Survival and Physiological Responses of Hatchling Blanding’s Turtles (Emydoidea blandingii) to Submergence in Normoxic and Hypoxic Water under Simulated Winter Conditions

Development-specific transcriptomic profiling suggests new mechanisms for anoxic survival in the ventricle of overwintering turtles

Oxygen deprivation swiftly damages tissues in most animals, yet some species show remarkable abilities to tolerate little or even no oxygen. Painted turtles exhibit a development-dependent tolerance that allows adults to survive anoxia ∼4x longer than hatchlings: adults survive ∼170 days and hatchlings survive ∼40 days at 3°C. We hypothesized this difference is related to development-dependent differences in ventricular gene expression. Using a comparative ontogenetic approach, we examined whole transcriptomic changes before, during, and five days after a 20-day bout of anoxic submergence at 3°C. Ontogeny accounted for more gene expression differences than treatment (anoxia or recovery): 1,175 vs. 237 genes, respectively. Of the 237 differences, 93 could confer protection against anoxia and reperfusion injury, 68 could be injurious, and 20 may be constitutively protective. Especially striking during anoxia was the expression pattern of all 76 annotated ribosomal protein (R-protein) mRNAs, which decreased in anoxia-tolerant adults, but increased in anoxia-sensitive hatchlings, suggesting adult-specific regulation of translational suppression. These genes, along with 60 others that decreased their levels in adults and either increased or remained unchanged in hatchlings, implicate antagonistic pleiotropy as a mechanism to resolve the long-standing question about why hatchling painted turtles overwinter in terrestrial nests, rather than emerge and overwinter in water during their first year. In sum, developmental differences in the transcriptome of the turtle ventricle revealed potentially protective mechanisms that contribute to extraordinary adult-specific anoxia tolerance, and provide a unique perspective on differences between the anoxia-induced molecular responses of anoxia-tolerant or anoxia-sensitive phenotypes within a species.

Water exchange relationships predict overwintering behavior in hatchling turtles

Neonatal ectotherms face a wide range of environmental hazards because of the diverse habitats that they inhabit and their small body sizes; this is especially true among turtles that live in temperate zones and experience cold winter conditions after hatching. Such hatchlings must balance challenges involving desiccation, freezing, and predation, among other threats. Turtle hatchlings either overwinter in water, terrestrially in relatively shallow nests, terrestrially deep below nests, or terrestrially outside of the nest entirely, and these different microhabitats are associated with different desiccation and freezing risks. We measured desiccation tolerance of individuals of six turtle species, including two (Diamondback Terrapins, Malaclemys terrapin (Schoepff, 1793), and Eastern Box Turtles, Terrapene carolina (Linnaeus, 1758)) that use a strategy that has not previously been explored, along with Wood Turtles (Glyptemys insculpta (Le Conte, 1830)), whose overwintering microhabitat is uncertain. We found additional support for the hypothesis that desiccation resistance is associated with overwintering strategies in hatchling turtles. Further investigation into the overwintering strategies of M. terrapin and T. carolina would be productive.

Freshwater turtle hatchlings that stay in the nest: strategists or prisoners?

Hatchlings of several species of freshwater turtles have been reported to remain in subterranean nests for extended periods following hatching from the egg. It has been suggested that this delayed emergence, including overwintering in the nest in populations at temperate latitudes, is an evolved adaptation that enables hatchlings to enter the aquatic environment at the most propitious time for survival and growth. I monitored nests of a temperate-zone population of the freshwater Australian eastern long-necked turtle (Chelodina longicollis) for up to a year after nest construction in fine-grained soils adjacent to oxbow lakes and farm ponds. An estimated 84% of nests were preyed on, probably mainly by non-native red foxes (Vulpes vulpes), whereas hatchlings emerged from autumn to spring from an estimated 5% of nests. The remaining 11% of nests were neither preyed on nor had emergence by a year after nest construction. Live hatchlings were present in some nests with no emergence up to 10 months after nest construction, but substantial numbers of dead hatchlings were present beyond nine months. It therefore seems unlikely that emergence occurs more than a year after nest construction. Delayed emergence of this species in this environment appears less likely to be an adaptive strategy than to be a consequence of imprisonment in the nest by hard soil that is difficult for hatchlings to excavate.

