Saturday, March 26, 2016

Mekong Mud Snake, Enhydris subtaeniata (Bourret, 1934)

 Hypsirhina jagorii – Morice, 1875 Coup d'Oeil sur la Fauna de la Cochinchine Francaise, p.        58.
Hypsirhina Iagorii (sic) – Mocquard, 1907 Revue Coloniale, p. 51.
 Hypsirhina enhydris subtaeniata Bourret, 1934 Bulletin Genéral de l’Instruction Publique, 9-10, Figure 3. Lectotype herein designated: MNHN 1958.04.74. Type locality: Kompong Speu, Cambodia. 1936 Bourret, 1936:278 [in part]. Bourret (1934) described the lectotype from Kompong Speu [Cambodia] (~11°27’N 104°32’E, elevation 39m) and illustrated it in Figure 3. He then listed four other specimens from Soc Trang, Vietnam (~09°36’N 105°58’E, elevation 0.9 m) and gave ventral and subcaudal counts as well as measurements. He does not apply the term “type” to any specific specimen nor does he exclude any of these specimens from being a type. Ivan Ineich (personal communication) has suggested that this specimen be designated the lectotype. The type locality is therefore Kompong Speu, Cambodia.
Enhydris jagorii – Smith, 1943: Fauna of British India, Reptilia and Amphibia, 3:384.
Enhydris jagori – Taylor, 1965 The serpents of Thailand and adjacent waters, p. 917. 
Hypsirhina enhydris enhydris – Deuve, 1970 Serpents du Laos, pp. 173-175, Pl. 18, Figures 4-5.

Comment: This species has been long confused with Enhydris jagorii (Peters) and most of the literature references, particularly those outside of Thailand, to Enhydris jagorii actually refer to this species.

Etymology: The name subtaeniata may be derived from the Latin sub meaning under, and taenia meaning ribbon. Thus it most likely refers to the several stripes that form a pattern on the belly.

Distribution:This snake is widely distributed in the Mekong drainage, and inhabits Laos, Thailand’s Korat Plateau, Vietnam, and Cambodia. It escaped the Mekong drainage basin, possibly with human assistance, and colonized the Bung Boraphet area (Chao Phraya drainage) in central Thailand.

Redescription of the Lectotype of Hypsirhina enhydris subtaeniata
            Juvenile female (MNHN 1958.0474, Bourret’s field number, M.388) total length of 202 mm, tail 35 mm (17.3%). Rostral pentagonal, broader than tall. Nasals semi-divided; cleft contacts first labial on each side. Prefrontal scales paired; scale on the right longitudinally divided, as illustrated in Bourret’s Figure 3; prefrontal scales about equal in diameter to nasal scales. Frontal pentagonal, slightly less than the length of the parietal. Loreal single; contacted by first three upper labials on both sides (contrary to Bourret’s Figure 3 showing only the first two upper labials contacting the loreal). Ocular ring composed of a supraocular, a preocular scale, two postocular scales, and upper labial number four enters orbit. Temporal formula 1 + 2 + 3. Upper labials eight with 6th tallest; lower labials 10, the 6th largest. First three lower labials contact the anterior pair of chin shields. Three pairs of chin shields, anterior pair greatest in length. Gular scales 10 between chin shields and first ventral. Dorsal scale rows 23 - 21 - 21. Ventral scales rounded, 139; subcaudals 54/55. Colour and pattern of specimen obscure; nape stripe present; head uniform brown. Ventral scales brown with zigzag stripe on outer edge and edge of dorsal scale row one; ventrals on posterior third of the body have a central spot which combine to form a medial stripe. Scale rows 1 - 3 a light stripe; scale rows 4 - 6 have dark lateral spots; spots involve 2 - 4 scales and fuse into a stripe on the tail, the spots number 52/47. Dorsum has scattered pigment suggesting distinct spots present in life.

