Dragonsnakes and Filesnakes Revisited

Click here to read this post in Spanish
Haga clic aquí para leer este mensaje en español

I've written about both filesnakes (family Acrochordidae) and dragonsnakes (part of the family Xenodermidae¹) before. Traditional snake taxonomy suggests that, although they branch off from the main stem of the snake family tree at about the same time, they're not very closely related. But, new evidence emphasizes the uniqueness of dragonsnakes and thickens the plot in the unfolding story of the evolution of snakes.

Two hypotheses about the relationships of the major groups of snakes.
Left: tree based on nuclear genes, showing Acrochordidae and Xenodermidae
as successive outgroups to core Colubroidea
Right: tree based on mitochondrial genes, showing a sister relationship
between Acrochordidae and Xenodermidae
From Oguiura et al. 2009

Most phylogenetic analyses are pretty consistent in classifying both filesnakes and dragonsnakes as caenophidians, or "advanced" snakes. But, they differ in their placement of dragonsnakes and other xenodermids, including the truly strange and obscure odd-scaled snakes (Achalinus), bearded snakes (Fimbrios), stream, earth, or red snakes (Stoliczkia²), wood, mountain, or narrow-headed snakes (Xylophis), and a new genus, just described in 2015 and still without a common name, Parafimbrios. Most analyses group xenodermids with the colubroids (pareids¹, vipers, homalopsids, colubrids, lamprophiids, and elapids), albeit as the most basal branch. Many textbooks actually define Caenophidia as Colubroidea + Acrochordidae (aka Acrochordoidea), distinctly separating the colubroids from the filesnakes on the basis of shared, derived characteristics such as wide ventral scales, as well as features of the skull, hemipenes, and the muscles, cartilages, and arteries between the ribs. However, several recent trees based on DNA sequences suggest instead that filesnakes and dragonsnakes might be one another's closest living relatives.

Study	Acrochordid-Xenodermid Relationship	Support	How many species?	What data were used?
Boulenger 1893	X&A	-	-	Morphology
Kraus & Brown 1998	X+A	8%	37	ND4
Kelly et al. 2003	X+A	98%	98	4 mitochondrial genes
Vidal et al. 2007	A,X	not reported	25	7 nuclear genes
Wiens et al. 2008	A,X	>95%	50	20 nuclear genes
Wang et al. 2009	A,X*	100%	30	12 nuclear genes
Zaher et al. 2009	A,X	94%	131	2 mitochondrial genes + 1 nuclear gene
Pyron et al. 2011	A,X	97-100%	761	3 mitochondrial genes + 2 nuclear genes
Gauthier et al. 2012	X,A	95-100%	141 extant + 51 extinct	610 morphological characters
Wiens et al. 2012	A,X	100%	161	44 nuclear genes
Pyron et al. 2013	X+A	95%	4,161	5 mitochondrial genes + 7 nuclear genes
Pyron et al. 2014	A,X*	99%	32	333 nuclear loci with 100% coverage
Zheng & Wiens 2016	A,X	91%	4,162	5 mitochondrial genes + 47 nuclear genes

A selection of studies that have examined the relationship between acrochordids and xenodermids.

X+A means that the two are each other's closest relatives; A,X means that acrochordids are more distantly
related to colubroids than xenodermids; X,A means that xenodermids are more distant
*Relationships differed depending on which methods were used

Arafura Filesnake (Acrochordus arafurae)

For example, the first study to use DNA to examine the relationships of these two groups of snakes found some support for each hypothesis, concluding that the "potential sister-taxon relationship of acrochordids and xenodermines [is] a reasonable hypothesis requiring future testing." In 2003, data from three more mitochondrial genes resulted in the same relationship, causing the authors to suggest that xenodermids should be excluded from Colubroidea. However, since that time, numerous studies have not repeated this result. In 2009, one research group predicted that "these differences...are due to taxonomic sampling issues", predicting that as DNA was collected from more species of snakes, the basal position of Acrochordus would be confirmed.

