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 cav ity. 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.
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