Gerardo A. Cordero

An Enhanced Developmental Staging Table for the Painted Turtle, Chrysemys picta (Testudines: Emydidae)

Normal developmental staging tables often undergo expansion and enhancement in response to advancing research paradigms and technologies. The Painted Turtle, Chrysemys picta, has long been a preferred reference taxon for comparative embryology and recently became the first turtle species to feature a sequenced genome. However, modern descriptive studies on embryogenesis are lacking and an earlier developmental staging table has been ignored. To address these problems, we re‐evaluated descriptions of developmental stages by studying embryos under standardized laboratory conditions. We created an enhanced normal developmental staging table that clarifies and validates previous descriptions of developmental processes in this species. Moreover, we emphasized description of turtle‐specific developmental characters such as the carapacial ridge. We demonstrated that embryo growth rate, length of incubation period, and timing to developmental stages are predictable under controlled environmental conditions. The appearance of characters associated with eye, limb, and shell anatomy was congruent with observations made in other turtle species. To reduce experimental bias, we recommend the use of our enhanced staging table when describing embryogenesis in the Painted Turtle. J. Morphol. 275:442–455, 2014.

Reptile Embryos Lack the Opportunity to Thermoregulate by Moving within the Egg

Historically, egg-bound reptile embryos were thought to passively thermoconform to the nest environment. However, recent observations of thermal taxis by embryos of multiple reptile species have led to the widely discussed hypothesis that embryos behaviorally thermoregulate. Because temperature affects development, such thermoregulation could allow embryos to control their fate far more than historically assumed. We assessed the opportunity for embryos to behaviorally thermoregulate in nature by examining thermal gradients within natural nests and eggs of the common snapping turtle (Chelydra serpentina; which displays embryonic thermal taxis) and by simulating thermal gradients within nests across a range of nest depths, egg sizes, and soil types. We observed little spatial thermal variation within nests, and thermal gradients were poorly transferred to eggs. Furthermore, thermal gradients sufficiently large and constant for behavioral thermoregulation were not predicted to occur in our simulations. Gradients of biologically relevant magnitude have limited global occurrence and reverse direction twice daily when they do exist, which is substantially faster than embryos can shift position within the egg. Our results imply that reptile embryos will rarely, if ever, have the opportunity to behaviorally thermoregulate by moving within the egg. We suggest that embryonic thermal taxis instead represents a play behavior, which may be adaptive or selectively neutral, and results from the mechanisms for behavioral thermoregulation in free-living stages coming online prior to hatching.

Sex-specific survival to maturity and the evolution of environmental sex determination

Four decades ago, it was proposed that environmental sex determination (ESD) evolves when individual fitness depends on the environment in a sex‐specific fashion—a form of condition‐dependent sex allocation. Many biological processes have been hypothesized to drive this sex asymmetry, yet a general explanation for the evolution of sex‐determining mechanisms remains elusive. Here, we develop a mathematical model for a novel hypothesis of the evolution of ESD, and provide a first empirical test using data across turtles. ESD is favored when the sex‐determining environment affects annual survival rates equivalently in males and females, and males and females mature at different ages. We compare this hypothesis to alternative hypotheses, and demonstrate how it captures a crucially different process. This maturation process arises naturally from common life histories and applies more broadly to condition‐dependent sex allocation. Therefore, it has widespread implications for animal taxa. Across turtle species, ESD is associated with greater sex differences in the age at maturity compared to species without ESD, as predicted by our hypothesis. However, the effect is not statistically significant and will require expanded empirical investigation. Given variation among taxa in sex‐specific age at maturity, our survival‐to‐maturity hypothesis may capture common selective forces on sex‐determining mechanisms.