Dragonsnake (Xenodermus javanicus)

So, it was a real surprise when a 2013 analysis, the largest yet, including samples from 80% of all snake genera, placed Acrochordidae and Xenodermidae as sister groups. Neither a follow-up analysis combining that dataset with one containing data from many more genes nor an analysis using only the most complete data have settled the issue. The latter study compared several methods for generating phylogenetic trees and found that the relationship between acrochordids and xenodermids depended a lot on which methods were used. Part of the problem is that, even if they are each others' closest relatives, they still diverged between 70 and 80 million years ago, making them susceptible to a problem in phylogenetics known as long-branch attraction, which happens when the amount of evolutionary change within a lineage causes that lineage to appear similar (and thus closely related) to another long-branched lineage, solely because they have both undergone a lot of change, rather than because they are actually related.

Bearded Snake (Fimbrios klossi)

The truth is that both acrochordids and xenodermids are obscure snakes, and we don't have that much data on either one of them. They are both found in areas of the world that are hard to get to. Morphologically, they appear superficially similar, and an association between them was first hypothesized in 1893. But, even the most comprehensive morphological trait database for snakes is missing crucial data on their anatomy, such as whether or not their hemipenial spines are mineralized. This would be helpful to know because  the hemipenial spines of basal snakes such as boas and pythons are not mineralized, whereas those of definitive colubroids are heavily mineralized.

Parafimbrios lao
From Teynié et al. 2015

Within the past year, two new studies on the chromosomes of dragonsnakes (Xenodermus javanicus) have been published. In the first, the karyotype (the number of chromosomes and their shape) of dragonsnakes was reported for the first time. In humans, each cell normally contains 23 pairs of chromosomes, for a total of 46. In most snakes, each cell normally contains 18 pairs of chromosomes, for a total of 36. Usually, eight of these pairs are relatively large (called macrochromosomes), and the other ten are somewhat small (called microchromosomes). Dragonsnakes have 16 pairs of chromosomes, for a total of 32, of which seven are large and nine are small. The dragonsnake karyotype probably evolved by two fusion events, one of two macrochromosomes and the other between a macrochromosome with a microchromosome. There are some other exceptions to the 18-pair pattern; some snakes have as few as 12 or as many as 25 pairs, including the only other xenodermid to have been karyotyped, the Sichuan Odd-scaled Snake (Achalinus meiguensis), which has just 12 pairs of chromosomes.

Amami Odd-scaled Snake (Achalinus werneri)

From the 1960s to the 1980s, before DNA sequencing became cheap and easy, scientists invested heavily in collecting karyotypes from a diversity of species for comparative purposes, so we can say with pretty good certainty that the ancestral state for all snakes is 36 (18 pairs). That's the number in filesnakes, pareids, most vipers, homalopsids, and many colubrids, lamprophiids, and elapids, although there are lots of exceptions in the latter three groups.  The fusions in xenodermids emphasize their uniqueness, but unfortunately don't shed any new light on their phylogenetic placement.

Stoliczkia borneensis

The other study focused on the sex chromosomes. In humans, sex is determined by which combination of sex chromosomes a baby receives from its parents: two X chromosomes make a female, whereas an X and a Y chromosome make a male. It's pretty similar in snakes, with a twist: the sex chromosomes are called Z and W instead of X any Y, and females are the heterogametic sex (meaning that a Z and a W chromosome make a female, and two Z chromosomes make a male). Birds and many other reptiles also have ZW sex determination. In many colubroid snakes, the W chromosome is about twice the size of the Z,  and it is often unusual in other ways as well, such as having sections of highly condensed chromatin or a different centromere position. In contrast, filesnakes, boids, and other more basal snakes have morphologically indistinguishable Z and W chromosomes, although they still contain different genes and perform different functions.

Perrotet's Narrow-headed Snake (Xylophis perroteti)
Are members of this genus really xenodermids? Or, like the
former xenodermids Oxyrhabdium and Nothopsis, will they
prove to be more closely related to something else?