Systematic Status of Hypsirhina enhydris subtaeniata Bourret 1934
            Morice (1875) reported H. jagorii from Cochinchine (=Vietnam) as did Müller (1887), and Mocquard (1907). Bourret (1934) described Hypsirhina enhydris subtaeniata on the basis of a specimen from Kompong Speu, Cambodia and four specimens from Soc Trang, Cochinchine (= Vietnam). Smith (1943) placed Hypsirhina enhydris subtaeniata Bourret in the synonymy of Enhydris jagorii. He considered E. jagorii to have 116 - 145 ventrals and 38 - 61 subcaudal scales, ranges that encompass subtaeniata and E. chanardi. A collection of snakes purchased at the markets around Tonlé Sap, Cambodia contained Enhydris enhydris and a few specimens labeled “Enhydris jagorii.” A collection of snakes made in central Thailand contained specimens labeled “Enhydris jagorii” but in both cases the snakes agree well with Hypsirhina enhydris subtaeniata Bourret. Examination of additional museum material labeled “Enhydris jagorii” found most of them to be conspecific with Bourret’s Hypsirhina enhydris subtaeniata. St. Girons (1972a) discussed “Enhydris jagori” from Cambodia and described it as having 127 - 149 ventrals and 48 - 68 subcaudals. His Plate 32 (a photograph) shows specimen no. 70-555, clearly a specimen conspecific with subtaeniata. Voris et al. (2002) used tissues from (FMNH 252506) “Enhydris jagorii,” in a molecular study, which suggested it is the sister species to Enhydris enhydris; this specimen is also conspecific with Bourret’s subtaeniataHypsirhina enhydris subtaeniata Bourret 1934 is a valid species. It is sympatric with E. enhydris at many locations in Thailand, Vietnam, and Cambodia. Its confusion with E jagorii undoubtedly occurred because of the similar scale counts, scale arrangements, and a pattern that includes lateral spots.

Diagnosis: There are 21 scale rows at midbody; ventral scales number 136 - 153 (136 - 146 in females and 143 - 153 in males), subcaudal scale counts were 46 - 69 (46 - 60 in females and 54 - 69 in males). The ventral counts will usually separate this species from its sister species Enhydris enhydris which has 153 - 174 ventral scales. Enhydris subtaeniata can have two or three pairs of chin shields that are elongate, rather than the flared condition found in jagorii and its relatives (chanardiinnominata, and longicauda). Both E. enhydris and E. subtaeniata have a stripe on scale rows 1 - 3, and both have striated dorsal scales, but only E. subtaeniata has a row of spots on scale rows 4 - 6. Each spot involves two or three scales, and these may fuse on the neck to form a stripe, but are distinct at midbody. Additionally, scale row two in subtaeniata is salmon pink, this is absent in all other known taxa except E. enhydrisEnhydris chanardi, E. jagoriiE. innominata, and E. longicauda, also all have 21 rows of scales, but they all have 136 or fewer ventral scales, none of these have a longitudinal stripe in their dorsal pattern above scale row 3, and none have striated scales. E. subtaeniata has a bold zigzag stripe at the outer edge of the ventral scales and dorsal scale row one, and it has a series of midline spots on each ventral scale on the posterior half of the body.

Size: The largest female (CUMZ 2002.227) had a total length of 870 mm with a 127 mm tail. The largest male (CUMZ 2002.234) had a total length of 659 mm, with a 120 mm tail. The smallest specimen was a female (MNHN 1938.0142) with a total length of 159 mm with a 31 mm tail, this is probably at, or near the size of neonates at birth. The range of 18 female SVLs was 128 - 743 mm (= 402 mm), the range of 19 males SVLs was 133 - 490 mm (= 397 mm). Females had tails that were 17 - 23% of the SVL, and males had tails that were 22 - 30% of the SVL. The largest specimens examined were from the Bung Boraphet population in Thailand.

