Pleated turtle escapes the box--shape changes in Dermochelys coriacea

Marine turtles are only minimally sexually size dimorphic, a pattern that is distinct from most nonmarine aquatic turtles

Turtles have been prominent subjects of sexual size dimorphism (SSD) analyses due to their compact taxonomy, mating systems, and habitat diversity. In prior studies, marine turtles were grouped with fully aquatic non‐marine turtles (NMATs). This is interesting because it is well‐established that the marine environment imposes a distinct selective milieu on body form of vagile vertebrates, driven by convergent adaptations for energy‐efficient propulsion and drag reduction. We generated a comprehensive database of adult marine turtle body sizes (38,569 observations across all species), which we then used to evaluate the magnitude of SSD in marine turtles and how it compares to SSD in NMAT. We find that marine turtles are only minimally sexually size dimorphic, whereas NMAT typically exhibit female‐biased SSD. We argue that the reason for this difference is the sustained long‐distance swimming that characterizes marine turtle ecology, which entails significant energetic costs incurred by both sexes. Hence, the ability of either sex to allocate proportionately more to growth than the other is likely constrained, meaning that sexual differences in growth and resultant body size are not possible. Consequently, grouping marine turtles with NMAT dilutes the statistical signature of different kinds of selection on SSD and should be avoided in future studies.

The macroevolutionary and developmental evolution of the turtle carapacial scutes

The scutes of the carapace of extant turtles exhibit common elements in a narrow range of topographical arrangements. The typical arrangement has remained constant since its origin in the clade Mesochelydia (Early Jurassic), after a period of apparent greater diversity in the Triassic. This contribution is a review of the development and evolutionary history of the scute patterns of the carapace, seen through the lens of recent developmental models. This yields insights on pattern variations in the fossil record. We reinterpret the “supracaudal” scute and propose that Proganochelys had five vertebral scutes. We discuss the relationship between supramarginal scutes and Turing processes, and we show how a simple change during embryogenesis could account for origin of the configuration of the caudal region of the carapace in mesochelydians. We also discuss the nature of the decrease in number of scutes over the course of evolution, and whether macroevolutionary trends can be discerned. We argue that turtles with complete loss of scutes (e.g., softshells) follow clade-specific macroevolutionary regimes, which are distinct from the majority of other turtles. Finally, we draw a parallel between the variation of scute patterns on the carapace of turtles and the scale patterns in the pileus region (roof of the head) of squamates. The size and numbers of scales in the pileus region can evolve over a wide range, but we recognized tentative evidence of convergence towards a typical configuration when the scales become larger and fewer. Thus, typical patterns could be a more general property of similar systems of integumentary appendages.

Morphometrics and blood analytes of leatherback sea turtle hatchlings (Dermochelys coriacea) from Florida: reference intervals, temporal trends with clutch deposition date, and body size correlations

The northwest Atlantic leatherback sea turtle (Dermochelys coriacea) population is exhibiting decreasing trends along numerous nesting beaches. Since population health and viability are inherently linked, it is important to establish species- and life-stage class-specific blood analyte reference intervals (RIs) so that effects of future disturbances on organismal health can be better understood. For hatchling leatherbacks, the objectives of this study were to (1) establish RIs for morphometrics and blood analytes; (2) evaluate correlations between hatchling morphometrics, blood analytes, and hatching success; and (3) determine temporal trends in hatchling morphometrics and blood analytes across nesting season. Blood samples were collected from 176 naturally emerging leatherback hatchlings from 18 clutches. Reference intervals were established for morphometrics and blood analytes. Negative relationships were noted between hatchling mass and packed cell volume, total white blood cells, heterophils, lymphocytes, and total protein and between body condition index (BCI) and immature red blood cells (RBC), RBC polychromasia and anisocytosis, and total protein. Clutch deposition date showed positive relationships with lymphocytes and total protein, and negative relationships with hatchling mass and BCI. Hatching success was positively correlated with mass, and negatively with total protein and glucose, suggesting that nutritional provisions in eggs, incubation time, and/or metabolic rates could change later in the season and affect survivorship. These various observed correlations provide evidence for increased physiological stress (e.g., inflammation, subclinical dehydration) in hatchlings emerging later in nesting season, presumably due to increased nest temperatures or other environmental factors (e.g., moisture/rainfall). Data reported herein provide morphometric and blood analyte data for leatherback hatchlings and will allow for future investigations into spatiotemporal trends and responses to various stressors.