One reason the W chromosome looks so different from the Z in colubroids is that it contains repetitive elements called Bkm ('banded krait minorsatellite') repeats, which consist of the sequence "GATA" (sometimes "GACA") repeated thousands of times. Mammalian X chromosomes and avian W chromosomes also have these repeats. Cell biologists think that these repeats function to inactivate all the genes on the W chromosome except for those that determine sex³. Both mammalian X chromosomes and snake W chromosomes become very dense in body cells, so that none of the genes on them can be expressed. They only decondense and plays their brief, female-determining roles, in maturing eggs that are destined to become females. Unlike in mammals, the sex chromosomes of snakes span the gamut from completely identical to markedly differentiated, allowing biologists to study the evolution of chromosomal sex determination. The new study showed that female dragonsnakes have two different-looking sex chromosomes, with many Bkm repeats in the W, whereas the two Z sex chromosomes of male dragonsnakes look similar and lacked Bkm repeats, bolstering the relationship between xenodermids and other colubroids and diminishing the relationship between xenodermids and filesnakes.

The other major finding of the new study is the documentation that at least part of the sex chromosomes are homologous across all families of caenophidian snakes, suggesting that snake sex chromosomes emerged in the common ancestor of Caenophidia some 60-80 million years ago. One gene that is only on the Z chromosome in all caenophidians, including dragonsnakes, is also found on the W chromosome in filesnakes. The Z-chromosome-specific genes in caenophidians were on both the Z and W chromosomes in boas, pythons, and sunbeam snakes (Xenopeltidae), as well as in bearded dragons and anoles. Other toxicoferan lizards with ZW sex chromosomes, including chameleons and monitor lizards, seem to have evolved them independently.

---

1 A recent article in the journal Herpetological Review pointed out that the grammatical rules for structuring family and subfamily names from genus names have recently been incorrectly applied in two cases involving snakes which concern this article: 1) Xenodermatidae/inae for the family/subfamily containing Xenodermus, the root of which is "dermus", a masculine noun with which the masculine specific epithet javanicus is correctly coupled (not the neuter javanicum; in contrast think of the neuter Heloderma horridum in family Helodermatidae). The correct family or subfamily name is thus Xenodermidae/inae. 2) Pareatidae or Pareatinae for the family containing Pareas, which is also masculine, making the correct family/subfamily name Pareidae/inae.^↩

---

2 Don't confuse this snake genus (Stoliczkia) with a genus of extinct ammonite (Stoliczkaia), both named for Czech biologist Ferdinand Stoliczka. The extra "a" was added to the original spelling of the snake genus by Boulenger in 1899, probably by accident, and this genus is still widely misspelled today (e.g., on GenBank and on Wikipedia before I fixed it while writing this article).^↩

---

3 It's also thought that "GATA" is a particularly potent regulatory sequence, with the power to turn nearby genes on and off. In a way, the sex genes have essentially 'hijacked' the W chromosome, turning off all its other genes, and simultaneously creating a concentrated source of mutation-causing elements. Chromosomal sex determination may therefore constitute a unique and potentially very powerful genotypic mechanism for abruptly enhancing evolutionary rates, which might have contributed to the explosive radiations of species in clades with chromosomal sex determination, such as mammals, birds, squamates, and certain groups of insects.^↩

ACKNOWLEDGMENTS

Thanks to Thomas Calame, Sam Howard, Konrad Mebert, Zeeshan Mirza, Takehito Sato, and Stephen Zozaya for the use of their photos.

REFERENCES

Boulenger, G. A. 1893. Catalogue of the Snakes in the British Museum (Natural History). Volume I., containing the families Typhlopidae, Glauconiidae, Boidae, Ilysiidae, Uropeltidae, Xenopeltidae, and Colubridae Aglyphae, Part. Trustees of the British Museum, London. <link>

Boulenger, G. A. 1899. Description of three new reptiles and a new batrachian from Mt. Kina Balu, North Borneo. Annals and Magazine of Natural History 7:451-453 <link>

Gauthier, J. A., M. Kearney, J. A. Maisano, O. Rieppel, and A. D. B. Behlke. 2012. Assembling the squamate Tree of Life: perspectives from the phenotype and the fossil record. Bulletin of the Peabody Museum of Natural History 53:3-308 <link>

Jerdon, T. C. 1870. Notes on Indian Herpetology. Proceedings of the Asiatic Society of Bengal 1870:66-85 <link>

Jones, K., and L. Singh. 1985. Snakes and the evolution of sex chromosomes. Trends in Genetics 1:55-61 <link>

