Batagur trivittata Duméril & Bibron, 1835

Platt, Steven G., Lwin, Tint, Win, Myo Min, Platt, Kalyar, Haislip, Nathan A., Dijk, Peter Paul Van & Rainwater, Thomas R., 2020, First description of neonate Batagur trivittata (Testudines: Geoemydidae), Zootaxa 4821 (2), pp. 394-400 : 394-399

Batagur trivittata Duméril & Bibron, 1835

Our sample (N = 392) included neonate B. trivittata sourced from the captive-breeding group at YZG (N = 261) and hatched from eggs deposited by wild females along the Chindwin River and head-started at LV (N = 131; Fig. 1 ). Neo- nates at both sites were examined and measured 12–36 hours after hatching. Using a pair of dial calipers we measured (to the nearest 1.0 mm) CL (Method D of Iverson & Lewis 2018), maximum carapace width (CW), mid-line plastron length (PL; from base of anal notch to anterior margin of gular scute), and shell depth (SD; from plastron to highest point of carapace) of each neonate. We used a Pesola® spring scale (± 5.0 g) and Camry® digital balance (± 0.001 g) to determine body mass (BM) of neonates at YZG and LV, respectively. We photographed neonates with a Canon 7D EOS Mark II (with Canon Macro lens EF 100 mm 1:2.8 USM) and Sony A6000 digital cameras. We made comparisons with older cohorts being head-started at LV to document the ontogeny of certain shell attributes.

We obtained measurements of shell attributes from all neonates in our sample, but report values for body mass only for neonates at LV because the wide margin of error associated with spring scales used at YZG precluded a sufficiently precise measure of this attribute. Mean ± 1SD and range (in parentheses) of shell attributes and body mass of neonate B. trivittata are as follows: CL = 55.3 ± 4.0 mm (43.0–65.0 mm); CW = 51.7 ± 4.6 mm (35.0– 61.8 mm); PL = 49.4 ± 4.3 mm (35.0–59.0 mm); SD = 30.4 ± 3.7 mm (20.0–48.0 mm); BM = 47.7 ± 4.4 mm (36.3–60.0 mm). Assuming an upper asymptotic CL of 580–620 mm for adult female B. trivittata ( Smith 1931; Platt et al. 2019), the mean CL of neonates is 8.9–9.5% of the maximum adult body size. Mean neonate body mass was 74.7% of mean egg mass for eggs collected during the 2017–18 and 2018–19 nesting seasons (mean egg mass = 63.8 ± 4.0 g; range = 50.3–70.0 g; N = 137; Platt et al. unpubl. data).

In neonate B. trivittata the overall shape of the carapace is ovoid to almost circular (mean CW = 93% mean CL), being widest at the third vertebral and sixth marginal pair, and tapering posteriorly ( Fig. 2 ). The carapace is green to brown with dark elongated blotches that straddle adjacent pleural scutes 1–4. The terminal blotch on the fourth pleural scute does not extend onto the adjacent fifth vertebral, but instead is present at the junction of the pleural, vertebral, and marginal scutes, and much reduced. Similar dark pleural blotches are also present in hatchling B. borneoensis and B. dhongoka , but absent in B. affinis , B. baska and B. kachuga ( Moll 1986; Praschag et al. 2009; Moll et al. 2015; Guntoro 2017; Turtle Survival Alliance, unpublished photographs). Notably, molecular analyses indicate that B. borneoensis and B. dhongoka are successive sister taxa to B. trivittata , while B. affinis , B. baska , and B. kachuga constitute a separate clade of Batagur ( Minh et al. 2007; Praschag et al. 2007, 2009). Thus, patterns of carapace coloration among hatchings appear to reflect the underlying phylogenetic relationships in Batagur .

The medial keel is well-pronounced on vertebrals 1–4 with sharp, spine-like posterior-pointing projections on vertebrals 2–4. The posterior edge of marginals 1–7 are tipped with a sharp rearward-pointing spine that becomes increasingly larger, reaching its greatest development on marginal 7. Marginal 6 is weakly serrated; marginals 7–12 are deeply serrated with the posterior edge of each scute (except 12) curving rearwards. Faint grey-black elongated blotches are present on marginals 4–10, overlapping the intermarginal seams; these fade rapidly and are no longer evident within a few weeks of hatching.

