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The crab-eating snake, Fordonia leucobalia, is a widespread coastal species. At the western edge of its range Fordonia is known from India’s Nicobar Islands and on the coastal mainland from Bangladesh and Myanmar it ranges eastward along the coasts of islands and the mainland throughout Southeast Asia to the Philippines, and southward to New Guinea and Northern Australia. Like other coastal marine homalopsids this snake probably does not wander far inland or in open ocean environments accept as waifs.
Common Names for this species include: crab-eating water snake, white-bellied freshwater snake, the Fordonia, the plain Fordonia; the crab-eater, white-bellied water-snake, and Fordon’s water snake.
In Southeast Asia Fordonia is almost always a uniform black with a white belly, however in Australia, New Guinea and possibly in extreme eastern Indonesia (Timor) it has multiple color morphs. Fordonia has 23 - 27 scale rows at midbody but the most frequently encountered count is 25 rows (85%); it also has 5 - 7 (usually six) upper labials, a low count for a homalopsid; and it usually lacks a loreal scale. Fordonia also has an internasal scale completely separating the two nasal scales, a character state that will immediately separate if from all Enhydris. In its mangrove and mud flat habitat it is most likely to be confused with Gerarda prevostiana which also has a uniform black dorsum over most of its range. However, Gerarda has 17 scale rows at midbody. Fordonia has also been mistaken for Enhydris punctata that has a similar number of scale rows at midbody, but it possesses a loreal scale, has 12 or more upper labials, and nasal scales that contact each other.
Rooij (1917) mentioned a specimen that had a total length of 930 mm and a 110 mm tail. Smith (1943) reported a male that had a total length of 780 mm with a 100 mm tail and a female that was 1065 mm in total length with a 125 mm tail. Bergman (1960) suggested newborns are less than 180 mm SVL, and adults are sexually mature at 330 mm SVL. His largest male had a total length of 606 mm, and a 95 mm tail; his largest female had a total length of 637 mm with a 72 mm tail. Bergman (1960) noted males have longer tails than females, and reported that Kopstein found females to have 32 - 36 subcaudal scales; males have 39 - 45 subcaudal scales in the Java population. The tail/SVL ratio for the New Guinea population determined during this study was 14 - 18% in males and 12 - 15% in females (males n = 10, females n = 8).
The crab-eating snake inhabits mangrove forest and associated mud flats. It may on occasion enter surrounding habits such as monsoon forest or open ocean but it is unlikely that these areas are supporting populations of this species. Fordonia uses the intertidal burrow system. Karns et al. (2002) monitored the movements of three male snakes over a period of five weeks using radiotelemetry in Singapore’s Pasir Ris Park, a mangrove forest. Snakes monitored for 7 - 10 days were relatively sedentary and when they did move it was only for short distances (1.8 - 14.0 m). Two of the snakes were always located in mud lobster mounds (100% of the telemetric locations), but they not show a preference for lobster mounds of a particular size. A third snake used the mud-root tangle of the mangrove 59% of the time, and an area under a boardwalk the other 41% of the time. It was usually associated with two mud lobster mounds. While these individual snakes frequented the mud lobster mounds and the landward edge of the mangrove, they were observed foraging on tidal mud flats. Body temperatures (26.3 - 29.0°C, = 28°C) for these three snakes were consistently above the temperature of the microhabitat they were using and significantly different. Of the three monitored snakes only one moved once during the day, and all other activity was nocturnal, and the snakes were active throughout the night.
Ever since Günther (1864) recognized that Fordonia feeds on crustaceans, most authors writing about this snake have discussed the unusual carcinophagus diet. And, several authors report their own observations. In Australia, Gow (1989) wrote, “It feeds upon small crustaceans and when hunting forages amongst dense mangrove roots, broken pools and channels. The author has recorded it feeding on fiddler crabs (genus Uca) and a mud lobster Thalassina anomala.” And, Shine (1991a) dissected 75 specimens and found 60 crabs, many were Uca, he also found one that contained a mud lobster, and two unidentified shrimp. In New Guinea, Parker (1982) reported small red and black mud crabs as food. McDowell (in Parker) reported nematodes in almost every stomach, and consistently found crabs, with the exception of one orthopteran insect in one specimen. Voris and Murphy (2002) reported the following crab taxa from Fordonia: Sarmiatium germaini, Macrophthalmus sp., and a sesarmine crab (Grapsidae) as well as Dotillopsis brevitarsis and Uca sp. (Octpodidae). Crab remains from five stomachs suggest these snakes use relatively small prey, 0.5 - 7.4% of the predator’s mass (Voris and Murphy, 2002). One specimen (MAGNT R.5270) a 44 cm SVL male was found carrying a 29.2 g mud lobster (Thalassina anamala) (carapace length 44.9 mm, total length 132 mm) in its mouth. Nobbs and Blamires (2004) describe diurnal feeding, and observed F. leucobalia coiling around a crab (Uca flammula), apparently in an attempt to secure it. They also observed two instances of F. leucobalia swallowing a mud lobster tail and separating the tail from the cephalothorax and report hearing audible crunching. In one of these instances one snake attempted to steal a mud lobster from another snake. Separation of mud lobster tails clearly does not occur all the time since this author has removed whole Thalassina from snake digestive systems.
