Overview

Brief Summary

Gerard’s Water Snake, Gerarda prevostiana

, is also known as: cat-eyed fishing snake, and the glossy marsh snake. Wall (1921) stated that this species was named in honor of Monsieur Prevost, Superintendent of the Paris Museum at the time specimen was named.

            Gerarda prevostiana is distributed in coastal areas from the vicinity of Bombay, India westward to the Philippines; this distribution may be discontinuous; only recently have specimens been reported from Indonesia. It seems absent from tropical coastal China. The distribution may be disjunct but this species should be expected wherever mangrove forest is found. Due to its secretive habitats it has been considered rare by many authors, but because of its habits it cannot be easily deleted from a faunal list from any area with mangrove forest between Bombay and the Philippines. Its distribution overlaps with several other widespread coastal homalopsids, including its sister species Fordonia leucobalia and members of the genus Cerberus, except that Gerarda is absent from the Sahul Shelf (Australia and New Guinea).

Boulenger (1896) gave its maximum length as 520 mm. Twelve specimens were measured for this study; the largest was a male with a total length of 514 mm, and a 65 mm tail. The largest female had a total length of 472 mm with a 57 mm tail. The smallest specimen examined had a total length of 230 mm with a 29 mm tail. An examination of 12 museum specimens from the eastern portion of the range (India to Thailand, five males and seven females) suggest that the tail/SVL ratio tends to be higher in males (14.4 - 17.6), than in females (13.7 - 15.8). This was also supported by data collected on the Singapore population by Karns et al. (2002).

            This is a small homalopsid with a dorsum that is uniform gray-brown or black. The nasal scales are separated by an internasal scale, and it has 17 rows of dorsal scales at midbody, the lowest count of any homalopsid. Its scales lack keels or striations, but may be pitted with irregular erosions of the scale surface. Its close relative, Fordonia leucobalia has 23 - 27 rows of dorsal scales at midbody, but it also has smooth scales and the nasal scales separated by the internasal.

            The crown of the head is uniform gray. Each of the upper labials is yellow or has a large yellow spot. The rostral is gray. The chin is mostly yellow with some scales having darker edges. The first three scale rows are mostly yellow, the rest are gray. Each ventral scale has diffuse chromatophores concentrated near the anterior edge of the scale. The ventral surface of the tail has pigmentation similar to the ventral scales.

            This is a snake of the mangrove forest, although it may occasionally stray into adjacent environments. It is restricted to coastal areas and occurs in rivers that empty into the ocean, on mud flats, and along rocky shorelines, but it most likely reaches its greatest abundance in mangroves. Karns et al. (2002) found G.  prevostiana uses a variety of microhabitats from the landward edge of the mangal to the mud pools in the lower tidal zone at Pasir Ris Park in Singapore. It has been excavated from mud lobster (Thalassinia anomala and T. gracilis) mounds (Karns et al., 2002; Voris and Murphy, 2002) which occur toward the landward edge of the mangal. However, Karns et al. (2002) found that it was not restricted to the area around mud lobster mounds. It is active throughout the night and more active at spring tides than at other times during the tidal cycle.

            Contrary to the literature none of the five museum specimens examined with stomach contents contained fish, they all contained crab remains (Voris and Murphy, 2002). Jayne et al. (2002) later found that it specializes in feeding on recently molted crabs, and that the snake will tear larger crabs apart in order to ingest them. This is the only species of snake known to tear its food into pieces that can be swallowed, all other snakes consume their prey whole (Fordonia is also a possible exception but in need of documentation). Tearing the prey into pieces is accomplished by a “loop and pull” behavior that involves the snake forming a loop of its body around the prey and pulling the prey with its mouth through the loop. The advantage of this is that it allows the snake to swallow a much larger prey, than it would otherwise be able to handle. G. prevostiana is known to feed on the crabs Dotillopsis sp. and Episesarma versicolor (Voris and Murphy, 2002; Jayne et al., 2002).

