(Duméril and Bibron, 1835)
Black spiny-necked swamp turtle
Recognition
Acanthochelys spixii is a medium-sized (to 17 cm) side-necked turtle with several elongated pointed tubercles on its neck. Its flattened elliptical carapace has a shallow dorsal groove between the posterior part of the 1st vertebral and the anterior part of the 5th vertebral. In adults, the 1st and 5th vertebrals are much broader than long, with the 1st broadest of all; the 2nd to 4th vertebrals are about as broad as long or longer than broad. Juveniles do not have the medial vertebral groove found in adults, but instead may actually have a low medial keel. Anterior- and posteriormost marginals are broadest, those lateral narrowest. Posterior marginals are unserrated, or only slightly serrated. Carapacial scutes often contain concentric and radiating striations. The carapace is highest just anterior of the 2nd intervertebral seam, and broadest at the level of the 7th or 8th marginals. The adult carapace is dark gray to black. Occasionally some yellow occurs at the base of the pleurals. Such would appear to be paedomorphic coloration, as neonates have yellow or orange spots on the plastron. In adults, the plastron and bridge are usually uniformly dark gray or black, but some individuals have small yellow spots along the plastral midseam. The forelobe is broader than the hindlobe and slightly upturned. The hindlobe contains a wide posterior notch. The intergular scute is over half (55-60%) as long as the length of the plastral forelobe. The plastral formula is: intergul > fem > abd > an > gul > hum > pect. The head is olive to gray and the unnotched jaws are yellow to tan. Dorsally, the head is covered with numerous, variably shaped scales which become arranged into three or four lateral rows above the tympanum. The snout is short and only slightly projecting. Two small gray barbels are present on the chin. The iris is white. On the dorsal surface of the neck are numerous long pointed tubercles, which become fewer and shorter on the sides; these tubercles are absent in hatchlings (Lehmann, 1988). The toes are webbed, and the anterior surfaces of the limbs are covered with large scales. Several rows of spiny tubercles occur on the thighs. The tail is relatively short, and all soft parts of the body are colored olive to gray.
Males have concave plastra and longer, thicker tails with the vent beyond the carapacial rim; females have flat plastra and shorter tails with the vent beneath the carapace. Females have the anal scutes upturned. All adult females (but no males) found in Uruguay by Buskirk (1991b) bore signs of old, healed injuries, typically having one truncated limb.
Distribution
Acanthochelys spixii ranges from the vicinity of São Paulo southward through Rio Grande do Sul, Brazil, to the provinces of Rocha and Tacuarembó in Uruguay, and westward to the territories of Formosa and Chaco in Argentina; it probably also occurs in Paraguay. It has been introduced near Mendoza, Argentina (Waller, 1988).
Geographic Variation
No subspecies have been described.
Habitat
This turtle lives in slow-moving and standing waters with soft bottoms and abundant aquatic vegetation, including roadside ditches (Buskirk, 1991b). Freiberg (1967) collected Acanthochelys spixii on land in vegetation that had been flooded after intense rains. Buskirk (1991b) found it only at depths not exceeding 30 cm.
Natural History
Courtship behavior in captivity was described by Lehmann (1988). The male slowly approaches the female, often in a face to face position. If she bites at him, the male slightly withdraws, but shortly after approaches her again. This event may be repeated several times, until the male sniffs the female's cloacal region and then mounts her. If she is receptive, the male hooks himself to the female's carapace using his feet and the spiny tubercles on his legs. He then shortly (for approximately 30 s) bobs his head. The intromission itself was not observed by Lehmann (1988). Two courting pairs were observed by Buskirk (1991b) in coastal Uruguay, both at midday in November. One of these was seen from afar as the carapace of the mounted male protruded at a 30° angle from the surface of a marshy lake. The head of the male was extended and could be seen making violent, jerky movements. The pair remained joined as they swam to the bottom upon the author's approach. The other pair, facing one another in 12 cm of water in a roadside ditch, remained motionless, perhaps owing to the author's sudden appearance.
A clutch of seven spherical eggs, 25 mm in diameter, was shown by Freiberg (1981), and Lehmann (1988) reported clutches of four to five 27 mm eggs. A resident of coastal Uruguay told Buskirk (1991b) of finding an intact shell of an A. spixii within which were nine spherical, white eggs in the month of January. Lehmann (1988) removed two vital hatchlings from their egg after 152 and 159 days of incubation (at 29-30°C); they were 31 and 32 mm and weighed 4.8 and 5.0 g respectively. Hatchlings have a dark brown carapace with an orange-red outer rim; the plastron and bridge are black with orange-red spots.
In the wild, Acanthochelys spixii feeds on snails and larval amphibians, but accepts fish and meat in captivity. When first captured, A. spixii does not attempt to bite, but usually emits a characteristic odor similar to that of wet alfalfa (Buskirk, 1991b). The same author encountered Uruguayan specimens molting their dorsal scutes in November. Some turtles bore heavy growths of algae, including on the neck tubercles and chin barbels.
Captives are often extremely shy, keeping their head, neck, and limbs tucked under the shell for long periods. When in this position they resemble small pincushions owing to the elongated spiny tubercles on their neck and thighs, and possibly these evolved as an adaptation to discourage predators.
IUCN Red List Status (1996)
Lower risk: near threatened.