Cerberus Cuvier 1829

Murphy, John C., Voris, Harold K. & Karns, Daryl R., 2012, The dog-faced water snakes, a revision of the genus Cerberus Cuvier, (Squamata, Serpentes, Homalopsidae), with the description of a new species, Zootaxa 3484, pp. 1-34 : 7-8

Cerberus Cuvier 1829

Hydrus Schneider 1799 1:233246 (in part).

Enhydris Sonnini and Latreille 1801 (in part), 4:202.

Elaps Schneider, 1801 (in part), 2:301.

Coluber (non-Linneaus, 1766) Daudin, 7:167.

Python Merrem, 1820:89 . Daudin used the combination Python rhynchops .

Cerberus Cuvier, 1829 , 2, 2:81. Type species Coluber cerberus = Hydrus rynchops Schneider. Note that Swainson (1839) spelled this Cerebus , a lapsus.

Homalopsis Schlegel, 1837 (in part), 2:332.

Identification. Distinguishing Cerberus species on the basis of morphology alone can be difficult which in part accounts for the fact that C. rynchops has been described 11 separate times since 1799. Table 2 summarizes some of the morphological data discussed here that can be used to separate these taxa. The variable dorsal scale row counts within and between species, the fragmentation of head shields, and the relatively similar coloration and pattern, make these species somewhat cryptic. This can be highlighted by Loveridge’s (1948) confusion when he named the New Guinea population Cerberus rynchops novaeguineae instead of recognizing its morphology as more similar to australis ; and Cogger et al. (1983) synonymizing Cerberus australis and Cerberus rynchops , two species that differ genetically by 6.4–9.5% ( Alfaro et al. 2004). The five species recognized here are morphologically distinct, with the differences often subtle but consistent. Four of these species have been previously compared using mitochondrial and nuclear genes ( Alfaro et al. 2004, 2008).

Diagnosis. Cerberus can be distinguished from all other homalopsid snakes with nasal scales in contact by: keeled, striated dorsal scales; absence of rostral appendages; and parietal scales that are partially or completely fragmented. It differs from its sister, Homalopsis , in having fewer scale rows (21–31 at midbody in Cerberus , 33– 49 rows in Homalopsis ) and fragmented parietal scales. Most specimens are less than one meter, and the maximum size for most species is less than 1.2 m. The tail is 16–29% of the snout-vent length (SVL). Females are larger than males in the species where both sexes could be examined. Head plates (prefrontal, frontal, and parietals) show a strong tendency to fragment into smaller scales. One or more large posterior upper labials are horizontally divided. Subocular scales may or may not be present, even within a single population. Temporal scales are small and not plate-like. Dorsal scale rows at midbody range from 21–31; however most specimens of most species have 23, 25, or 29 rows at midbody. Dorsal scale ornamentation consists of a strongly striated surface with central keel when viewed with a binocular light microscope with 10x magnification, and some individuals have tubercles on head scales. Ventral scales range from 134–172 and subcaudal scales range from 42–75. Anterior maxillary teeth number 12–19 and are followed by a diastema and a pair of deeply grooved fangs. Three pairs of chin shields are present and the first pair is usually the largest.

Content: Here we recognize 5 species: C. australis ( Gray, 1849) ; C. dunsoni sp. n.; C. microlepis Boulenger, 1896 ; C. rynchops ( Schneider, 1799) , and C. schneiderii ( Schlegel, 1837) new combination.

Tables

TABLE 2. A comparison of morphological characters commonly used to distinguish five species of Cerberus.

	australis	dunsoni	microlepis	rynchops schneiderii
n=	14	6	16	52 123
Distribution	Australasia	Palau, Micronesia	Lake Buhi, Philippines	South Asia Southeastern Asia
Dorsal scale rows	21–25 (usually 23)	23	27–31 (usually 29)	21–27 21–27 (usually 25) (usually 23)
Ventrals males	146–154	No data	160–172	134–152 140–170
Ventral females	142–153	146–147	159–165	135–152 141–152
Subcaudals males	49–63	No data	60–70	60–67 53–75
Subcaudals females	42–51	50–52	54–61	48–59 47–61
Divided rear upper labials	one	one	one	two one
Keeled scales on crown	no	no	yes	yes yes
Lateral stripe on scale rows	1–3	1–2	1–2	1–3 or 1–4 1–3
Upper labials @ loreal	2–3 or 2–4	1–4	1–4	1–3, 1–4 1–3, 1–4
Plate-like parietal scale fragments	no	yes	no	no no
ventral pattern	mottled	uniformly dark	mottled	mottled, lateral mottled bars