A Baseline Model For Estimating the Risk of Gas Embolism in Sea Turtles During Routine Dives

Sea turtles, like other air-breathing diving vertebrates, commonly experience significant gas embolism (GE) when incidentally caught at depth in fishing gear and brought to the surface. To better understand why sea turtles develop GE, we built a mathematical model to estimate partial pressures of N2 (PN2), O2 (PO2), and CO2 (PCO2) in the major body-compartments of diving loggerheads (Caretta caretta), leatherbacks (Dermochelys coriacea), and green turtles (Chelonia mydas). This model was adapted from a published model for estimating gas dynamics in marine mammals and penguins. To parameterize the sea turtle model, we used values gleaned from previously published literature and 22 necropsies. Next, we applied this model to data collected from free-roaming individuals of the three study species. Finally, we varied body-condition and cardiac output within the model to see how these factors affected the risk of GE. Our model suggests that cardiac output likely plays a significant role in the modulation of GE, especially in the deeper diving leatherback turtles. This baseline model also indicates that even during routine diving behavior, sea turtles are at high risk of GE. This likely means that turtles have additional behavioral, anatomical, and/or physiologic adaptions that serve to reduce the probability of GE but were not incorporated in this model. Identifying these adaptations and incorporating them into future iterations of this model will further reveal the factors driving GE in sea turtles.

Adaptive Patterns of Mitogenome Evolution Are Associated with the Loss of Shell Scutes in Turtles

The mitochondrial genome encodes several protein components of the oxidative phosphorylation (OXPHOS) pathway and is critical for aerobic respiration. These proteins have evolved adaptively in many taxa, but linking molecular-level patterns with higher-level attributes (e.g., morphology, physiology) remains a challenge. Turtles are a promising system for exploring mitochondrial genome evolution as different species face distinct respiratory challenges and employ multiple strategies for ensuring efficient respiration. One prominent adaptation to a highly aquatic lifestyle in turtles is the secondary loss of keratenized shell scutes (i.e., soft-shells), which is associated with enhanced swimming ability and, in some species, cutaneous respiration. We used codon models to examine patterns of selection on mitochondrial protein-coding genes along the three turtle lineages that independently evolved soft-shells. We found strong evidence for positive selection along the branches leading to the pig-nosed turtle (Carettochelys insculpta) and the softshells clade (Trionychidae), but only weak evidence for the leatherback (Dermochelys coriacea) branch. Positively selected sites were found to be particularly prevalent in OXPHOS Complex I proteins, especially subunit ND2, along both positively selected lineages, consistent with convergent adaptive evolution. Structural analysis showed that many of the identified sites are within key regions or near residues involved in proton transport, indicating that positive selection may have precipitated substantial changes in mitochondrial function. Overall, our study provides evidence that physiological challenges associated with adaptation to a highly aquatic lifestyle have shaped the evolution of the turtle mitochondrial genome in a lineage-specific manner.

Crying a river: how much salt-laden jelly can a leatherback turtle really eat?

Leatherback turtles (Dermochelys coriacea) are capital breeders that accumulate blubber (33 kJ g wet mass−1) by hyperphagia on a gelatinous diet at high latitudes; they breed in the tropics. A jellyfish diet is energy-poor (0.1–0.2 kJ g wet mass−1), so leatherbacks must ingest large quantities. Two published estimates of feeding rate (50% body mass d−1 (on Rhizostoma pulmo), 73% body mass d−1 (on Cyanea capillata)) have been criticised as too high. Jellyfish have high salt and water contents that must be removed to access organic material and energy. Most salt is removed (as NaCl) by paired lachrymal salt glands. Divalent ions are lost via the gut. In this study the size of adult salt glands (0.622 kg for a 450kg turtle; relatively 3 times the size of salt glands in cheloniid turtles) is measured for the first time by CT scanning. Various published values for leatherback field metabolic rate (FMR), body fluid composition and likely blubber accumulation rates are combined with known jellyfish salt, water and organic compositions to calculate feasible salt gland secretion rates and feeding rates. The results indicate that leatherbacks can produce about 10–15 ml secretion g salt gland mass−1 h−1 (tear osmolality 1800 mOsm kg−1). This will permit consumption of 80 % body mass d−1 of Cyanea capillata. Calculations suggest that leatherbacks will find it difficult/impossible to accumulate sufficient blubber for reproduction in a single feeding season. Rapid jellyfish digestion and short gut transit times are essential.

Topsy-turvy: turning the counter-current heat exchange of leatherback turtles upside down

Counter-current heat exchangers associated with appendages of endotherms feature bundles of closely applied arteriovenous vessels. The accepted paradigm is that heat from warm arterial blood travelling into the appendage crosses into cool venous blood returning to the body. High core temperature is maintained, but the appendage functions at low temperature. Leatherback turtles have elevated core temperatures in cold seawater and arteriovenous plexuses at the roots of all four limbs. We demonstrate that plexuses of the hindlimbs are situated wholly within the hip musculature, and that, at the distal ends of the plexuses, most blood vessels supply or drain the hip muscles, with little distal vascular supply to, or drainage from the limb blades. Venous blood entering a plexus will therefore be drained from active locomotory muscles that are overlaid by thick blubber when the adults are foraging in cold temperate waters. Plexuses maintain high limb muscle temperature and avoid excessive loss of heat to the core, the reverse of the accepted paradigm. Plexuses protect the core from overheating generated by muscular thermogenesis during nesting.

