Acoustic abundance of fish depends directly on the target strength (TS) of the fish surveyed. We analyzed 70 and 120 kHz acoustic data from two lakes with abundant rainbow-smelt (Osmerus mordax) populations. Using repeated surveys through the summer growing season, we derived a relationship between TS (dB) and fish length L (cm) at 120 kHz (TS = 19.9 log10 L − 67.8). Values for 70 kHz were similar. In situ TS increased with fish density, indicating a bias from accepting multiple targets at high fish densities. Correcting for this bias increased estimates of smelt abundance by up to 18% in Lake Erie and up to 100% in Lake Champlain. Multiple modes in the TS distributions observed for older fish do not reflect different size groups, as the same modes can be observed from measurements from a single fish. Smelt released gas bubbles during the evening ascent, and these bubbles had TS (−60 to −58 dB) within the range of TS observed from the fish. Gas-bubble release occurred mostly during the migration. Conducting surveys after the ascent is completed will decrease bias associated with counting bubbles as fish.

The eggshell of lizards is a complex structure composed of organic and inorganic molecules secreted by the oviduct, which protects the embryo by providing a barrier to the external environment and also allows the exchange of respiratory gases and water for life support. Calcium deposited on the surface of the eggshell provides an important nutrient source for the embryo. Variation in physical conditions encountered by eggs results in a tradeoff among these functions and influences eggshell structure. Evolution of prolonged uterine egg retention results in a significant change in the incubation environment, notably reduction in efficiency of gas exchange, and selection should favor a concomitant reduction in eggshell thickness. This model is supported by studies that demonstrate an inverse correlation between eggshell thickness and length of uterine egg retention. One mechanism leading to thinning of the eggshell is reduction in size of uterine shell glands. Saiphos equalis is an Australian scincid lizard with an unusual pattern of geographic variation in reproductive mode. All populations retain eggs in the uterus beyond the embryonic stage at oviposition typical for lizards, and some are viviparous. We compared structure and histochemistry of the uterus and eggshell of two populations of S. equalis, prolonged egg retention, and viviparous to test the hypotheses: 1) eggshell thickness is inversely correlated with length of egg retention and 2) eggshell thickness is positively correlated with size of shell glands. We found support for the first hypothesis but also found that eggshells of both populations are surprisingly thick compared with other lizards. Our histochemical data support prior conclusions that uterine shell glands are the source of protein fiber matrix of the eggshell, but we did not find a correlation between size of shell glands and eggshell thickness. Eggshell thickness is likely determined by density of uterine shell glands in this species.

Cold environmental temperature is detrimental to reproduction by oviparous squamate reptiles by prolonging incubation period, negatively affecting embryonic developmental processes, and by killing embryos in eggs directly. Because low soil temperature may prevent successful development of embryos in eggs in nests, the geographic distributions of oviparous species may be influenced by the thermal requirements of embryos. In the present study, we tested the hypothesis that low incubation temperature determines the northern distributional limit of the oviparous lizard Sceloporus undulatus. To compare the effects of incubation temperature on incubation length, egg and hatchling survival, and hatchling phenotypic traits, we incubated eggs of S. undulatus under temperature treatments that simulated the thermal environment that eggs would experience if located in nests within their geographic range at 37 degrees N and north of the species' present geographic range at latitudes of 44 and 42 degrees N. After hatching, snout-vent length (SVL), mass, tail length, body condition (SVL relative to mass), locomotor performance, and growth rate were measured for each hatchling. Hatchlings were released at a field site to evaluate growth and survival under natural conditions. Incubation at temperatures simulating those of nests at 44 degrees N prolonged incubation and resulted in hatchlings with shorter SVL relative to mass, shorter tails, shorter hind limb span, slower growth, and lower survival than hatchlings from eggs incubated at temperatures simulating those of nests at 37 and 42 degrees N. We also evaluated the association between environmental temperature and the northern distribution of S. undulatus. We predicted that the northernmost distributional limit of S. undulatus would be associated with locations that provide the minimum heat sum (approximately 495 degree-days) required to complete embryonic development. Based on air and soil temperatures, the predicted northern latitudinal limit of S. undulatus would lie at approximately 40.5-41.5 degrees N. Our predicted value closely corresponds to the observed latitudinal limit in the eastern United States of approximately 40 degrees N. Our results suggest that soil temperatures at northern latitudes are not warm enough for a sufficient length of time to permit successful incubation of S. undulatus embryos. These results are consistent with the hypothesis that incubation temperature is an important factor limiting the geographic distributions of oviparous reptile species at high latitudes and elevations.

The evolution of viviparity requires modifications to multiple integrated physiological features to support embryonic development during pregnancy. Embryonic growth during pregnancy is dependent upon the capacity of the uterine vascular system to satisfy increasing embryonic oxygen demand throughout gestation. We tested the hypothesis that total surface area of uterine blood vessels increases in concert with embryonic growth, and hence its oxygen demand, during gestation. We used immunofluorescence and laser-scanning confocal microscopy to quantify uterine microvascular density and morphology during gestation in the oviparous skink Ctenotus taeniolatus and in Saiphos equalis, a skink species with prolonged egg retention. For C. taeniolatus, vessel density (Nv) and vessel length-density (Lv) in the embryonic hemisphere of the uterus is 23% and 17% less, respectively, than that of S. equalis and vascular surface-area does not differ as a function of embryo stage. For S. equalis, overall Nv, Lv, and vessel diameter (Dv), does not change during the first half of gestation but increases by 36% (Nv), 44% (Lv), and 60% (Dv) by near-term embryo stages late in gestation. The chorioallantoic membrane of S. equalis increases in absolute size but vascular density does not differ as a function of embryo stage. The marked increase in uterine vascular density during late gestation coincides with the phase of rapid growth in embryo mass and concomitant increase in metabolic rate. Expansion of the uterine vascular bed in concert with embryo size and metabolism is likely to be an important transitional step in the evolution of viviparity. Anat Rec,

