Aquatic Snakes: Australian Bockadam, Cerberus australis (Gray, 1842)

Homalopsis australis Gray, 1842: 59–68. Holotype: BMNH 1946.1.2.40. Type locality: Port Essington, Northern Territory, Australia.

Cerberus australis: Gray, 1849a: 65.

Cerberus rynchops novaeguineae Loveridge,1948: 388. Holotype: MCZ 22818. Type locality: Merauke, Dutch New Guinea.

Cerberus rynchops australis: Loveridge, 1948: 389.

Cerberus rynchops [in part]: Cogger et al.,1983: 210.

Distribution: Northern coastal Australia and thesouth coast of New Guinea. A single specimen is known from Waingapu, Sumba Island, Indonesia. It is unclear if this is a waif or represents an established population (Murphy et al., 2012b).

Diagnosis: Dorsal scales in 23 rows at mid-body, upper labials 2–4 contact loreal, scales on the crown imbricate; those anterior to the angle of jaw lack keels; first labial does not contact loreal; last upper labial horizontally divided; venter is mottled.

Size: The largest specimen Shine (1991a) reported was a female with a 75.6 cm SVL, and he reported a male with a 54 cm SVL (tail lengths not available). The largest specimen measured for this study was a female with a total length a 737 mm with a 106 mm tail. The largest male examined had a total length of 665 mm and a 117 mm tail. The smallest individual examined had a total length of 382 and a 53 mm tail. Eleven males had tails that were 21 - 26.7% of the SVL. Eight females had tails that were 19 - 22.6% of the SVL.

Head scalation of Cerberus australis.

External Morphology: The head is distinct from the neck, elongated and slightly depressed. The eyes are dorsolateral, round, and raised above the surface of the head. This may be due in part to the high subocular scale. This characteristic is also useful in distinguishing this species from C. rynchops. The body is slightly compressed, the tail is more compressed. The lower jaw is not countersunk; the snout only slightly protrudes over the lower jaw.

On the head the rostral scale is visible from above, about 1.2 times broader than high, and pentagonal. The nasal scales are in contact, they are as large as or larger than the rostral in diameter. The nasal scales may be divided or semi-divided with the nasal cleft touching the second labial, the internasal, and or the loreal. The internasal scale is divided, and slightly penetrates the nasals from behind. They are small, about one third the size of a nasal scale in area. The internasal scales are either not in contact with the loreal scales, or in very narrow contact. In 14 specimens seven (50%) had very narrow contact. And seven (50%) had no contact with the loreal scales. The prefrontal scales contact the loreal scale. The frontal scale is relatively small; its length is about equal to the greatest length of the prefrontal scales. The parietal scales are fragmented and barely distinguishable from the surrounding scales. There are two, relatively small, primary temporal scales. The loreal may be in contact with upper labials 2 - 3 or 2 - 4. This characteristic was suggested by Gyi as one that would separate this species from rynchops, which usually has 1 - 3 touching the loreal. On 14 specimens (28 sides) of australis 18 sides had labials 2 - 3 touching the loreal and 10 sides had labials 2 - 4 touching the loreal. In 21 (42 sides) specimens of rynchops only six (14%) sides had labials 2 - 3 or 2 - 4 contacting the loreal. The ocular ring is composed of one supraocular, one preocular, and two postocular scales. There is one subocular, thus the labials do not enter the orbit, but in some specimens the postocular and subocular scales may be fused. Upper labials number 8 - 10 and the last two or three are divided; the sixth or fifth is the largest.

On the chin the lower labials number 13 - 15, the fifth or the sixth is the largest. The first pair of chin shields is in contact with lower labials 1 - 2, 1 - 3, or 1 - 4. The second pair of chin shields is greatly reduced in size . Gular scales number 8 - 13.