           

  • Jayne, B. C, H. K. Voris, and P. K. L. Ng. 2002. Snake circumvents constraints on prey size. Nature 418:143.
  • Karns, D. R., H. K. Voris, T. G. Goodwin. 2002. Ecology of the Oriental-Australian rear-fanged water snakes (Colubridae: Homalopsidae) in the Pasir Ris Park Mangrove Forest, Singapore. The Raffles Bulletin of Zoology 50:487-498.
  • Voris, H. K. and J. C. Murphy. 2002. Prey and predators of homalopsine snakes. Journal of Natural History 36(13):1621-1632.
  • Wall, F. 1921. Ophida Taprobanica or the Snakes of Ceylon. Colombo: H. R. Cottle.
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Comprehensive Description

Brief

Scales 17:17:15 rows. Ventrals 144-157; anal divided; subcaudals 29-36 paired.
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Distribution

Coasts and tidal rivers
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Continent: Asia
Distribution: India, Myanmar (= Burma), Sri Lanka, Thailand, Philippine Islands (?) Bangladesh; West Malaysia; Singapore.  
Type locality: Manila (in error)
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Ecology

Habitat

Habitat and Ecology

Habitat and Ecology
This species is most abundant in mangrove forests, but may stray into adjacent areas. It is restricted to coastal areas and may be found in rivers that empty into the ocean, on mud flats, and along rocky shorelines. It is nocturnal. This species specializes in feeding on recently moulted crabs and is known to tear apart larger crabs in order to ingest them. (Murphy 2007).

Systems
  • Terrestrial
  • Freshwater
  • Marine
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coastal
  • UNESCO-IOC Register of Marine Organisms
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General Habitat

"Coastal ares, mangroves and estuaries"
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Conservation

Conservation Status

IUCN Red List Assessment


Red List Category
LC
Least Concern

Red List Criteria

Version
3.1

Year Assessed
2010

Assessor/s
Murphy, J.

Reviewer/s
Livingstone, S.R., Elfes, C.T., Polidoro, B.A. & Carpenter, K.E. (Global Marine Species Assessment Coordinating Team)

Contributor/s

Justification
This species had a wide distribution and is common in the areas where it is found. There are no known threats to this species. This species is listed as Least Concern
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Population

Population
This species is considered common in the habitats where it is found.

Population Trend
Unknown
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Threats

Major Threats
There are no known major threats specific to this species, but threats to associated habitat such as mangrove forests may be detrimental to the species.
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Management

Conservation Actions

Conservation Actions
There are no known conservation measures in place for this species.
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Wikipedia

Gerarda prevostiana

Gerard's water snake or the cat-eyed water snake (Gerarda prevostiana) is a species of water snake found in Asia.

Diet[edit]

They feed almost exclusively on crabs, which they tear into bite-sized pieces by pulling them through their coils, in contrast to most other snakes which swallow their prey whole.[3]

Description[edit]

Frontal a little longer than broad, shorter than its distance from the end of the snout, or than the parietals; loreal slightly longer than deep, a little smaller than the nasal; one pre- and two post-oculars; temporals 1+2; upper labials 8, fourth entering the eye; 4 lower labials in contact with the anterior chin-shields, which are much larger than the posterior. Scales in 17 rows. Ventrals 146–158; anal divided; subcaudals 31–34. Uniform dark olive above; three outer rows of scales whitish; upper lip white, rostral dark olive; ventrals and subcaudals whitish, with dark edges.[4]

Total length 41 cm (16 inches); tail 5 cm (2 inches).

Distribution[edit]

India, Myanmar (= Burma), Sri Lanka, Thailand, Philippine Islands ( = mindanao), Bangladesh, west Malaysia, and Singapore

References[edit]

  1. ^ Eydoux, F. & P. Gervais 1822 Voyage de la Favourite. Reptiles. Mag. Zool. Guérin, Paris, 111: 1 - 10
  2. ^ Boulenger, G.A. 1896. Catalogue of the Snakes in the British Museum (Natural History), Volume III. London.
  3. ^ Jayne, B. C., Voris, H. K. & Ng, P. K. L. 2002 Snake circumvents constraints on prey size. Nature 418: 143
  4. ^ Boulenger, G. A. 1890. Fauna of British India. Reptilia and Batrachia. Taylor & Francis, London. p. 379
  1. ^ Eydoux, F. & P. Gervais 1822 Voyage de la Favourite. Reptiles. Mag. Zool. Guérin, Paris, 111: 1 - 10
  2. ^ Boulenger, G.A. 1896. Catalogue of the Snakes in the British Museum (Natural History), Volume III. London.
  3. ^ Jayne, B. C., Voris, H. K. & Ng, P. K. L. 2002 Snake circumvents constraints on prey size. Nature 418: 143
  4. ^ Boulenger, G. A. 1890. Fauna of British India. Reptilia and Batrachia. Taylor & Francis, London. p. 379
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