Intraspecific variation in aerobic and anaerobic locomotion: gilthead sea bream (Sparus aurata) and Trinidadian guppy (Poecilia reticulata) do not exhibit a trade-off between maximum sustained swimming speed and minimum cost of transport

Intraspecific variation and trade-off in aerobic and anaerobic traits remain poorly understood in aquatic locomotion. Using gilthead sea bream (Sparus aurata) and Trinidadian guppy (Poecilia reticulata), both axial swimmers, this study tested four hypotheses: (1) gait transition from steady to unsteady (i.e., burst-assisted) swimming is associated with anaerobic metabolism evidenced as excess post exercise oxygen consumption (EPOC); (2) variation in swimming performance (critical swimming speed; Ucrit) correlates with metabolic scope (MS) or anaerobic capacity (i.e., maximum EPOC); (3) there is a trade-off between maximum sustained swimming speed (Usus) and minimum cost of transport (COTmin); and (4) variation in Usus correlates positively with optimum swimming speed (Uopt; i.e., the speed that minimizes energy expenditure per unit of distance traveled). Data collection involved swimming respirometry and video analysis. Results showed that anaerobic swimming costs (i.e., EPOC) increase linearly with the number of bursts in S. aurata, with each burst corresponding to 0.53 mg O2 kg−1. Data are consistent with a previous study on striped surfperch (Embiotoca lateralis), a labriform swimmer, suggesting that the metabolic cost of burst swimming is similar across various types of locomotion. There was no correlation between Ucrit and MS or anaerobic capacity in S. aurata indicating that other factors, including morphological or biomechanical traits, influenced Ucrit. We found no evidence of a trade-off between Usus and COTmin. In fact, data revealed significant negative correlations between Usus and COTmin, suggesting that individuals with high Usus also exhibit low COTmin. Finally, there were positive correlations between Usus and Uopt. Our study demonstrates the energetic importance of anaerobic metabolism during unsteady swimming, and provides intraspecific evidence that superior maximum sustained swimming speed is associated with superior swimming economy and optimum speed.

Skeletal remodelling suggests the turtle's shell is not an evolutionary straitjacket

Recent efforts to decipher the enigma of the turtles shell revealed that distantly related turtle species deploy diverse processes during shell development. Even so, extant species share in common a shoulder blade (scapula) that is encapsulated within the shell. Thus, evolutionary change in the correlated development of the shell and scapula probably underpins the evolution of highly derived shell morphologies. To address this expectation, we conducted one of the most phylogenetically comprehensive surveys of turtle development, focusing on scapula growth and differentiation in embryos, hatchlings and adults of 13 species. We report, to our knowledge, the first description of secondary differentiation owing to skeletal remodelling of the tetrapod scapula in turtles with the most structurally derived shell phenotypes. Remodelling and secondary differentiation late in embryogenesis of box turtles (Emys and Terrapene) yielded a novel skeletal segment (i.e. the suprascapula) of high functional value to their complex shell-closing system. Remarkably, our analyses suggest that, in soft-shelled turtles (Trionychidae) with extremely flattened shells, a similar transformation is linked to truncated scapula growth. Skeletal remodelling, as a form of developmental plasticity, might enable the seemingly constrained turtle body plan to diversify, suggesting the shell is not an evolutionary straitjacket.

Long Distance Aquatic Movement and Home-Range Size of an Eastern Mud Turtle, Kinosternon subrubrum, Population in the Mid-Atlantic Region of the United States

Abstract The aquatic movement patterns and home-range size of the Eastern Mud Turtle, Kinosternon subrubrum, have received little attention. We radio-tracked 5 adult females and 5 adult males during 2 yrs in the Mid-Atlantic region of the United States. Our mean estimates of home-range size (18.6 ± 23 ha in 2008 and 16.3 ± 16.3 ha in 2009) and maximum aquatic distance traveled (815 ± 455 m in 2008 and 774 ± 331 m in 2009) suggest that K. subrubrum is highly adept at movement in aquatic environments.

Effects of low-oxygen conditions on embryo growth in the painted turtle, Chrysemys picta.

