Mary J. Packard

Influence of Moisture, Temperature, and Substrate on Snapping Turtle Eggs and Embryos

Flexible-shelled eggs of common snapping turtles (Chelydra serpentina) were incubated on two substrates (sand and vermiculite) at each of three temperatures (26.0°, 28.5°, 31.0°C) and three moisture regimes (water potentials initially -150 kPa, -550 kPa, -950 kPa) in a factorial experiment assessing the influence of these variables on the water relations of eggs and the development of embryos. Hatching success was high on wet substrates at 26.0° and 28.5°, but declined at the highest temperature and on drier media. Net absorption of water by viable eggs, duration of incubation by embryos, and size of hatchlings were positively correlated with wetness of substrates and negatively correlated with temperature. Turtles hatching from eggs at 26.0° were males regardless of the wetness of the medium, whereas those emerging from eggs at 28.5° and 31.0° were females. These patterns of response characterized eggs incubated on sand as well as those on vermiculite. Findings from this study indicate that temporal and spatial variations in moisture and temperature within and among natural nests probably elicit ecologically important variation in size and sex of hatchling snapping turtles.

Water Relations of Chelonian Eggs

Eggs of painted turtles (Chrysemys picta) absorb water across that portion of the eggshell in contact with the substrate and simultaneously lose water by transpiration from that part of the eggshell exposed to air inside the nest chamber. Depending upon the rates of influx and efflux, eggs may experience increases or decreases in mass during incubation, or mass may remain essentially constant between oviposition and hatching. Water exchanges are especially sensitive to such factors as substrate water potential, relative surface exposed to the nest atmosphere, hydraulic conductance of the eggshell, and conductance of the eggshell to water vapor. Hatchlings emerging from eggs absorbing and storing large quantities of water are heavier than hatchlings emerging from eggs taking up smaller quantities of water from the substrate. Furthermore, water absorption equal to, or in excess of, water loss by transpiration assures that the original shape of the egg will be preserved, thereby guaranteeing that sufficient space is available within the egg for normal development of the embryo.

Influence of Water Exchanges by Flexible-Shelled Eggs of Painted Turtles Chrysemys picta on Metabolism and Growth of Embryos

Flexible-shelled eggs of painted turtles (Chrysemys picta) were incubated under controlled conditions eliciting different patterns of net water exchange between eggs and the environment. The temporal patterns of decline in dry mass of yolks and of increase in dry mass of embryos did not vary among eggs incubated in different hydric environments, indicating that rates of metabolism and growth of embryos were largely unaffected by variation in the amount of water available inside eggs to support embryogenesis. Nevertheless, embryos in wet environments assimilated more water during incubation than did embryos in dry conditions, and this extra water apparently enabled them to develop longer before hatching than was possible for embryos in the drier settings. Because of their longer incubation, hatchlings emerging from eggs in wet environments were larger (both in mass and in carapace length) and contained less residual yolk than turtles coming from eggs incubated in drier surroundings. Embryos accumulated three times more excretory nitrogen in the form of urea than in the form of ammonia, but the patterns of nitrogen accumulation did not vary among embryos exposed to different hydric conditions. Water potential of the yolk increased during the first 10 days of incubation, as water flowed from the albumen into the vitelline sac, and decreased linearly thereafter, as water was transferred from the yolk to the developing embryo. The predicted water potential of the yolk at the time of hatching was the same for eggs incubated in wet and dry environments, thereby raising the possibility that water potential of some compartment inside eggs provides the cue for hatching. Water exchanges by eggs of painted turtles incubating in natural nests probably affect survival of embryos to hatching as well as body size and level of tissue hydration in young turtles.