Lawson, R., J. B. Slowinski, B. I. Crother, and F. T. Burbrink. 2005. Phylogeny of the Colubroidea (Serpentes): new evidence from mitochondrial and nuclear genes. Molecular Phylogenetics and Evolution 37:581-601 <link>

Kelly, C. M. R., N. P. Barker, and M. H. Villet. 2003. Phylogenetics of advanced snakes (Caenophidia) based on four mitochondrial genes. Systematic Biology 52:439-459 <link>

Kraus, F., and W. M. Brown. 1998. Phylogenetic relationships of colubroid snakes based on mitochondrial DNA sequences. Zoological Journal of the Linnean Society 122:455-487 <link>

Oguiura, N., H. Ferrarezzi, and R. Batistic. 2009. Cytogenetics and molecular data in snakes: a phylogenetic approach. Cytogenetic and Genome Research 127:128-142 <link>

O’Meally, D., H. R. Patel, R. Stiglec, S. D. Sarre, A. Georges, J. A. M. Graves, and T. Ezaz. 2010. Non-homologous sex chromosomes of birds and snakes share repetitive sequences. Chromosome Research 18:787-800 <link>

Pokorna, M., and L. Kratochvíl. 2009. Phylogeny of sex‐determining mechanisms in squamate reptiles: are sex chromosomes an evolutionary trap? Zoological Journal of the Linnean Society 156:168-183 <link>

Pyron, R. A., F. T. Burbrink, G. R. Colli, A. N. M. de Oca, L. J. Vitt, C. A. Kuczynski, and J. J. Wiens. 2011. The phylogeny of advanced snakes (Colubroidea), with discovery of a new subfamily and comparison of support methods for likelihood trees. Molecular Phylogenetics and Evolution 58:329-342 <link>

Pyron, R. A., F. Burbrink, and J. J. Wiens. 2013. A phylogeny and revised classification of Squamata, including 4161 species of lizards and snakes. BMC Biology 13:53 <link>

Pyron, R. A., C. R. Hendry, V. M. Chou, E. M. Lemmon, A. R. Lemmon, and F. T. Burbrink. 2014. Effectiveness of phylogenomic data and coalescent species-tree methods for resolving difficult nodes in the phylogeny of advanced snakes (Serpentes: Caenophidia). Molecular Phylogenetics and Evolution 81:221-231 <link>

Rovatsos, M., M. Johnson Pokorná, and L. Kratochvíl. 2015. Differentiation of sex chromosomes and karyotype characterisation in the Dragonsnake Xenodermus javanicus (Squamata: Xenodermatidae). Cytogenetic and Genome Research 147:48-54 <link>

Rovatsos, M., J. Vukić, P. Lymberakis, and L. Kratochvíl. 2015. Evolutionary stability of sex chromosomes in snakes. Proceedings of the Royal Society B: Biological Sciences 282:20151992 <link>

Savage, J. M. 2015. What are the correct family names for the taxa that include the snake genera Xenodermus, Pareas, and Calamaria? Herpetological Review 46:664-665 <link>

Sharma, G., and U. Nakhasi. 1980. Karyological studies on six species of Indian snakes (Colubridae: Reptilia). Cytobios 27:177-192 link>

Teynié, A., P. David, A. Lottier, M. D. Le, N. Visal, and T. Q. Nguyan. 2015. A new genus and species of xenodermatid snake (Squamata: Caenophidia: Xenodermatidae) from northern Lao People’s Democratic Republic. Zootaxa 3926:523-540 <link>

Vicoso, B., J. Emerson, Y. Zektser, S. Mahajan, and D. Bachtrog. 2013. Comparative sex chromosome genomics in snakes: differentiation, evolutionary strata, and lack of global dosage compensation. PLoS Biology 11:e1001643 <link>

Vidal, N., A. S. Delmas, P. David, C. Cruaud, A. Couloux, and S. B. Hedges. 2007. The phylogeny and classification of caenophidian snakes inferred from seven nuclear protein-coding genes. Comptes Rendus Biologies 330:182-187 <link>

Wang, G., S. He, S. Huang, M. He, and E. Zhao. 2009. The complete mitochondrial DNA sequence and the phylogenetic position of Achalinus meiguensis (Reptilia: Squamata). Chinese Science Bulletin 54:1713-1724 <link>