External Morphology
            The head is moderate in size and depressed, the body is cylindrical, and increases in diameter posteriorly to a point near the vent. The eyes are dorsolateral, and the orbit diameter is less than the eye-mouth distance, and about equal to the eye-nostril distance.
            On the head the rostral scale is pentagonal and broad, about twice as wide as it is tall . The nasal scales are in contact, and they are semi-divided with the nasal cleft extending to the first labial. They are slightly larger than the diameter of the eye. The internasal is single, posterior to the nasals and slightly penetrating between them. The prefrontals are paired and about equal in their greatest diameter to the diameter of the eye. The frontal length is about equal to the interocular distance and the length of the parietals. It is hexagonal in shape. The loreal is singular and more or less quadrangular. The supraocular is single; the preocular is single; the postocular is usually divided into two scales with the more dorsal scale being taller; and there are no subocular scales, upper labial number four enters the orbit. Temporal scales number 1 + 2 + 3, or 1 + 2 + 4. The primary is about as tall as it is broad and relatively small. The secondary and tertiary temporals are small and similar to the occipital scales. The upper labials usually number eight; the largest is usually the sixth. The first three or four upper labials contact the loreal.
            On the chin the lower labials number 10 - 12, usually 11. The first three are in contact with the anterior pair of chin shields. There are two or three pairs of chin shields with the anterior pair being the largest, a characteristic that will readily separate this species from Enhydris enhydris. The gular scales number 9 - 11.
            On the body the dorsal scales are in 23 rows on the neck (rarely 24); dorsal scales at midbody number 21; dorsal scales at posterior body number 19 - 21. The ventrals number 134 in a Laotian specimen, but 140 - 153 in specimens from elsewhere. These numbers are higher than those given by Bourret (132 - 140) for his series from Soc Trang, Vietnam. Ventral scales at the anterior of the body are 2 - 3 times the height of a nearby ventral, while ventrals at mid-body are 3 - 4 times the height of a nearby ventral, as are those posteriorly. Eighteen females had 134 - 146 ventral scales (= 139), and 19 males had 143 - 153 ventral scales (= 146). Thus, the ventral scale counts are most likely sexually dimorphic in this species. Subcaudal scales number 46 - 60 in 16 females (= 51.9) and 54 - 69 (= 62.8) in 17 males; the subcaudal counts are also sexually dimorphic in this species when examined by individual population, although they overlap when all populations are combined.
The anal plate is divided and about the same length as the preceding ventral.
            On the tail the subcaudal scales are divided and number 46 - 69, a number that agrees with Bourrett’s specimens. At the base of the tail the width is 81% of the height, based upon the average of four specimens.
            Color and pattern. The outer edge of the ventral scales has dark brown pigment and when combined with the dark brown pigment on the anterior edge of the first scale row forms a zigzag stripe on each side of the belly. The posterior half of the first scale row is cream. The second scale row and sometimes the bottom portion of scale row three have a salmon pink or red stripe with some brown pigment. In some specimens the salmon pink coloration is absent. Scale rows 4 - 18 are solid dark brown. Starting behind the parietal scales, there is a darker stripe on four rows of scales, these become indistinct posteriorly. The midline of the ventral scales frequently has a series of spots that form a midline stripe on the posterior of the body, this is rarely absent, and rarely does it extend to the anterior quarter of the ventral scales. There is a dark stripe on the midline of the tail bordered by cream stripes above and below.

Habitat: This aquatic snake uses streams impoundments, padi, ditches, klongs, ponds, and probably many other aquatic microhabitats. There are several localities in central Thailand where E. enhydris and E. subtaeniata co-exists in similar numbers and both species have been collected in the same gill net. The population around Tonlé Sap, Cambodia also coexists with E. enhydris (Saint Girons and Pfeffer, 1972; Stuart et al. 2000). In Cambodia, Saint Girons (1972) and Saint Girons and Pfeffer (1972) considered this snake to be strictly aquatic and diurnal (see their comments about E. jagorii) and they suggest it migrates with the water as the water levels in the lake undergo season fluctuations. This snake has been collected from gill nets set in padi and in reservoirs.

Diet: One specimen was found to contain the remains of an unidentified fish; another contained the remains of an anuran.

Reproduction: Litter Size. Three females contained 7, 12, 20 oviductal eggs. The smallest gravid female has a SVL of 572 mm. Deuve (1970) reported young born in April and May with the first rains. This assumes his E. enhydris account pertains to this species and not E. enhydris.

Relationships: E. subtaeniata is the sister species to E. enhydris (Voris et al. 2002).

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