Upper Miocene and Lower Pliocene epochs of Pakistan.

The phylogenetic relationships between acrochordids and other snakes are unclear. The snakes in this family retain some primitive characteristics but have evolved numerous specialized traits. Some morphologic features are so different from those of other snakes that file snakes have been joined in a separate superfamily, the Acrochordoidea. They appear to be related most closely to advanced snakes: colubrids (a large family including many common and familiar snakes such as garter snakes, rat snakes, and king snakes), elapids (cobras and their relatives), viperids (vipers and pit vipers), and a group of African snakes known as Atractaspididae.

Taxonomy for these species: Acrochordus arafurae McDowell, 1979, Papua New Guinea, Western Province, Lake Daviumbo; commonly known as the Arafura file snake. Acrochordus granulatus Schneider, 1799, "India"; commonly known as the little file snake. Acrochordus javanicus Hornstedt, 1787, Java; commonly known as the Java file snake, wart snake, or elephant trunk snake.

Physical characteristics

File snakes exhibit many fascinating and novel features of morphology, physiology, and behavior. The little file snake is the smallest of the three species. Adults average 20–28 in (50–70 cm) and grow to a maximum length of approximately 40 in (1 m). The other species are nearly twice this size, the Arafura file snake reaching a maximum length of approximately 67 inches (1.7 m) and the Java file snake reaching a maximum length of nearly 80 inches (2 m). All three species

of file snake show sexual dimorphism, females having larger heads, shorter tails, and generally heavier bodies than males of the same species.

A number of the unusual features of anatomy can be interpreted as adaptations for aquatic life and a specialized diet of fishes. File snakes are distinguished by a very loose skin and supple musculature that allow strong lateral compression and enable these snakes to seize and hold struggling fish. The ventral scutes are very small and project downward at the midline to form a compressed ventral keel during swimming. The flabby skin enhances mobility beneath water but sags noticeably when a snake is out of water. The skin is prominent in being roughened by spines or tubercles that project from each of the numerous small scales covering the body. These scales enable grasping of fish and are sensory. A bundle of bristle-like structures is present in the dome of the tubercles, and the base of this structure is richly supplied with nerves. The skin between the small scales may be developed into smaller bristle-bearing tubercles. These are presumed to be sense organs that detect mechanical stimuli and aid in movement, orientation, and the capture of fish in waters where visibility can be extremely limited.

The tail is laterally compressed to assist swimming, although this feature is not as pronounced as it is in sea snakes. The nostrils are valved and located at the dorsal aspect of the snout. This feature enables these snakes to periodically breathe atmospheric air while the remainder of the body remains underwater. The vertebrae are relatively short and have a small condyle that is partially freed such that flexibility is enhanced for both swimming and constriction of fishes. The skull is flexible, quadrate bones are elongated, and features of articulation are well adapted for swallowing fishes.

File snakes have a heart similar to that of other snakes, but the position is more central (mid-body) than it is in many terrestrial species of snakes, in which the heart is closer to the head. Unlike that of terrestrial snakes, the lung of file snakes contains vascularized tissue for respiratory gas exchange that extends almost the entire length of the body cavity. The veins are capacious and accommodate a relatively large volume of blood. This characteristic is presumed to be adaptive with respect to storage of oxygen in support of prolonged dives.

Some interesting metabolic, respiratory, and cardiovascular adaptations are related to aquatic habits. The metabolic rate is relatively low compared with that of other snake species, and the low rate of energy use appears related to the generally sluggish lifestyle of file snakes. Laboratory studies of the Arafura file snake indicate that the capacity for generating metabolic energy is low and cannot sustain vigorous activity for more than a few minutes. These snakes are lung breathers but can remain submerged for several hours. The skin functions as an accessory respiratory organ and exchanges a considerable fraction of oxygen and carbon dioxide when snakes are in well-oxygenated water. Relatively long submergence times are related to the low metabolic rate, cutaneous gas exchange, sluggish behavior, and large oxygen store attributed to the elongated lung and to the presence of a large volume of circulating blood, which contains large amounts of red blood cells and hemoglobin.

File snakes prefer the high body temperatures achieved in shallow tropical waters. Body temperature typically is 77–86°F (25–30°C). These snakes largely conform to the temperatures that prevail in surrounding water, but there is evidence that the Arafura file snake selects specific thermal microhabitats where variation of body temperature is minimized. The little file snake can tolerate a range of water salinity from freshwater to seawater, and the other species tolerate water conditions ranging from fresh to brackish. The little file snake has a sublingual salt gland that is presumed to function in osmoregulation. Little is known, however, about the importance of this gland. Marine populations of this snake need fresh water, which they obtain from surface lenses of fresh water that form temporarily during rainstorms.