On the body the anterior dorsal scale rows are ovate in the first row, and become more lanceolate toward the midline. Scale shape is also useful in distinguishing this species from C. rynchops which has more lanceolate scales. The scales in the first 1 - 3 rows are smooth and slightly striated; the scales in the fourth row to the midline are keeled and striated. The dorsal scale rows number 23 - 25 on the neck. The dorsal scales at midbody number 21 - 25 (usually 23, the type specimen has 25 scale rows at midbody) are striated in the first two rows, the third row to midline are striated and keeled. The first two rows are ovate and the other rows become more lanceolate toward the midline. The dorsal scales at posterior body are more ovate than the anterior scales, and become lanceolate toward the midline; all rows are keeled and striated, and these rows number 19 - 21. The ventral scale count was 146 - 154 in males and 142 - 153 in females; the anterior ventral scales are about twice the length of a nearby dorsal; the ventral scales at midbody are about three times as wide as the height of a nearby dorsal scale; and the ventral scales at posterior body are about 2.5 times as wide as the height of a nearby dorsal. The anal plate is divided.

On the tail the dorsal scales are ovate, keeled and striated. The subcaudal scales are divided and number 49 - 63 in males and 42 - 51 in females.

Color and pattern. In one adult female there are no obvious head markings. In a young female there is a canthal streak and a postocular streak as well as a stripe from the angle of the jaw onto the first few ventral scales (the adult female also has the latter). Gyi (1970) suggested the eye stripe is useful in distinguishing this species from rynchops; of 14 preserved specimens examined for this study the eye stripe was missing in eight (57%) of the specimens. The chin and gular region are spotted in both specimens, and both have a lateral nape stripe. The dorsum has 30 - 50 spots, crossbars, or a combination of each that are 1 - 2 scales wide. There are dark ventrolateral crossbars or blotches that extend onto the first two or three scale rows and number 28 - 32. The underside of the tail is a uniform brown. There are two color morphs, a gray morph and a red morph. Greer (1997) pointed out that color change with growth is unstudied in this snake, but that Shine (1991b) found small individuals from Darwin tended to be gray, and larger specimens tended to be gray-brown. The illustration of this snake from Boulenger (1896) shows a snake with a uniform dorsal coloration and a mottled venter.

Red and gray color morphs of

C. australis. Northern Territory,

Australia.

Habitat: Cerberus australis uses mud flats and mangrove forests, and streams that pass through these habitats. Literature descriptions of the habitat: Kinghorn (1929) wrote, “It [C. australis] may be found in the sea, in estuaries, or in fresh-water streams, and is said to be fond of basking on mud-flats.” Heatwole (1977) reported one 3 km upstream from the Gulf of Carpentaria at the mouth of the Norman River. Gow (1989) wrote, “A nocturnal aquatic species which is normally found in estuaries and mangrove flats, sometimes in small colonies. At low tide it burrows into soft mud, leaving just the top of its head visible.”

Diet and Feeding Behavior: This species feeds on fish. Ten specimens with stomach contents were examined for this study; they contained only fish. Six (60%) of these contained two or more prey items, and in nine (90%) of these the prey was less than 10% of the snake’s mass. The other specimen contained prey that was 10.3% of the snake’s mass. Gow (1989) stated, “It preys principally upon mudskippers (family Periophthalmidae), but also feeds upon other small fishes and crustaceans.” Worrell (1963) wrote, “Frequents mangrove roots around estuaries and the shelving banks of tidal streams, where it feeds upon small crabs and fish.” The assertions that it feeds on crustaceans needs verification.

Reproduction: Gow (1989) wrote, “A live-bearer, it may have up to 26 in a litter; newborns measure about 180 mm in length.” Shine (1991a) examined three litters that ranged from 6 - 8 young (= 7), and he reports that gravid females were collected in February and August.

Predators and Parasites: Lyle and Timms (1986) found one in the stomach of the nervous shark, Carcharhinus caustus, caught in Darwin Harbor, Northern Territory. This species undoubtedly falls prey to a variety of bird, reptile, fish and invertebrate predators with which it shares its environment, but documentation is needed. Jones (1978, 1980) reported the nematode Camallanides cerberi(Spirurida: Camallanoidae) from the esophagus and stomach of this species.

Behavior: Kinghorn (1929) described movement that “…reminds one of the action of the American sidewinder.” Homalopsid snakes are capable of this locomotor behavior which is used when the snake finds itself on smooth surfaces, such as mud flats.

Venom: Gow (1989) reported a stinging sensation from the bite of a large specimen. Guinea et al. (1992) collected venom from the fang surface using capillary tubes; the yields were usually about 10 µl per snake, but one specimen produced about 200 µl. The venom increased the clotting times of free platelets by 12 - 40%; and at least four major proteins that were acidic were detected.