The physiology of lipid storage and use in reptiles

Lipid metabolism is central to understanding whole-animal energetics. Reptiles store most excess energy in lipid form, mobilise those lipids when needed to meet energetic demands, and invest lipids in eggs to provide the primary source of energy to developing embryos. Here, I review the mechanisms by which non-avian reptiles store, transport, and use lipids. Many aspects of lipid absorption, transport, and storage appear to be similar to birds, including the hepatic synthesis of lipids from glucose substrates, the transport of triglycerides in lipoproteins, and the storage of lipids in adipose tissue, although adipose tissue in non-avian reptiles is usually concentrated in abdominal fat bodies or the tail. Seasonal changes in fat stores suggest that lipid storage is primarily for reproduction in most species, rather than for maintenance during aphagic periods. The effects of fasting on plasma lipid metabolites can differ from mammals and birds due to the ability of non-avian reptiles to reduce their metabolism drastically during extended fasts. The effect of fasting on levels of plasma ketones is species specific: β-hydroxybutyrate concentration may rise or fall during fasting. I also describe the process by which the bulk of lipids are deposited into oocytes during vitellogenesis. Although this process is sometimes ascribed to vitellogenin-based transport in reptiles, the majority of lipid deposition occurs via triglycerides packaged in very-low-density lipoproteins (VLDLs), based on physiological, histological, biochemical, comparative, and genomic evidence. I also discuss the evidence for non-avian reptiles using 'yolk-targeted' VLDLs during vitellogenesis. The major physiological states - feeding, fasting, and vitellogenesis - have different effects on plasma lipid metabolites, and I discuss the possibilities and potential problems of using plasma metabolites to diagnose feeding condition in non-avian reptiles.

VALIDATION OF ULTRASOUND AS A NONINVASIVE TOOL TO MEASURE SUBCUTANEOUS FAT DEPTH IN LEATHERBACK SEA TURTLES (DERMOCHELYS CORIACEA)

Leatherback turtles (Dermochelys coriacea) undergo substantial cyclical changes in body condition between foraging and nesting. Ultrasonography has been used to measure subcutaneous fat as an indicator of body condition in many species but has not been applied in sea turtles. To validate this technique in leatherback turtles, ultrasound images were obtained from 36 live-captured and dead-stranded immature and adult turtles from foraging and nesting areas in the Pacific and Atlantic oceans. Ultrasound measurements were compared with direct measurements from surgical biopsy or necropsy. Tissue architecture was confirmed histologically in a subset of turtles. The dorsal shoulder region provided the best site for differentiation of tissues. Maximum fat depth values with the front flipper in a neutral (45-90°) position demonstrated good correlation with direct measurements. Ultrasound-derived fat measurements may be used in the future for quantitative assessment of body condition as an index of health in this critically endangered species.

Leatherback sea turtle shell: A tough and flexible biological design

The leatherback sea turtle is unique among chelonians for having a soft skin which covers its osteoderms. The osteoderm is composed of bony plates that are interconnected with collagen fibers in a structure called suture. The soft dermis and suture geometry enable a significant amount of flexing of the junction between adjacent osteoderms. This design allows the body to contract better than a hard-shelled sea turtle as it dives to depths of over 1,000 m. The leatherback turtle has ridges along the carapace to enhance the hydrodynamic flow and provide a tailored stiffness. The osteoderms are of two types: flat and ridged. The structure of the two types of osteoderms is characterized and their mechanical properties are investigated with particular attention to the failure mechanisms. They both are bony structures with a porous core sandwiched between compact layers that form the outside and inside surfaces. The compressive strength is highly anisotropic by virtue of the interaction between loading orientation and arrangement of porous and compact components of osteoderms. The angle of interpenetration at the suture of osteoderms is analyzed and compared with analytical predictions. The sutures have a triangular shape with an angle of ∼30° which provides a balance between the tensile strength of the osteoderms and shear strength of the collagen fiber layer and is verified by Li-Ortiz-Boyce in a previous study. This is confirmed by an FEM analysis. A calculation is developed to quantify the flexibility of the carapace and plastron as a function of the angular displacement at the sutures, predicting the interdependence between geometrical parameters and flexibility.