Distribution

Species of Acrochordidae are distributed in tropical waters, but there are important habitat distinctions. The range of the little file snake extends from the western coast of India through tropical Asia to the Philippines, south to Timor, and east to Papua New Guinea, northern Australia, the Bismarck Archipelago, and the Solomon Islands. The Java file snake ranges from Thailand through Malaysia and the Greater Sunda Islands of Indonesia. The range of the Arafura file snake appears confined to the freshwater drainages of Papua New Guinea and those of Australia connected to the Gulf of Carpentaria and the Arafura Sea.

Habitat

The smallest species, the little file snake, is primarily marine but tolerates water of varying salinity, including freshwater. These snakes are found in the sea but live more typically in mangroves or other areas of shallow coastal waters, including estuaries. Little file snakes have been captured at depths of 13–66 ft (4–20 m) as far as 1–6 mi (2–10 km) offshore, but shallow waters of a few feet (meters) or less are more typical of primary habitat. Populations enter rivers throughout the range, and a few populations are known to live in freshwater lakes in Papua New Guinea and the Philippines. The inland distribution of this species is probably limited by waterfalls rather than by lack of tolerance for freshwater.

The Java file snake is largely an inhabitant of lagoons and streams as well as other areas of permanent freshwater. The species also enters estuaries and the sea, but permanent occupation of marine habitats is unlikely. The Java file snake should be considered a freshwater species.

The Arafura file snake is a freshwater species that occupies tropical rivers and billabongs (dry streambeds that fill only seasonally), reaching high population densities in some of the drainage systems of northern Australia. Much of this habitat is subject to periodic fluctuation in water level caused by seasonal aridity in parts of the range. The snakes live in billabongs during the dry season but disperse into inundated grassland with the onset of wet-season flooding.

Behavior

All three species of file snake are aquatic and appear to be nocturnal. Nighttime activity seems related largely to foraging, and snakes have been observed patrolling shallow tidal pools, where fish tend to become concentrated. During daylight hours these snakes are very reclusive, hiding among tangled mangrove roots, river edges, or in holes and burrows. In captivity, these snakes bury themselves in mud. Java file snakes have been observed to burrow in riverbanks beneath the roots of trees, where they are sometimes found in large aggregations. Java file snakes are occasionally seen swimming, mostly because of the periodic need to breathe air at the water's surface. Although they are adept swimmers, file snakes often move by crawling sluggishly over muddy substrates at the bottom of streams or swamps. Younger, smaller snakes are seldom seen, and very little is known about their ecology.

Although movements of file snakes are generally described as sluggish, data on the Arafura file snake indicate these snakes sometimes make extensive daily excursions. Occasional specimens of the little file snake have been found on tidal mud flats, and it seems likely that file snakes might occasionally leave water to travel between bodies of water during times of tidal or seasonal fluctuation of water level. In at least some instances, however, file snakes are known to remain within a limited area year after year. Because file snakes have low energy requirements and exist in areas where fishes tend to be concentrated, population densities may be very high and are reported to be at least 100 snakes for every 2 acres (1 hectare) in some Australian billabongs.

Little is known about predation on file snakes. They may be eaten by crocodiles, various birds, and other animals and be captured by humans. It seems likely that the physiological and behavioral adaptations for prolonged submergence are related to avoidance of predation in shallow-water habitats.

Feeding ecology and diet

All three species of file snake feed almost exclusively on fishes, including carrion and a large variety of species. The little file snake appears to specialize on gobiid and goby-like fishes. Stomach contents indicate that crustaceans may be eaten occasionally. Australian Arafura file snakes consume a diversity of fish species and act as scavengers as well as predators in watercourse systems where they have been studied. Sleepy cod and barramundi are important prey. Java file snakes specialize on freshwater eels and catfishes in Malaysia. The spines of catfishes occasionally perforate the digestive tracts of these snakes and can cause wounds and abscesses.