Movements, overwintering, and mortality of hatchling Diamond-backed Terrapins (Malaclemys terrapin) at Jamaica Bay, New York

As with other turtles, the postemergent movements, overwintering behaviours, and survivorship of hatchling Diamond-backed Terrapins ( Malaclemys terrapin (Schoepff, 1793)) are poorly known, but anecdotal reports suggest that they may spend more time on land than most aquatic turtles. We investigated this behaviour using drift fences with pitfall traps on the island of Ruler’s Bar, Jamaica Bay, New York, fall 2006 to spring 2008. We captured 324 live hatchling Terrapins, 95 were recaptured at least once, and we found 43 dead. After emergence from nests in the fall, most hatchlings moved upland away from the water; this pattern was reversed in the spring. Hatchling body sizes shrank during winter, probably owing to desiccation, and hatchlings were more likely to move on warmer days and days without precipitation. We recaptured some hatchlings on land as long as 9 months after emergence. As a result, hatchling M. terrapin were seen on land from April to December, well outside fall and spring during which they emerge from nests, and we found strong evidence that hatchling M. terrapin overwinter on land outside their nests. One important nest predator (raccoons, Procyon lotor (L., 1758)) was also an important hatchling predator, as were Norway rats ( Rattus norvegicus (Berkenhout, 1769)). Future work should investigate the terrestrial microhabitats used by hatchling M. terrapin, and management should protect hatchlings during this life stage.

Physiological ecology of overwintering in hatchling turtles

Temperate species of turtles hatch from eggs in late summer. The hatchlings of some species leave their natal nest to hibernate elsewhere on land or under water, whereas others usually remain inside the nest until spring; thus, post-hatching behavior strongly influences the hibernation ecology and physiology of this age class. Little is known about the habitats of and environmental conditions affecting aquatic hibernators, although laboratory studies suggest that chronically hypoxic sites are inhospitable to hatchlings. Field biologists have long been intrigued by the environmental conditions survived by hatchlings using terrestrial hibernacula, especially nests that ultimately serve as winter refugia. Hatchlings are unable to feed, although as metabolism is greatly reduced in hibernation, they are not at risk of starvation. Dehydration and injury from cold are more formidable challenges. Differential tolerances to these stressors may explain variation in hatchling overwintering habits among turtle taxa. Much study has been devoted to the cold-hardiness adaptations exhibited by terrestrial hibernators. All tolerate a degree of chilling, but survival of frost exposure depends on either freeze avoidance through supercooling or freeze tolerance. Freeze avoidance is promoted by behavioral, anatomical, and physiological features that minimize risk of inoculation by ice and ice-nucleating agents. Freeze tolerance is promoted by a complex suite of molecular, biochemical, and physiological responses enabling certain organisms to survive the freezing and thawing of extracellular fluids. Some species apparently can switch between freeze avoidance or freeze tolerance, the mode utilized in a particular instance of chilling depending on prevailing physiological and environmental conditions.

Hypoxia tolerance in reptiles, amphibians, and fishes: life with variable oxygen availability

The ability of fishes, amphibians, and reptiles to survive extremes of oxygen availability derives from a core triad of adaptations: profound metabolic suppression, tolerance of ionic and pH disturbances, and mechanisms for avoiding free-radical injury during reoxygenation. For long-term anoxic survival, enhanced storage of glycogen in critical tissues is also necessary. The diversity of body morphologies and habitats and the utilization of dormancy have resulted in a broad array of adaptations to hypoxia in lower vertebrates. For example, the most anoxia-tolerant vertebrates, painted turtles and crucian carp, meet the challenge of variable oxygen in fundamentally different ways: Turtles undergo near-suspended animation, whereas carp remain active and responsive in the absence of oxygen. Although the mechanisms of survival in both of these cases include large stores of glycogen and drastically decreased metabolism, other mechanisms, such as regulation of ion channels in excitable membranes, are apparently divergent. Common themes in the regulatory adjustments to hypoxia involve control of metabolism and ion channel conductance by protein phosphorylation. Tolerance of decreased energy charge and accumulating anaerobic end products as well as enhanced antioxidant defenses and regenerative capacities are also key to hypoxia survival in lower vertebrates.

