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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).