Wiens, J. J., C. A. Kuczynski, S. A. Smith, D. G. Mulcahy, J. W. Sites, T. M. Townsend, and T. W. Reeder. 2008. Branch lengths, support, and congruence: testing the phylogenomic approach with 20 nuclear loci in snakes. Systematic Biology 57:420-431 <link>

Wiens, J. J., C. R. Hutter, D. G. Mulcahy, B. P. Noonan, T. M. Townsend, J. W. Sites, and T. W. Reeder. 2012. Resolving the phylogeny of lizards and snakes (Squamata) with extensive sampling of genes and species. Biology Letters 8:1043-1046 <link>

Zaher, H., F. G. Grazziotin, J. E. Cadle, R. W. Murphy, J. C. Moura-Leite, and S. L. Bonatto. 2009. Molecular phylogeny of advanced snakes (Serpentes, Caenophidia) with an emphasis on South American Xenodontines: A revised classification and descriptions of new taxa. Papeis Avulsos de Zoologia (Sao Paulo) 49:115-153 <link>

Zheng, Y., and J. J. Wiens. 2016. Combining phylogenomic and supermatrix approaches, and a time-calibrated phylogeny for squamate reptiles (lizards and snakes) based on 52 genes and 4162 species. Molecular Phylogenetics and Evolution 94:537-547 <link>

Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.

Filesnakes, Wartsnakes, or Elephant Trunksnakes

Click here to read this post in Spanish
Haga clic aquí para leer este blog en español

Arafura Filesnake (Acrochordus arafurae)

In the swamps, marshes, streams, and estuaries of northern Australia and southeastern Asia live ancient snakes as thick as your arm, with tongues as thin as a thread, skin as rough as a file, and a disposition as gentle as a lamb. These snakes comprise the family Acrochordidae (from the Greek akrochordon, wart), and are known as filesnakes¹, wartsnakes, or elephant trunksnakes. In Indonesian they are known as karung, which means 'sack'; in Thai, as ngū nguang-cĥāng, 'elephant-trunk snake'. There are three species, all in the genus Acrochordus: Javan (A. janavicus), Arafura (A. arafurae), and Little (A. granulatus). The largest, female Javans, grow up to 8 feet and over 20 pounds. Acrochordids are an old and highly distinct group of snakes, distantly related to colubroids, with which they share a common ancestor between 50 and 70, but possibly as long as 90 million years ago.

Close-up of A. arafurae scales

Filesnakes have strongly-keeled scales with the texture of sandpaper or a coarse file, after which they are named. They have very loose, baggy skin. I held one once and it felt like a human arm inside the sleeve of a very sturdy, very baggy rain poncho made out of chain mail. This loose skin is likely an adaptation that allows filesnakes to withstand the great force of the initial dash for freedom of their fish prey. Their sharp scales are used to help gain purchase on slimy fish skin during constriction. They have sensory, bristle-bearing tubercles on the skin between their scales, as well as sensory organs on the scales themselves, both of which presumably help them sense the underwater movements of nearby prey, analogous to the lateral line systems of fishes and some amphibians. Acrochordids sometimes ambush their prey, but more often they forage by searching slowly along the edges of mangroves, billabongs, and other water bodies at night, looking for sleeping fishes and crustaceans (although they don't tear them apart like some southeast Asian snakes). During the day, filesnakes hide in the shadows of overhanging trees, moving with them to remain concealed from predatory birds. They are nearly incapable of moving on land, and shed in the water using a knotting behavior similar to that of Pelamis platurus, the most completely aquatic sea snake.

Acrochordus arafurae regurgitates
an eel-tailed catfish (Tandanus tandanus)
whose spine has pierced its neck