Viviparity in reptiles is hypothesized to evolve in cold climates at high latitudes and high elevations through selection for progressively longer periods of egg retention. Oxygen consumption of embryos increases during development and therefore longer periods of egg retention should be associated with maternal or embryonic features that enhance embryonic oxygen availability. We tested the hypotheses that embryos of the oviparous lizard Sceloporus undulatus from a high-latitude population in New Jersey are oviposited at more advanced developmental stages and have a higher growth rate at low oxygen partial pressures ( p O 2 ) than embryos from a low-latitude population in South Carolina. These hypotheses were rejected; embryos from the two populations did not differ in embryonic stage at oviposition, survival, rate of differentiation or growth in mass when incubated under simulated in utero conditions at low oxygen concentrations. We also estimated the effective p O 2 experienced by lizard embryos in utero . At an effective p O 2 of 8.6 kPa (9% O 2 ), development of S. undulatus embryos is arrested at Dufaure and Hubert stage 30 and at a dry mass of 0.8 mg. Physiological and morphological features of gravid females, embryos, or both, that facilitate oxygen uptake for developing embryos appear to be a critical early step during the evolution of reptilian viviparity.

A prominent scenario for the evolution of viviparity and placentation in reptiles predicts a step-wise pattern with an initial phase of prolonged oviductal egg retention accompanied by progressive reduction in eggshell thickness culminating in viviparity; calcium placentotrophy evolves secondarily to viviparity. Saiphos equalis is an Australian scincid lizard with a reproductive mode that is uncommon for squamates because eggs are retained in the oviduct until late developmental stages, and the embryonic stage at oviposition varies geographically. We studied calcium mobilization by embryos in two populations with different oviductal egg retention patterns to test the hypothesis that the pattern of nutritional provision of calcium is independent of the embryonic stage at oviposition. Females from one population are viviparous and oviposit eggs containing fully formed embryos, whereas embryos in oviposited eggs of the second population are morphologically less mature, and these eggs hatch several days later. The reproductive mode of this population is denoted as prolonged oviductal egg retention. Yolk provided the highest proportion of calcium to hatchlings in both populations. Eggs of both populations were enclosed in calcified eggshells, but shells of the population with prolonged egg retention had twice the calcium content of the viviparous population and embryos recovered calcium from these eggshells. Placental transfer accounted for a substantial amount of calcium in hatchlings in both populations. Hatchling calcium concentration was higher in the population with prolonged egg retention because these embryos mobilized calcium from yolk, the eggshell and the placenta. This pattern of embryonic calcium provision in which both a calcified eggshell and placentotrophy contribute to embryonic nutrition is novel. The reproductive pattern of S. equalis illustrates that calcified eggshells are compatible with prolonged oviductal egg retention and that viviparity is not requisite to calcium placentotrophy.

Reptilian viviparity evolves through selection for increasingly prolonged egg retention within the oviduct. In the majority of sceloporine lizard species, however, egg retention past the normal time of oviposition results in retarded or arrested embryonic development. In this study, we tested the hypothesis that the amount of embryonic development normally attained in utero is directly related to in utero oxygen partial pressure (PO(2)). The three species of sceloporine lizards we used are characterized by developmental arrest (Urosaurus ornatus), retarded development (Sceloporus virgatus), and normal development (Sceloporus scalaris) when eggs are retained. We incubated eggs of these species for 10 d under conditions that simulated retention in the oviduct at a range of experimental oxygen partial pressures (PO(2)). We estimated in utero PO(2) from a standard curve generated from the stage and dry mass of experimental embryos incubated for 10 d at known PO(2). The standard curve was then used to predict the PO(2) associated with the observed rate of development of embryos retained in utero. The results of this study showed that the degree of embryonic development attained in utero during egg retention was positively associated with in utero PO(2). The results indicate that oxygen availability in utero is associated with interspecific differences in the capacity to support intrauterine development in sceloporine lizards.

We used immunofluorescent confocal microscopy and scanning electron microscopy to quantify uterine vascularity and to describe uterine surface morphology during gestation in pregnant females of the lecithotrophic lizard Niveoscincus coventryi. As uterine angiogenesis and epithelial cell morphology are thought to be under progesterone control, we studied the effect of a progesterone receptor antagonist (mifepristone) on uterine and chorioallantoic microvasculature and features of the uterine epithelial surfaces. Although intussuceptive angiogenesis was observed in both, uterine and chorioallantoic, vascular beds during gestation, the only significant increases were in the diameters of the uterine vessels. An ellipsoid vessel-dense area grows in the mesometrial hemisphere of the developing conceptus, which parallels the expansion of the allantois to form the chorioallantoic placenta. Uterine surface topography changed during gestation. In particular, uterine blood vessels bulge over the luminal surface to form marked ridges on the uterine embryonic hemisphere, especially during the last stage of pregnancy, and ciliated cells are maintained in the embryonic and abembryonic hemispheres but disappear in both the mesometrial and antimesometrial poles. This distinct regionalization of uterine ridges and ciliated cells in the uterine surface and in the shape of the epithelial component of the chorion might be related to the function of both chorioallantoic and yolk sac placentae during gestation. There was no significant difference between females treated with or without mifepristone, which may be related to the partial function of mifepristone as a progestin antagonist and/or with the function and time of action of progesterone in the uterus during gestation in N. coventryi. Differences in the pattern of angiogenesis and uterine surface morphology during gestation among squamates may be related to the functional diversity of the uterine component of the different placentae and probably reflect its diverse evolutionary history.