STATEMENT OF SIGNIFICANCE

The leatherback turtle is a magnificent chelonian whose decreasing numbers have brought it to the brink of extinction in the Pacific Ocean. This first study of the structure of its shell provides important new insights that explain its amazing capacity for diving: depths of over 1,000 m have been recorded. This is enabled by the flexibility between the bony plates comprising its shell, which is covered by a skin and not by hard keratin as all other turtles. We use the arsenal of Materials Science characterization techniques to probe the structure of the shell and explain its amazing structure and capacity for flexing, while retaining its protection capability.

Behavioral and metabolic contributions to thermoregulation in freely swimming leatherback turtles at high latitudes

Leatherback turtles in the Northwest Atlantic Ocean have a broad geographic range that extends from nesting beaches near the equator to seasonal foraging grounds as far north as Canada. The ability of leatherbacks to maintain core body temperature (Tb) higher than that of the surrounding water is thought to be a key element of their biology that permits them to exploit productive waters at high latitudes. We provide the first recordings of Tb from freely swimming leatherbacks at a northern foraging ground, and use these data to assess the importance of behavioral adjustments and metabolic sources of heat for maintenance of the thermal gradient (Tg). The mean Tb for individual leatherbacks ranged from 25.4±1.7 to 27.3±0.3°C, and Tg ranged from 10.7±2.4 to 12.1±1.7°C. Variation in mean Tb was best explained by the amount of time that turtles spent in the relatively warm surface waters. A diel trend in Tb was apparent, with daytime cooling suggestive of prey ingestion and night-time warming attributable to endogenous heat production. We estimate that metabolic rates necessary to support the observed Tg are ~3 times higher than resting metabolic rate, and that specific dynamic action is an important source of heat for foraging leatherbacks.

Mercury and selenium concentrations in leatherback sea turtles (Dermochelys coriacea): population comparisons, implications for reproductive success, hazard quotients and directions for future research

Leatherback sea turtles (Dermochelys coriacea) are long-distance migrants that travel thousands of km from foraging grounds to breeding and nesting grounds. These extensive journeys are fueled by ingestion of an estimated 300-400 kg of prey/d and likely result in exposure to high concentrations of environmental toxicants (e.g., mercury compounds). Increased bodily concentrations of mercury and its compounds in nesting female turtles may have detrimental effects on reproductive success. Leatherbacks have relatively low reproductive success compared with other sea turtles (global average hatching success ~50-60%). To assess toxicants and necessary nutrients as factors affecting leatherback turtle reproductive success at Sandy Point National Wildlife Refuge (SPNWR), St. Croix, U.S. Virgin Islands, we collected blood from nesting female leatherbacks and tissues from their hatchlings (blood from live turtles, liver and yolk sac from dead turtles). We compared the concentrations in those tissues to hatching and emergence success. We found that on SPNWR, hatching and emergence success were more closely related to seasonal factors than to total mercury and selenium concentrations in both nesting females and hatchlings. Selenium concentrations of nesting females were positively correlated with those of their hatchlings. Mercury and selenium in the liver of hatchlings were positively correlated with one another. Turtles with greater remigration intervals tended to have higher blood selenium concentrations, suggesting that selenium accumulates in leatherbacks through time. Through hazard quotients, we found evidence that selenium may be at or above concentrations that may cause physiologic harm to hatchlings. We also found evidence that population level differences exist for these trace elements. The concentrations of mercury and selenium established in this manuscript form a baseline for future toxicant studies.

Shape and material characteristics of the trachea in the leatherback sea turtle promote progressive collapse and reinflation during dives

The leatherback turtle regularly undertakes deep dives and has been recorded attaining depths in excess of 1,200 m. Its trachea is an almost solid, elliptical-section tube of uncalcified hyaline cartilage with minimal connective tissue between successive rings. The structure appears to be advantageous for diving and perfectly designed for withstanding repeated collapse and reinflation. This study applies Boyle's law to the respiratory system (lungs, trachea and larynx) and estimates the changes in tracheal volume during a dive. These changes are subsequently compared with the results predicted by a corresponding finite element (FE) structural model, itself based on laboratory studies of the trachea of an adult turtle. Boyle's law predicts that the trachea will collapse progressively with greater volume change occurring in the early stages. The FE model reproduces the changes extremely well (agreeing closely with Boyle's law estimations) and provides visual representation of the deformed tracheal luminal area. Initially, the trachea compresses both ventrally and dorsally before levelling ventrally. Bulges are subsequently formed laterally and become more pronounced at deeper depths. The geometric configuration of the tracheal structure confers both homogeneity and strength upon it, which makes it extremely suited for enduring repeated collapse and re-expansion. The structure actually promotes collapse and is an adaptation to the turtle's natural environment in which large numbers of deep dives are performed annually.