The plastron, bridge, and ventral surface of the marginals are whitish to cream. An umbilical scar is present on the mid-line of the abdominal scute of very young neonates, but disappears rapidly with age ( Fig. 3A ). A prominent keel-like bony ridge extends along the lateral edge of the plastron from the humeral to anal scutes, forming a distinct angle between the plastron and bridge regions of the pectoral and abdominal scutes. The posterior edge of this ridge on each scute is tipped with a sharp rearward projecting spine. This spine is well-developed on the pectoral, abdominal, and femoral scutes; less so on the anal scute, and weakly developed on the humeral scute ( Fig. 3B ).

Soft parts of neonates vary in coloration from uniform green-brown to dark grey dorsally, with most individuals tending towards the latter; the ventral surface of the legs, tail and gular region are lighter (white-yellowish). The rostrum extends horizontally from the snout with no upturn. The keratinized sheath of the maxilla is slightly serrated while that of the mandible is smooth; a caruncle (egg-tooth) is present just below the rostrum in very young neonates ( Fig. 4 ). Neonates hatched in mid- to late May at LV retained the caruncle through June, but the structure was no longer present in any individual at the end of July. The fore- and hind-feet are fully webbed with sharp claws (five and four on each fore- and hind-foot, respectively) extending just beyond the light-colored margin. The iris is greenish-yellow with a dark pupil.

A number of notable ontogenetic changes occur as neonate B. trivittata mature. The dark elongated blotches on the pleural scutes of neonates are absent from older females, which have a solid dark brown-gray carapace ( Smith 1931; Platt et al. 2017b). In adult males, the pleural blotches probably coalesce to form the black lateral stripes on the pleural scutes ( Smith 1931), although we have not yet monitored cohorts long enough to confirm the origin of these stripes. The medial carapace keel is present but lower and less pronounced in adults. Our observations of successive cohorts being headstarted at LV indicate the vertebral spines are blunt after two years of growth, with the exception of the spine on vertebral 4, which remains pointed and sharp to the touch until 3–4 years of age. The rearward pointing spines on marginals 1–7 remain sharp to the touch in one-year-old turtles, become less so in two-year-olds, and are blunt and rounded by age three. Marginals 7–12 become progressively less serrated as turtles mature, with the posterior margin of the carapace becoming smooth by age five.

Coloration of the plastron, plastral bridge, and ventral surface of the marginals in females change from whitish-cream to dark brown-black with increasing age and by age five these surfaces are heavily pigmented ( Platt et al. 2017b). In contrast, the plastron, plastral bridge, and ventral surface of the marginals in males remain whitish-cream into maturity ( Platt et al. 2017b). The bony ridge extending along the distal edge of the plastron becomes smoother with age and is no longer evident in five-year-old B. trivittata . The spines adorning this ridge remain sharp to the touch in two-year-old turtles, become blunt by age three, and have largely disappeared in four-year-olds. The functional significance of the spines on the vertebrals and plastral ridge of neonate B. trivittata is unknown; however, the spines on other turtles are generally interpreted as an important physical defense that deters consumption by gape-limited predators such as large fish and wading birds ( Inbar & Lev-Yadun 2005). The fact that spines on B. trivittata disappear as the turtle matures and body size increasingly provides safety from predation seems consistent with the anti-predator defense hypothesis.

Figures

FIGURE 1. Neonate Batagur trivittata moments after emerging from an egg collected from a sandbank along the Chindwin River and incubated at a head-starting facility in Limpha Village, Sagaing Region, Myanmar (Photographed by Myo Min Win).

FIGURE 2. Carapace of neonate Batagur trivittata approximately one month post-hatching. Note the prominent medial keel on vertebrals 2–4 and serrated marginal scutes. The faint blotches visible on some marginal scutes disappear rapidly with age. (Photographed by Myo Min Win).

FIGURE 3. Plastron of neonate Batagur trivittata showing umbilical scar and prominent keel-like bony ridge extending along the lateral edge from humeral to anal scutes (A). Lateral view of the bony ridge (B). Note the sharp rearward pointing spines on the bony ridge, which become blunt by age three and disappear by age four. The bony ridge recedes with age and is no longer evident by age five. Note also the spine-like posterior-pointing projections on the medial keel of the carapace (Photographed by Myo Min Win).

FIGURE 4. Head of neonate Batagur trivittata. Note caruncle (egg-tooth) and eye color. The caruncle is lost during the two months after hatching. (Photographed by Myo Min Win).