Filesnakes occupy a unique phylogenetic position, not closely related to anything but somewhere in-between the colubroids ("advanced snakes") and the "henophidians" (boas, pythons, and uropeltoids). A few recent papers reanimate an old hypothesis that they might be closely related to dragonsnakes, but historically acrochordids have been considered the sister group to all colubroids, a group of >2,850 species (>80% of all living snakes) that includes dragonsnakes, asymmetrical slug-eaters, vipers, homalopsids, elapids, lamprophiids, and strict colubrids. Colubroidea and the three Acrochordus species together form the Caenophidia ("recent snakes"). Acrochordids share unspecialized head scales and undifferentiated ventral scales with boas and pythons, but they are united with colubroids in that they totally lack vestigial limbs and have spines on their hemipenes, a well-developed vomeronasal system, and several particular characteristics of skull morphology, including a coronoid bone. Other features of the skull and skeleton are unique to acrochordids, including the aforementioned skin sense organs, a passive joint between the frontal and parietal skull bones, the presence of certain holes in the vertebrae, the shape of the head of the ribs, and an ear region that most closely resembles the ears that other snakes have as embryos, but which forms in a different way. Acrochordids also have an unusual lung morphology, with a double row of holes leading from the trachea into individual small lunglets, and a more tangled intestinal tract than other snakes.

Acrochordus granulatus with algae growing on its back

Filesnakes have incredibly low metabolic rates, even for a snake, and cannot sustain rigorous physical activity for very long. In captivity, they "epitomize sluggishness in snakes", although radio telemetry has shown that in the wild they move around wetlands slowly but steadily, covering up to 450 feet per night. They can remain submerged for over an hour (record 2 h 20 min), and surface to take about 5 breaths, about one per minute. The first several of these breaths oxygenate the blood, and the last one fills the multi-chambered lung. In addition, Little Filesnakes have about twice as much blood as other snakes, and this voluminous blood is about twice as thick with red blood cells as even that of other diving snakes. Their hemoglobin has a very high affinity for oxygen, which results in their being able to store between three and fifteen times as much oxygen in their blood as a similarly-sized sea snake, and release it slowly over a long period of time. Many turtles also use this strategy. Also like turtles, filesnakes can both obtain oxygen from and release acidic carbon dioxide into the water through their skin, which helps prolong their dives.² In fact, filesnakes are so well-adapted to sitting still that they are practically incapable of exercise, and get tired out quickly.

This slow theme carries over into filesnake life history. Male filesnakes mature around six years old, females around nine, and 8-10 years may elapse between consecutive births. Studies from northern Australia found that only a small proportion of females are reproductive in any given year, and that only the very largest females reproduce relatively frequently. Large Javan Filesnakes give birth to as many as 52 young at once, although the average is closer to 30. Arafura Filesnakes average about 16 (as few as 9 and as many as 25 have been reported), and Little Filesnakes about 6 (as few as 1 and as many as 12). Female filesnakes are courted by up to eight males at a time in shallow water. Their population dynamics are driven by rainfall in northern Australia. One captive filesnake gave birth to a single young after seven years of isolation, suggesting that filesnakes are either capable of parthenogenesis or of very prolonged sperm storage.

The Little Filesnake (Acrochordus granulatus) has
a banded pattern like a sea krait (Laticauda colubrina)

At first glance the three extant Acrochordus species seem quite similar, but in fact they exhibit striking differences in both anatomy and ecology. The Little Filesnake (Acrochordus granulatus) was described in 1799 and used to be classified in a separate, monotypic genus (Chersydrus). As its name suggests, it is the smallest acrochordid (~ 3 feet in total length) and the most widely distributed. It is found along the coast from northwestern India throughout southeast Asia and Indonesia, reaching east to the Solomon Islands. Its diverse habitats include freshwater lakes, rivers, mangroves, mudflats, reefs, and the open ocean, up to 6 miles offshore and over 60 feet deep. It is the most marine of the filesnakes, the most brightly patterned, and has a shorter, more laterally-compressed tail, more granular scales, more dorsally-oriented nostrils, and a salt excretion gland beneath its tongue.³

Acrochordus javanicus

The Javan Filesnake (Acrochordus javanicus) was the first to be described, in 1787, and is the largest and heaviest filesnake, sometimes reaching 8 feet and over 20 pounds. It is found in fresh and brackish water on the Malay Peninsula and on the islands of Sumatra, Java, and Borneo (and was introduced to Florida in the 1980s, although it does not appear to have established there). It is harvested for meat and for its skin, out of which is made fine leather; up to 2 million are exported from Indonesia annually. Unlike other filesnakes, the posterior-most teeth in its lower jaw have sharp edges. The Arafura Filesnake (Acrochordus arafurae) was thought to be the same species as the Javan until 1979. It grows as long but at the same size is only about half as heavy-bodied. It is found only in freshwater habitats in northern Australia and southern New Guinea. Surprisingly, A. arafurae is more closely related to A. granulatus than either is to A. javanicus, a relationship that is supported by genetics as well as morphology.