The ecology of overwintering among turtles: where turtles overwinter and its consequences

Turtles are a small taxon that has nevertheless attracted much attention from biologists for centuries. However, a major portion of their life cycle has received relatively little attention until recently - namely what turtles are doing, and how they are doing it, during the winter. In the northern parts of their ranges in North America, turtles may spend more than half of their lives in an overwintering state. In this review, I emphasise the ecological aspects of overwintering among turtles, and consider how overwintering stresses affect the physiology, behaviour, distributions, and life histories of various species. Sea turtles are the only group of turtles that migrate extensively, and can therefore avoid northern winters. Nevertheless, each year a number of turtles, largely juveniles, are killed when trapped by cold fronts before they move to safer waters. Evidently this risk is an acceptable trade-off for the benefits to a population of inhabiting northern developmental habitats during the summer. Terrestrial turtles pass the winter underground, either in burrows that they excavate or that are preformed. These refugia must provide protection against desiccation and lethal freezing levels. Some burrows are extensive (tortoise genus Gopherus), while others are shallow, or the turtles may simply dig into the ground to a safe depth (turtle genus Terrapene). In the latter genus, freeze tolerance may play an adaptive role. Most non-marine aquatic turtles overwinter underwater, although Clemmys (Actinemys) marmorata routinely overwinters on land when it occurs in riverine habitats, Kinosternon subrubrum often overwinters on land, and several others may overwinter terrestrially on occasion, especially in more southern climates. For northern species that overwinter underwater, there are two physiological groupings, those that are anoxia-tolerant and those that are relatively anoxia-intolerant. All species fare well physiologically in water with a high partial pressure of oxygen (PO2). A lack of anoxia tolerance limits the types of habitats that a freshwater turtle may live in, since unlike sea turtles, they cannot travel long distances to hibernate. Hatchlings of some species of turtles spend their first winter in or below the nest cavity, while hatchlings of other species in the same area, including northern areas, emerge in the autumn and presumably hibernate underwater. All hatchlings are relatively anoxia-intolerant, and there are no studies to date of where hatchling turtles that do not overwinter in or below the nest cavity spend their first winter. Equally little is known of the ontogeny of anoxia tolerance, other than that adults of all species are more anoxia-tolerant than their hatchlings, probably because of their better ossified shells, which provide adults with more buffer reserves and a larger site in which to sequester lactate. The northern limits of turtles are most likely determined by reproductive limitations (time for egg-laying, incubation, and hatching) than by the rigors of hibernation. Mortality is typically lower in turtle populations during hibernation than it is during their active periods. However, episodic mortality events do occur during hibernation, due to freezing, prolonged anoxia, or predation.

Anoxia tolerance and freeze tolerance in hatchling turtles

Freezing survival in hatchling turtles may be limited by ischemic anoxia in frozen tissues and the associated accumulation of lactate and reactive oxygen species (ROS). To determine whether mechanisms for coping with anoxia are also important in freeze tolerance, we examined the association between capacities for freezing survival and anoxia tolerance in hatchlings of seven species of turtles. Tolerance to freezing (-2.5 degrees C) was high in Emydoidea blandingii, Chrysemys picta, Terrapene ornata, and Malaclemys terrapin and low in Graptemys geographica, Chelydra serpentina, and Trachemys scripta. Hatchlings survived in a N(2) atmosphere at 4 degrees C for periods ranging from 17 d (M. terrapin) to 50 d (G. geographica), but survival time was not associated with freeze tolerance. Lactate accumulated during both stresses, but plasma levels in frozen/thawed turtles were well below those found in anoxia-exposed animals. Activity of the antioxidant enzyme catalase in liver increased markedly with anoxia exposure in most species, but increased with freezing/thawing only in species with low freeze tolerance. Our results suggest that whereas oxygen deprivation occurs during somatic freezing, freeze tolerance is not limited by anoxia tolerance in hatchling turtles.