Patterns and Possible Significance of Water Exchange by Flexible-Shelled Eggs of Painted Turtles (Chrysemys picta)

Samples of eggs laid by nine painted turtles (Chrysemys picta) were half buried in substrates differing in water potential and incubated at 29 C. Other eggs from these clutches were incubated on wire platforms above the same substrates. Eggs on the wire platforms declined in mass between the outset of study and hatching owing to a net outward diffusion of water vapor, with eggs held above relatively dry substrates losing more vapor than eggs held above relatively wet substrates. Eggs half buried in the substrates increased in mass during the first half of incubation by amounts that were directly related to substrate water potential, but decreased in mass during the second half of incubation as the net flux of water shifted from inward to dutward. The conductance of painted turtle eggs to water vapor was 70 times higher than expected for avian eggs of comparable mass, yet transpirational loss of water from incubating turtle eggs nonetheless was small owing to the very small gradient in vapor pressure between eggs and air trapped inside the containers. Eggs exposed to wet substrates had longer incubation periods and higher hatching success than eggs exposed to drier substrates. Moreover, turtles hatching from eggs exposed to relatively wet conditions were larger than hatchlings emerging from eggs incubated in slightly drier settings.

Comparative aspects of calcium metabolism in embryonic reptiles and birds

Eggs of oviparous, amniotic vertebrates must be endowed at oviposition with all of the organic and most of the inorganic components required for embryonic growth. A major inorganic constituent of these eggs is calcium used for ossification of the skeleton. The two main sources of this element are the yolk and the eggshell, but the proportion of calcium supplied by these compartments varies among species. Some of the calcium absorbed from the eggshell by embryos of domestic fowl is stored in the yolk. causing the calcium content of this compartment to increase appreciably during incubation. In contrast, the calcium content of yolk declines throughout incubation in eggs of reptiles, and the yolk that is withdrawn into the abdominal cavity just prior to hatching contains only small quantities of this element. Thus, major differences in the pattern of calcium metabolism characterize avian and reptilian embryos, and studies of embryos of domestic fowl may not provide a broadly-based model with which to characterize calcium metabolism in embryos of other species. Control of calcium transport across the cellular epithelia (the yolk sac and chorioallantois) that separate embryos from their sources of calcium (yolk and eggshell) represents one aspect of control of calcium metabolism during embryogenesis, but this process has been examined only in eggs of domestic fowl and only in the chorioallantois. Calcium transport across the chorioallantois of embryonic chicks is influenced by a vitamin K-dependent calcium-binding protein, carbonic anhydrase, the level of calcium to which the chorioallantois is exposed, and vitamin D. However, a complete story concerning control of calcium transport across the chorioallantois and its relationship to calcium regulation in embryos of domestic fowl is not yet possible.

Water-vapor conductance of testudinian and crocodilian eggs (class reptilia)

Flexible-shelled eggs of snapping turtles (Chelydra serpentina) have conductances to water vapor that are 55 times higher than predicted for avian eggs of similar size, whereas rigid-shelled eggs of softshell turtles (Trionyx spiniferus) and American alligators (Alligator mississippiensis) have conductances that are only five times higher than expected for comparable eggs of birds. The differences between empirical and predicted values result from the much higher effective pore areas in reptilian eggshells than in those of birds. The relatively high porosities of these reptilian eggs presumably facilitate the transport of oxygen and carbon dioxide eggshells in later stages of incubation when air trapped inside nest chambers may become hypoxic and hypercapnic, yet seem not to lead to excessive transpiration of water vapor owing to the high humidities in nests where incubation occurs.

Environmentally induced variation in body size of turtles hatching in natural nests

Eggs from three snapping turtles (Chelydra serpentina) were divided between two natural nests in a factorial experiment assessing the role of the nest environment as a cause for variation in body size and energy reserves of hatchlings at our study site in northcentral Nebraska. Nest # 1 was located in an unshaded area on the south side of a high sandhill, whereas nest #2 was located in an unshaded area on level ground. Eggs in nest #1 increased in mass over the course of incubation, with eggs at the bottom of the nest gaining more mass than eggs nearer to the surface. In constrast, eggs in nest #2 lost mass during incubation, with eggs at the bottom declining less in mass than eggs at the top of the cavity. Hatchlings from nest #1 were much larger (but contained smaller masses of unused yolk) than hatchlings from nest #2. Additionally, eggs from the lower layers in both nests tended to produce larger hatchings (but with smaller masses of unused yolk) than eggs from the upper layers. Thus, ecologically important variation in body size and nutrient reserves of hatchling snapping turtles results from variation in the environment among and within nests.