Low-oxygen conditions (hypoxia; <21% O2 ) are considered unfavorable for growth; yet, embryos of many vertebrate taxa develop successfully in hypoxic subterranean environments. Although enhanced tolerance to hypoxia has been demonstrated in adult reptiles, such as in the painted turtle (Chrysemys picta), its effects on sensitive embryo life stages warrant attention. We tested the hypothesis that short-term hypoxia negatively affects growth during day 40 of development in C. picta, when O2 demands are highest in embryos. A brief, but severe, hypoxic event (5% O2 for 0.5 h) moderately affected embryo growth, causing a 13% reduction in mass (relative to a normoxic control). The same condition had no effect during day 27; instead, a nearly anoxic event (1% O2 for 72 h) caused a 5% mass reduction. All embryos survived the egg incubation period. Our study supports the assumption that reptilian embryos are resilient to intermittently low O2 in subterranean nests. Further work is needed to ascertain responses to suboptimal O2 levels while undergoing dynamic changes in developmental physiology.

Reptile embryos are not capable of behavioral thermoregulation in the egg

Reptile embryos have recently been observed moving within the egg in response to temperature, raising the exciting possibility that embryos might behaviorally thermoregulate analogous to adults. However, the conjecture that reptile embryos have ample opportunity and capacity to adaptively control their body temperature warrants further discussion. Using turtles as a model, we discuss the spatiotemporal constraints to movement in reptile embryos. We demonstrate that, as embryos grow, the internal egg space rapidly diminishes such that the temporal window for appreciable displacement is confined to stages that feature incomplete neuromuscular differentiation. During this time, muscles are insufficiently developed to actively and consistently control movement. These constraints are well illustrated by the Chinese softshelled turtle (Pelodiscus sinensis), the first reptile reported to behaviorally thermoregulate. Furthermore, sporadic embryo activity peaks after the temperature‐sensitive period in species with temperature‐dependent sex determination, thus nullifying the opportunity for embryos to exhibit control over this important phenotype. These embryonic constraints add to previously‐identified environmental constraints on behavioral thermoregulation by reptile embryos. We discuss alternative hypotheses to explain previously reported patterns of behavioral thermoregulation. Based on a holistic consideration of embryonic limitations, we conclude that reptile embryos are generally unable to adaptively behaviorally thermoregulate within the egg.

Physiology at near-critical temperatures, but not critical limits, varies between two lizard species that partition the thermal environment

The mechanisms that mediate the interaction between the thermal environment and species ranges are generally uncertain. Thermal environments may directly restrict species when environments exceed tolerance limits (i.e. the fundamental niche). However, thermal environments might also differentially affect relative performance among species prior to fundamental tolerances being met (i.e. the realized niche). We examined stress physiology (plasma glucose and corticosterone), mitochondrial performance and the muscle metabolome of congeneric lizards that naturally partition the thermal niche, Elgaria multicarinata (southern alligator lizards; SALs) and Elgaria coerulea (northern alligator lizards; NALs), in response to a thermal challenge to quantify variation in physiological performance and tolerance. Both NAL and SAL displayed physiological stress in response to high temperature, but neither showed signs of irreversible damage. NAL displayed a higher baseline mitochondrial respiration rate than SAL. Moreover, NAL substantially adjusted their physiology in response to thermal challenge, whereas SAL did not. For example, the metabolite profile of NAL shifted with changes in key energetic molecules, whereas these were unaffected in SAL. Our results indicate that near-critical high temperatures should incur greater energetic cost in NAL than SAL via an elevated metabolic rate and changes to the metabolome. Thus, SAL displace NAL in warm environments that are within NALs fundamental thermal niche, but relatively costly. Our results suggest that subcritical thermal events can contribute to biogeographic patterns via physiological differences that alter the relative costs of living in warm or cool environments.

Re-emergence of the Painted Turtle (Chrysemys picta) as a reference species for evo-devo

Evolutionary developmental biology requires integration of knowledge from multiple levels of organization. Information at the genome level is quickly outpacing crucial descriptive information on developmental processes at the cell, tissue, and organ levels. Unprecedented progress in genomics has enabled the rise of so‐called non‐traditional reference species. However, a substantial, though overlooked, amount of phenotypic information has long been available for species such as the Painted Turtle (Chrysemys picta), which recently became the first turtle to feature a fully sequenced and annotated nuclear genome. In this commentary, I highlight research on the comparative embryology of Chrysemys during the twentieth century and briefly discuss future directions for research on this re‐emerging species for evo‐devo.