Carettochelys insculpta is an aberrant freshwater species resembling the marine turtles in having paddlelike forelimbs. Its gray to olive carapace (to 56.3 cm) lacks scutes but has a rugose granulated surface. In this respect it resembles the softshells of the family Trionychidae, to which it is related. The adult carapace is smooth bordered and usually lacks a vertebral keel; that of juveniles has a serrated border and a pronounced knobby medial keel. The underlying bony carapace is fairly well-developed with six to seven slender neurals partly separated along the midline by costals, and 10 complete peripherals. Vertebral centra are opisthocoelous except the 8th which is biconvex. There may be a series of white blotches along the lower sides of the carapace. The white bridge and plastron are well-developed, but the entoplastron and epiplastra are only loosely attached to the hyoplastra. No scutes are present on the bridge or plastron, and the underlying bones and sutures may show through the skin covering. Emargination occurs posteriorly in the temporal region and the parietals no longer touch the squamosals; the skull is unique in having a well-developed concavity on the posterior surface of the articularis process. The quadratojugal extends forward to the maxilla, and the premaxillae are fused. The small jugal is separated from the parietal by a large postorbital, and only its medial process articulates with the palatine. Also, the vomer is so small it does not separate the palatines, allowing the choanae to be confluent. In the region behind the premaxillae, a large opening, the intermaxillary foramen, prevents the vomer from touching the premaxillae. Also the vomer does not touch the maxillae. The palatine touches the prefrontal and has a dorsal process that is incorporated into the lateral wall of the braincase. Palatines contact the basisphenoid which separates the broad pterygoids. The occipital condyle is composed only of exoccipitals. No ridge is present on the crushing surface of the white jaws. Dorsally, the head is granulated and gray to olive in color; it is whitish ventrally. A white blotch occurs behind each orbit. The thick snout protrudes like that of trionychids, but it is shorter, wrinkled, and has the nares somewhat laterally placed, giving it a piglike appearance. Each unpatterned, paddlelike forelimb contains two claws and is dark anteriorly but lighter posteriorly. The hind feet are heavily webbed and colored like the forelimbs. A series of large horizontal scutes lie on the dark dorsal surface of the tail.
The karyotype is 2n = 68; 32 macrochromosomes and 26 microchromosomes (Bickham and Carr, 1983; Bickham et al., 1983).
Males are smaller than females, and have longer, thicker tails with the vent near the tip.
Carettochelys insculpta is restricted to Lake Jamar and the southern rivers of New Guinea (Fly, Strickland, Morehead, Lorentz, Stekwa and Vailala), and the major rivers (Daly, Victoria, and South Alligator) of the Northern Territory of Australia.
No subspecies have been described, but those from Australia should be critically compared to the New Guinea populations.
Clear, shallow, continuously flowing rivers (including estuaries and deltas) and streams, and lakes, lagoons, and upland swamps with soft or gravel bottoms and slow currents seem to be preferred, but Carettochelys insculpta has also been taken in the estuary of the Fly River, and may be able to withstand brackish conditions.
Nesting is in the dry season from mid-July to November; probably more than one clutch is laid a year (Webb et al., 1986; Georges and Kennett, 1989; Georges and Wombey, 1993). In the evening, females crawl out onto mud or sand banks to excavate their nests. Nest sites are near the water, and usually exposed to the sun the majority of the daylight hours. Most nest are flooded near the time of hatching. Maximum nest depths are 15-24 cm (Georges, 1992); mean depth from the soil surface to the topmost egg was 10-18 cm in 10 nests examined by Webb et al. (1986). Six nests studied by Georges (1992) had the following thermal characteristics: 1) core temperatures of 26.1-33.7°C in early September, from 30.0-38.7°C in early October, and 28.4-36.8°C in early November; 2) a pronounced daily cycle in core temperatures up to 6.5°C in range; and 3) a difference in the temperatures experienced by the top and bottom eggs of up to 3.5°C. Hot nests produced only females, nests of intermediate temperature produced both males and females, and cool nests produced only males (Georges, 1992).
Four to 39 (Groombridge, 1982; Georges, 1992; Georges and Rose, 1993) spherical (36.2-46.0 mm), smooth, white, brittle-shelled eggs are laid at one time. The egg contains a distinct shell membrane composed of 4-5 individual layers (Erben, 1970). Incubation takes 64-74 days at 30°C, and the embryos go through a stage of arrested development (diapause) (Webb et al., 1986). Spreading shell opacity accompanies embryonic development (Webb et al., 1986). Incubation periods for natural nests from Papua New Guinea ranged from 86-102 days at an average temperature of 31.6°C (Georges and Rose, 1993). Hatchling carapaces are have a medial keel and are 52.4-58.6 mm.
Carettochelys is omnivorous. In the dry season it is predominately a plant eater, consuming the leaves, fruits and seeds of a wide variety of plants, including Ficus, Syzygium, and Pandanus, and mangroves (Sonneratia). Some aquatic vegetation (Valisneria, Najas), and algae are also eaten. Animal foods are taken more frequently during the wet season: mollusks, crustaceans, insects, fishes, and mammals (possibly as carrion) (Georges and Kennett, 1989; Georges and Rose, 1993; Georges and Wombey, 1993). Georges and Kennett (1989) found plant material in 92.1% (Ficus 70.8%), animal remains in 42.3%, and fish in 29.2% of the turtles they examined.
Due to the paddlelike structure of its forelimbs, Carettochelys does not swim like other freshwater turtles, but instead rows itself through the water by simultaneous movements of its forelimbs, as do marine turtles. Cann (1978) remarked that this species often aggressively bites when first captured and that its powerful jaws can inflict severe wounds; with time, however, captives become calmer.
IUCN Red List Status (1996)
Vulnerable (A1bd). Carettochelys is declining because of increased demand for its meat and eggs, and pollution from mining activities probably also takes its toll (Georges and Rose, 1993). Additional research is required to determine trends in population numbers and the levels of exploitation in New Guinea, and whether management leading to sustainable exploitation is necessary and attainable.