Effects of temperature and moisture during incubation on carcass composition of hatchling snapping turtles (Chelydra serpentina)

SummaryFlexible-shelled eggs of common snapping turtles (Chelydra serpentina) were incubated on each of two substrates (vermiculite, sand) at each of three temperatures (26.0°C, 28.5°C, 31.0°C) and three moisture regimes (wet, intermediate, dry). Embryos developing in cool, wet environments mobilized the largest amounts of protein from their yolk and attained the largest size before hatching, whereas turtles developing in warm, dry environments mobilized the smallest quantities of protein and were the smallest in body size at hatching. Embryos on wet substrates mobilized more lipid from their yolk than did embryos on dry media, but ambient temperature had no demonstrable influence on patterns of lipid mobilization. The total reserve of neutral lipid available in residual yolk plus carcass to sustain neonates in the interval prior to the beginning of feeding was largest in hatchlings from dry environments and smallest in animals from wet environments, but was unaffected by temperature during incubation. Hydration of tissues in hatchlings was higher when incubation was in cool, moist conditions than when incubation was in warm, dry settings, thereby indicating that some of the effects of moisture and temperature on mobilization of nutrients by embryos may be mediated by differences in intracellular water. Patterns of response to temperature and moisture recorded for turtles emerging from eggs on sand were similar to those recorded for hatchlings on vermiculite, so no important conclusion would have been affected by incubating eggs on one medium instead of the other.

Influence of Hydration of the Environment on Eggs and Embryos of the Terrestrial Turtle Terrapene ornata

Flexible-shelled eggs of the terrestrial turtle Terrapene ornata were incubated on wet (-150 kPa) and dry (-800 kPa) substrates at 29 C. Eggs on the wet medium absorbed water from the environment and increased in mass by 6% over the course of incubation, whereas eggs on the dry substrate lost water throughout development and weighed 17% less late in incubation than they did at oviposition. Availability of water to embryos had no apparent influence on hatching success, but embryos in the wet environment incubated longer, mobilized more of the nutrient reserve in their yolk, and grew larger before hatching than did those in the dry setting. Most of the ammonia released in catabolism of protein by embryonic Terrapene was detoxified by converting it to urea, which accumulated in eggs during incubation. In later stages of development urea attained concentrations inside eggs that may have been sufficient to inhibit metabolism of embryos, with the inhibition potentially being greater in dry settings than in wet environments. Differential inhibition of metabolism could have led to differences in rates of growth that contributed to the differences in size of turtles at hatching, The relatively large eggs of Terrapene ornata may represent the one adaptation of this species for development in terrestrial conditions, because large eggs of other turtles are more likely to hatch following incubation in a stressful hydric environment than are smaller eggs laid by conspecifics.

Structure of the shell and tertiary membranes of eggs of softshell turtles (Trionyx spiniferus)

Eggs of the turtle Trionyx spiniferus are rigid, calcareous spheres averaging 2.5 cm in diameter. The eggshell is morphologically very similar to avian eggshells. The outer crystalline layer is composed of roughly columnar aggregates, or shell units, of calcium carbonate in the aragonite form. Each shell unit tapers to a somewhat conical tip at its base. Interior to the crystalline layer are two tertiary egg membranes: the outer shell membrane and the inner shell membrane. The outer shell membrane is firmly attached to the inner surface of the shell, and the two membranes are in contact except at the air cell, where the inner shell membrane separates from the outer shell membrane. Both membranes are multi‐layered, with the inner shell membrane exhibiting a more fibrous structure than the outer shell membrane. Numerous pores are found in the eggshell, and these generally occur at the intersection of four or more shell units.