Hydrodynastes gigas is commonly referred to as the false water cobra, false cobra, South American water cobra, and Brazilian smooth snake. It is often referred to within the reptile hobby more simply as either a "falsy" or "falsie" or a "FWC", which is an abbreviation of the common name false water cobra. In South America, it is sometimes referred to as boipevassu.
H. gigas is a large colubrid that may exceed 300 cm (9 to 10 ft) in total length when adult. However most H. gigas reach approximately 2 m (6 to 7 ft) in length. It is of medium body, and is therefore neither particularly heavy nor slender-bodied, but is one of the heaviest colubrid species when full adult size is attained. The common name false water cobra is an allusion to its ability to flatten its neck, similar to a cobra as a defensive reaction to make it look larger and more intimidating. Unlike the true cobra, though, the false water cobra stays in a horizontal position when it hoods, rather than rearing into a vertical position. H. gigas can flatten not only its neck, but also lower down the body, which is not possible for a true cobra.
Additionally, the pattern and coloration of this Hydrodynastes species superficially resemble those of true water cobras (Boulengerina).
The false water cobra has large eyes with circular pupils, allowing good daytime vision. The tongue is black, and of the typical snake fashion.
The background colour of a mature specimen is an olive green or brown, with dark spots and bands covering much of its body. The background coloring and banding generally become darker towards the end tail. This colouring gives the false water cobra effective camouflage in its natural rainforest environment. The ventral scales are yellow or brown, spotted with dark flecks that make three dotted lines which appear to merge towards the tail. Mehrtens, 1987, suggested females are brown ventrally, whilst males are yellow. Females are suggesteed to have lighter bands and markings on their bodies. This is not an effective way of judging the sex of H. gigas, as coloring will differ slightly between all individuals. Hatchling and juveniles are much darker in coloration and do not have the typical dark eyes of the adults. They more resemble a garter or water snake than their mature counterparts. In captivity, hypomelanstic animals have been produced. These animals vary in coloration, from some having only slightly lighter colored saddles, to those that are almost patternless.