File snakes feed by seizing fish rapidly with the mouth or by swiftly ensnaring fish in coils of the body or tail. In either

case, the body is wrapped quickly around the fish to subdue and to hold it. These snakes have been labeled constrictors, but the body coils function to hold and immobilize prey rather than to cause death. Captured fish are swallowed very quickly, thus little water is ingested with prey. File snakes have been observed to forage nocturnally in areas of shallow water, and captive specimens feed more readily and capture fish more easily in shallow than in deep water. Stomach contents indicate file snakes feed infrequently and appear to grow slowly.

Reproductive biology

All three species of file snake appear to have seasonal reproductive cycles, even though they are active year-round in tropical habitats. These snakes are viviparous, and their young are born alive. Gestation begins in the middle of the year, and birth usually occurs in December. Evidence suggests that female snakes do not give birth every year, so the reproductive frequency is less than annual. Little file snakes and Java file snakes reproduce, on average, once every two years. Arafura file snakes reproduce less frequently because of greater climatic variability and the aridity of the Australian habitat. Litter size averages five for the little file snake, 17 for the Arafura file snake, and 26 for the Java file snake. Information on Java file snakes indicates that larger females produce both larger litters and larger offspring.

Conservation status

Although no species are listed in the IUCN Red Book, there are two areas of concern in relation to the conservation status of acrochordid snakes. One is commercial exploitation, largely in relation to the skin trade, and the other is depletion of habitat and its quality. Commercial hunting of file snakes and sale of the products are prohibited by law in some countries where these snakes exist, but such protection is either absent or ineffectively enforced throughout much of Asia. Because of slow growth and low reproductive frequency, file snakes may be quite sensitive to harvesting in areas where such exploitation is intensive. On the other hand, much of the habitat of these snakes is relatively inaccessible to large numbers of human hunters. Other potential threats to file snakes are related to physical disturbances of rivers, estuaries, and wetlands and to various forms of water pollution. The effect of such factors on the distribution and abundance of file snakes is almost totally unknown and is in need of evaluation.

Significance to humans

Female Arafura file snakes are hunted by indigenous peoples and used as food in parts of Australia. Because of the pronounced sexual dimorphism that characterizes this species, the larger females are more easily found and captured by aboriginal hunters. Such subsistence use of these snakes by relatively small numbers of people appears to have little effect on populations of this species.

The other two species of file snake are harvested commercially for their skins. The skin of the Java file snake is used for karung, ornamental leather, and is heavily harvested in the Indonesian archipelago. It has been difficult to estimate how widespread or intensive such exploitation has become.

Both the Java file snake and the little file snake have made occasional appearances in the pet trade, but use of these snakes as pets appears to be uncommon. Given the difficulties associated with maintaining these snakes in captivity, exploitation related to the pet trade seems to have little importance and may likely disappear altogether.

Resources

Periodicals

Dowling, Herndon.G. "The Curious Feeding Habits of the Java Wart Snake." Animal Kingdom 63 (1960): 13–15.

Lillywhite, Harvey B. "The Biology and Conservation of Acrochordid Snakes." Hamadryad 16 (1991): 1–9.

——. "Husbandry of the Little File Snake, Acrochordus granulatus." Zoo Biology 15 (1996): 315–327.

Lillywhite, Harvey B., and Tamir M. Ellis. "Ecophysiological Aspects of the Coastal-Estuarine Distribution of Acrochordid Snakes." Estuaries 17 (1994): 53–61.

McDowell, S. B. "A Catalogue of the Snakes of New Guinea and the Solomons, with Special Reference to Those in the Bernice P. Bishop Museum: Part III, Boinae and Acrochordoidea (Reptilia, Serpentes)." Journal of Herpetology 13 (1979): 1–92.

Seymour, Roger S., G. P. Dobson, and John Baldwin. "Respiratory and Cardiovascular Physiology of the Aquatic Snake, Acrochordus arafurae." Journal of Comparative Physiology 144 (1981): 215–227.

Shine, Richard. "Ecology of a Low-Energy Specialist: Food Habits and Reproductive Biology of the Arafura Filesnake (Acrochordidae)." Copeia 1986 (1986): 424–437.

Shine, Richard, and R. Lambeck. "A Radiotelemetric Study of Movements, Thermoregulation and Niche Utilization in an Aquatic Snake, Acrochordus arafurae." Herpetologica 41 (1985): 351–361.

Voris, Harold K., and G. S. Glodek. "Habitat, Diet and Reproduction of the File Snake, Acrochordus granulatus, in the Straits of Malacca." Journal of Herpetology 14 (1980): 105–108.

Harvey B. Lillywhite, PhD