Glauert (1950) and Worrell (1963) mention frogs in its diet, this seems highly improbable, although in Southeast Asia the crab-eating frog Rana [=Fejervarya] cancrivora inhabits mangroves and uses mud lobster mounds, thus the opportunity is present, and it is not inconceivable that a frog eating a crab or a crab eating a frog could be encountered by a snake and ingested accidentally. Similarly, Hoesel (1959), Worrell (1963) and Campden-Main (1970) included fish in this snake’s diet, prey that seems highly improbable, but accidental ingestion is always a possibility.
Several aspects of prey handling in Fordonia have gained attention because they seem to deviate from typical snake behavior. Hoesel (1959) wrote, “It is exciting to observe a Fordonia catching a crab. In a flash the crab is constricted and the snake waits in this position till the victim has quieted down by the influence of its poison.” Shine and Schwaner (1985) described Fordonia pressing crabs into the mud and holding crabs in a coil while removing the animal’s legs. And, O’Shea (1996) wrote, “Crabs are pinned in their burrows and ‘chewed’ until they are dismembered.” Shine and Schwaner (1985) and Voris and Murphy (2002) report them pinning crabs into the mud with their chin, and using the crab’s own defense behavior to immobilize the crustacean; swallowing small crabs with the strike.
Savitzky (1983) proposed that Fordonia crushes its prey using “hypertrophied cranial kinesis.” The fangs of this species are particularly robust and the heavy musculature of the skull may be used to apply enough pressure to the crab’s exoskeleton via the fangs so that the fang can puncture the exoskeleton and deliver venom and/or digestive enzymes to the crab’s tissues. These robust fangs and associated musculature may also be used to remove the mud lobster tail from the rest of the body as reported by Nobbs and Blamires (2004).
The range of litter sizes reported by Shine (1991) of 2 - 17 spans the ranges of all others reported in the other literature. Kopstein (1932; 1938) reported Indonesian females with 3 - 5 embryos (= 4); Gyi reported a specimen with 13 eggs; Parker (1982) reported the range of 2 - 11. Gravid female SVL’s reported by Parker (1982) ranged from 521 - 720 mm.
Timing of reproduction was discussed by McDowell (in Parker, 1982) based upon a series of snakes collected at Agats (Irian Jaya, Indonesia) in March, as well as specimens from the Western Province. Parker wrote,
“He [McDowell] has reconstructed the reproductive calendar for the south coast as: eggs becoming mature June-July, when breeding takes place. Perhaps August to September through to December and January, embryonic development taking place. February and March fetuses identifiable and birth of young. April to June, eggs developing.
He found that a Queensland specimen agreed well with this timetable, but that Broome (Western Australia) specimens differed considerably, and that the Broome population had many morphological differences from the populations of southern New Guinea and Queensland.”
One museum specimen examined (MAGNET R.6201), a female (425 mm SVL) gave birth to eight young on 24 November. This is the smallest gravid female reported to date. Parker (1982) reported neonates ranged in total length from 176 - 196mm, and had tails that were 22 - 24mm. Gow (1989) reported neonate size as 180 mm in length.
Lyle and Timms (1986) reported five snakes taken from the stomachs of four specimens of the nervous shark, Carcharhinus cautus, in Darwin Harbor, Northern Territory. An unnumbered MAGNT specimen of Varanus indicus, from Adelaide River Creek, NT contained a Fordonia leucobalia that was about 350 mm in total length. The lizard has an SVL length of about 40 cm. The lizard specimen is unregistered and on display at MAGNT. Guinea (in Greer, 1997) reported that the bird commonly called the jabiru (Xenorhynchus asiaticus) feeds on this snake. Loveridge (1948) reported the nematode Ortleppina longissima from the stomach of one specimen.
Parker (1982) wrote about the abundance of this species, “At times many hundreds of these snakes have been found in the mangroves of Daru and Bobo Islands [New Guinea]. At other times only odd individuals can be found. In some cases, these large numbers appeared to coincide with extra high tides. At those times, many of the snakes were gravid females with large eggs lacking embryonic development.” Karns et al. (2002) found no association between tides and the activity of this species at Pasar Ris, Singapore; but at this study site the tidal cycle had been modified by human alteration of the drainage system. However, they did get the impression that snake activity was higher on days with late afternoon or early evening showers.