(Agassiz, 1857)
Loggerhead musk turtle
Recognition
The oval carapace (to 13.5 cm) is generally deep, serrated posteriorly, and has a vertebral keel and two dorsolateral keels (which may disappear with age). Its sides always slope at an angle less than 100°, and the mean angle/height ratio is about 5: 1. The vertebral scutes overlap the one behind. The 1st vertebral is long, and never touches the 2nd marginals; the other four vertebrals are slightly broader than long, and the 5th is posteriorly expanded. The carapace is dark brown to orange with dark-bordered seams and often a pattern of scattered dark spots or radiating dark streaks which fade with age. The immaculate pink to yellowish plastron has a single gular scute, and contains an indistinct hinge between the pectoral and abdominal scutes. There is only a shallow posterior notch. The plastral formula is: an > abd > pect > fem > hum > gul. The head is moderate to large (especially in older turtles) with a protruding snout and a slightly hooked to nonhooked upper jaw. The rostral shield is furcated posteriorly, and there are two chin barbels. The head is grayish to pink with dark dots; jaws are tan with dark streaks. Other skin is pinkish, gray, or orange with dark streaks.
Chromosomes number 56: 12 macrochromosomes with median or submedian centromeres, 12 macrochromosomes with terminal or subterminal centromeres, and 16 additional pairs of chromosomes, including microchromosomes (Sites et al., 1979b).
Males have thick, long, spine-tipped tails with the vent behind the carapacial rim, and roughened patches of scales on their thighs and crura.
Distribution
Sternotherus minor ranges from southwestern Virginia, eastern Tennessee, and central Georgia south to central Florida and west to the Pearl River system in south-central Mississippi.
Geographic Variation
Two subspecies are recognized. Sternotherus minor minor (Agassiz, 1857), the loggerhead musk turtle, ranges from central Georgia and southeastern Alabama to central Florida. It usually lacks stripes on the head and neck, and the juvenile has three carapacial keels (these may disappear with age). S. m. peltifer Smith and Glass, 1947, the stripe-necked musk turtle, ranges from eastern Tennessee and Alabama to the Pearl River in south-central Mississippi. It has only a single, middorsal carapacial keel in juveniles (which disappears with age), and distinct wide stripes on the neck.
The subspecies peltifer was thought to hybridize with S. depressus in west-central Alabama (Mount, 1975); however, Ernst et al. (1988b) proved that these populations exhibit variation within the range of S. depressus and should be included in that species. They also reported that S. m. peltifer has a different slope to its growth curve than either S. m. minor or S. depressus and may represent a separate species; however, this needs additional verification.
Habitat
Sternotherus minor inhabits rivers, creeks, spring runs, oxbows, and swamps. It occurs most commonly around snags, fallen trees, or rocky outcrops.
Natural History
Males mature at a carapace length of 55-60 mm in 3-9 years (Etchberger and Stovall, 1990; Cox et al., 1991). Females mature at an average age of eight (5-9) years at about 80 mm carapace length (Tinkle, 1958a, Etchberger and Ehrhart, 1987; Etchberger, 1988; Cox et al., 1991). Spermatogonia proliferate and become abundant from March to June, there is a great increase in testes mass in July, and testes achieve their greatest mass in August when spermatozoa are abundant. September-October marks the peak in spermatogenesis, after which the sperm passes to the epididymides for storage over winter. Female follicular enlargement begins in August or September and continues through the next June. Ovulation and oviposition occur from October through June or July.
The following behavior was recorded during courtship and mating by S. minor observed by Bells and Crama (1994). All courtship occurs under water. The male approaches and pursues the female. With fully extended head and neck, he smells her cloacal region and bridge. The male may bite the female while performing some of the sniffing procedures. He then turns and moves either anteriorly toward her head or posteriorly toward her tail, while keeping his nose along her marginals. She usually tries to bite or flee at this point. If he turns anteriorly, the male positions himself in front of the female's head and swings his head and neck back and forth sideways. If the male proceeds posteriorly to a position behind the female, he will trail her as she moves and stops when she stops. Mounting occurs next, with the male grasping the marginals of the female's carapace with the claws of his forefeet. He then curls his tail under hers to bring the vents together and a mutual grasping and interlocking of the tails ensues. Intromission soon follows. During copulation the male is usually positioned almost perpendicular to the female's carapace. Mating may occur throughout most of the year, as Cox and Marion (1978) and Iverson (1978a) found oviducal eggs in females from September to July, but observations of copulation have only been in March and April (Cox et al., 1980).
Nests are often placed by a log or at the base of a tree. As many as 2-5 clutches of 1-5 eggs may be laid a year, and the number of eggs per clutch is directly correlated to female body size. The eggs are elongated (X = 28.5 x 17.2 mm, Iverson, 1978a) with brittle white shells. The young hatch after an incubation period of 61-119 days.
Hatchlings of both subspecies have a medial keel and two lateral keels. The carapace bears a pattern of dark streaks, blotches or spots (the typical Sternotherus juvenile pattern). Most hatchlings have carapace lengths of 22-27 mm.
Sternotherus minor may forage in shallow water of a few mm to depths of 12.5 m (Hensley, 1995). It is primarily a mollusk eater, but it also feeds on filamentous algae, vascular aquatic plants, aquatic insects, millipedes, spiders, crayfish, and fishes. In S. m. minor, increasing size brings a shift from an insectivorous to a molluscivorous (snail, clam) diet. This subspecies develops heavy lower jaw musculature and an expanded crushing surface on both jaws—apparently adaptations to eating mollusks.
Remark
Morphological and electrophoretic studies by Tinkle (1958a), Zug (1966), Iverson (1977b), Seidel and Lucchino (1981), and Seidel et al. (1981) have shown Sternotherus minor to be distinct, but more closely related to S. depressus and S. odoratus than to S. carinatus.
IUCN Red List Status (1996)
Not listed.