The long, thin tongue of Acrochordus javanicus
From Greene (1997)

Fossil Acrochordus have been found in Pakistan and Nepal, as well as within the extant range. These extinct filesnakes date from 5-20 million years ago during the Miocene, only a few million years after the Indian plate crashed into Asia to form the Himalayas. They grew larger than modern filesnakes, reaching at least 9 feet, and are the most well-represented snakes in the southern Asian fossil record, possibly because their habitat lends itself well to fossilization. The extinct species Acrochordus dehmi is represented by over 1000 fossils from over 100 different locations, and probably went extinct about 6 million years ago. Because it is so well-known, we can say with confidence that it is more closely related to A. javanicus than to the other two living species of Acrochordus. Molecular clock methods suggest that the three modern species of Acrochordus and A. dehmi diverged from one another 16-20 million years ago, a timescale that usually justifies separation into family-level or higher categories. Despite their superficial similarities, the ecological and morphological differences among the three living Acrochordus species have been considered equivalent to differences among genera in other groups of snakes. Because no fossil acrochordids have been found in Australia, it is assumed that they evolved in Asia and spread to Australia in the last 5 million years. It is also likely that the ancestors of the Little Filesnake entered the ocean before sea snakes (~7 mya) and kraits (~13 mya) and just after marine homalopsids (~18 mya).

Acrochordus in contemporary aboriginal artwork by Chris Liddy (Moonggun),
showing the embryos inside the snake in the northern Australian style

One of the most interesting things about filesnakes is that Aboriginal Australians collect and eat them in some areas. Mostly this is done at the end of the Australian dry season, in November, when water levels are lowest and the snakes are easiest to find and capture. Although the snakes themselves don't generally put up much resistance, the old women who hunt them do so by wading into murky waters filled with crocodiles and feel under overhanging banks, weed beds, and logs, sometimes collecting over 30 snakes per person-hour. Often the snakes are killed immediately by biting their necks. The pregnant females are highly prized for their embryos, which are cooked on hot ashes, eaten like popcorn, and called 'cookies' by Aboriginal children.

---

1 Not to be confused with African Filesnakes (genus Mehelya), which are so-named not for their texture but for their cross-sectional shape, which resembles a triangular file. ^↩

---

2 Although the warm, shallow, slow-moving waters in which they live are fairly oxygen-poor and oxygen is difficult to extract out of salty water, so augmenting their ability to hold their breath using their massive blood oxygen reservoir is almost certainly of greater importance. ^↩

---

3 Little Filesnakes can excrete salt but gradually get dehydrated, so they must have a source of fresh water. They drink rain that falls on the ocean or migrate to areas where rivers flow into estuaries. This is because, like other marine reptiles, filesnakes "pee like a fish": they excrete nitrogen as ammonia, rather than as uric acid like other snakes or as urea like mammals. This is much more wasteful of water than the uric acid method, and it's not clear why they do this.^↩

ACKNOWLEDGMENTS

Thanks to Chris Liddy, Matt Summerville, Darryl Houston, M. & P. Fogden, Jordan de Jong, Stephen Zozaya, Jason Isley, and Dick Bartlett for their photos, and to Rick Shine for information on tracking down Darryl Houston.

REFERENCES

Boulenger, G. A. 1893. Catalogue of the snakes in the British Museum (Natural History). Trustees of the British Museum, London <link>

Feder, M. E. 1980. Blood oxygen stores in the file snake, Acrochordus granulatus, and in other marine snakes. Physiological Zoology 53:394-401 <link>

Head, J. J. 2005. Snakes of the Siwalik Group (Miocene of Pakistan): systematics and relationship to environmental change. Palaeontologia Electronica 8:81-33. <link>

Head, J. J., D. M. Mohabey, and J. A. Wilson. 2007. Acrochordus Hornstedt (Serpentes, Caenophidia) from the Miocene of Gujarat, Western India: temporal constraints on dispersal of a derived snake. Journal of Vertebrate Paleontology 27:720-723. <link>

Heatwole, H. and R. Seymour. 1975. Pulmonary and cutaneous oxygen uptake in sea snakes and a file snake. Comparative Biochemistry and Physiology Part A: Physiology 51:399-405 <link>

Houston, D. and R. Shine. 1994. Movements and activity patterns of Arafura filesnakes (Serpentes: Acrochordidae) in tropical Australia. Herpetologica 50:349-357 <link>

Lillywhite, H. B. and T. M. Ellis. 1994. Ecophysiological aspects of the coastal-estuarine distribution of acrochordid snakes. Estuaries 17:53-61. <link>

Lillywhite, H. B., A. W. Smits, and M. E. Feder. 1988. Body fluid volumes in the aquatic snake, Acrochordus granulatus. Journal of Herpetology 22:434-438 <link>

Madsen, T. and R. Shine. 2000. Rain, fish and snakes: climatically driven population dynamics of Arafura filesnakes in tropical Australia. Oecologia 124:208-215 <link>

Magnusson, W. A. 1979. Production of an embryo by an Acrochordus javanicus isolated for seven years. Copeia 1979:744-745 <link>

McDowell, S. B. 1975. A catalogue of the snakes of New Guinea and the Solomons, with special reference to those in the Bernice P. Bishop Museum. Part II. Anilioidea and Pythoninae. Journal of Herpetology 9:1-79 <link>

McDowell, S. B. 1979. 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 13:1-92 <link>

Intertubercular papilla of Acrochordus granulatus
From Povel & Van Der Kooij 1996

Povel, D. and J. Van Der Kooij. 1996. Scale sensillae of the file snake (Serpentes: Acrochordidae) and some other aquatic and burrowing snakes. Netherlands Journal of Zoology 47:443-456 <link>

Pyron RA, Burbrink F, Wiens JJ, 2013. A phylogeny and revised classification of Squamata, including 4161 species of lizards and snakes. BMC Biology 13 <link>

Rasmussen, A. R., J. C. Murphy, M. Ompi, J. W. Gibbons, and P. Uetz. 2011. Marine Reptiles. PLoS ONE 6:e27373 <link>

Rieppel, O. and H. Zaher. 2001. The development of the skull in Acrochordus granulatus (Schneider)(Reptilia: Serpentes), with special consideration of the otico‐occipital complex. Journal of Morphology 249:252-266 <link>

Sanders KL, Mumpuni, Hamidy A, Jead J, Gower D, 2010. Phylogeny and divergence times of filesnakes (Acrochordus): inferences from morphology, fossils and three molecular loci. Molecular Phylogenetics and Evolution 56:857-867 <link>

Seymour, R., G. Dobson, and J. Baldwin. 1981. Respiratory and cardiovascular physiology of the aquatic snake, Acrochordus arafurae. Journal of Comparative Physiology 144:215-227 <link>

Shine R, 1995. Australian Snakes: A Natural History Ithaca, New York: Cornell University Press <link>

Shine, R. 1986. Sexual differences in morphology and niche utilization in an aquatic snake, Acrochordus arafurae. Oecologia 69:260-267 <link>

Shine, R. 1986. Ecology of a low-energy specialist: food habits and reproductive biology of the arafura filesnake (Acrochordidae). Copeia 10:424-437 <link>

Shine, R. 1986. Predation upon filesnakes (Acrochordus arafurae) by aboriginal hunters: selectivity with respect to body size, sex and reproductive condition. Copeia 10:238-239 <link>

Shine, R. and D. Houston. 1993. Acrochordidae. in C. Glasby, G. Ross, and P. Beesley, editors. Fauna of Australia. AGPS, Canberra <link>

Shine, R., P. Harlow, J. S. Keogh, and Boeadi. 1995. Biology and commercial utilization of acrochordid snakes, with special reference to karung (Acrochordus javanicus). Journal of Herpetology 29:352-360 <link>

Voris, H. K. and G. S. Glodek. 1980. Habitat, diet, and reproduction of the file snake, Acrochordus granulatus, in the straits of Malacca. Journal of Herpetology 14:108-111 <link>

Life is Short, but Snakes are Long by Andrew M. Durso is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.