Unexpected Effects of Predators Upon Their Prey: The Case of the American Alligator

Indirect trophic effects play important roles in ecosystem dynamics and can at times oppose and dominate the action of direct feeding linkages. Each predator directly exerts a negative effect upon its prey, but predators may also provide indirect benefits to their prey. In ecosystems, such benefits are effected via indirect trophic pathways that can provide a more than compensating positive influence. The ecosystem of the Big Cypress National Preserve (southwest Florida) appears to contain an unusually high number of such predators—most notably, the American alligator, Alligator mississippiensis. The trophic exchanges of carbon among the 68 principal taxa comprising the cypress wetland ecosystem have been quantified during both wet and dry seasons. The network analysis program IMPACTS identified predators that potentially have a positive influence on some of their prey. A total of 64 of these instances were recorded for the wet season and 44 for the dry. Taxa that, on balance, have positive effects upon their prey include fishes, turtles, snakes, birds, and, most significantly, alligators. The feeding habits of alligators benefit a conspicuous number (11) of their prey (invertebrates, frogs, mice, and rats). Further trophic analysis reveals that the predation by alligators on snakes and turtles accounts for most of the trophic benefits bestowed. The actions of alligators in modifying their physical environment has been cited elsewhere as contributing to the maintenance of biotic diversity. It appears that the trophic influence of this species adds further evidence to the important role it plays in the functional ecology of the cypress wetland. Received 4 March 1998; accepted 21 October 1998     

Energy production per circulatory cycle: a constant in resting land vertebrates?

Correcting for differences in blood flow, resting alligators, caimans, lizards, turtles, rats and dogs deaminated amino acids at the same rate. Each produced about 21 calories/kg during one complete circulatory revolution, irrespective of body temperature, size or species. Uniform O2 and substrate A-V differences are responsible for the phenomenon.     

Technics for blood collection and intravascular infusion of reptiles.

Several methods for external sampling of blood and administration of intravascular fluids are applicable to reptilian species. This paper reviewed some of the technics that have been previously reported and introduced several new procedures applicable to snakes, turtles, alligators, crocodiles, and lizards.     

Changes in femoral morphology during egg-laying in Alligator mississippiensis

Birds and many reptiles are egg-layers. Birds provide calcium for the formation of eggshells by resorbing medullary bone, which is laid down before ovulation. Turtles do not possess this mechanism and resorb structural bone to form eggshells. Femora from three groups of alligators (egg-laying females; quiescent, immature, or barren females; and males) were examined to determine if alligators, which are closely related to birds in evolution, resorb structural bone during the formation of eggshells as do turtles. Microradiographs of cross sections from femoral mid-shafts were analyzed for porosity, and the robusticity index of each femur was determined. Scanning electron micrographs of anorganic endosteal and periosteal femoral surfaces were analyzed to determine numbers of entrances of vascular canals, numbers of lacunae of osteoblasts, and types of femoral surfaces. Femora from egg-laying females were significantly less robust than those of other females or males, and sections of bone from the egg-layers were significantly more porous than those of the other groups. Scanning electron microscopy of anorganic femoral endosteal surfaces from egg-laying females revealed significantly more resorption areas when compared with males or non egg-laying females. Periosteal surfaces from egg-layers had significantly more resting and less bone-forming surface than those from the other groups. Results indicated that apposition of periosteal bone may have been reduced in egg-layers and that egg-laying alligators, like turtles, resorb endosteal structural bone, which may be used as a source of calcium for the formation of eggshells.     

Unusually high zinc concentrations in snake plasma, with observations on plasma zinc concentrations in lizards, turtles and alligators

We provide evidence that normal plasma zinc levels in snakes are in the same range as the elevated zinc levels associated with haemolytic anaemia and fatal zinc toxicosis in dogs, and with weight loss and anorexia in crocodiles that had ingested coins with high zinc content. Blood plasma samples from large representative groups of snakes, lizards, turtles and alligators were analysed for zinc content by atomic absorption spectrophotometry. Plasma zinc levels in all snake species were five to 50-fold greater than levels reported in mammals. Plasma zinc levels in lizards and turtles were also higher than those of mammals, but significantly lower than those of snakes. Plasma zinc levels in alligators were in the same range as mammals and birds. After prolonged dialysis of snake plasma, 76% of the zinc remains in the retentate, suggesting the presence of a plasma protein with a strong affinity for the metal. Zinc levels vary significantly among the taxonomic groups, emphasizing the common evolutionary origin of crocodiles and birds and their divergence from the other reptiles.     

Complete mitochondrial DNA sequences of the green turtle and blue-tailed mole skink: statistical evidence for archosaurian affinity of turtles.

Turtles have highly specialized morphological characteristics, and their phylogenetic position has been under intensive debate. Previous molecular studies have not established a consistent and statistically well supported conclusion on this issue. In order to address this, complete mitochondrial DNA sequences were determined for the green turtle and the blue-tailed mole skink. These genomes possess an organization of genes which is typical of most other vertebrates, such as placental mammals, a frog, and bony fishes, but distinct from organizations of alligators and snakes. Molecular evolutionary rates of mitochondrial protein sequences appear to vary considerably among major reptilian lineages, with relatively rapid rates for snake and crocodilian lineages but slow rates for turtle and lizard lineages. In spite of this rate heterogeneity, phylogenetic analyses using amino acid sequences of 12 mitochondrial proteins reliably established the Archosauria (birds and crocodilians) and Lepidosauria (lizards and snakes) clades postulated from previous morphological studies. The phylogenetic analyses further suggested that turtles are a sister group of the archosaurs, and this untraditional relationship was provided with strong statistical evidence by both the bootstrap and the Kishino-Hasegawa tests. This is the first statistically significant molecular phylogeny on the placement of turtles relative to the archosaurs and lepidosaurs. It is therefore likely that turtles originated from a Permian-Triassic archosauromorph ancestor with two pairs of temporal fenestrae behind the skull orbit that were subsequently lost. The traditional classification of turtles in the Anapsida may thus need to be reconsidered.     

Comparison of functional aspects of the coagulation cascade in human and sea turtle plasmas

Functional hemostatic pathways are critical for the survival of all vertebrates and have been evolving for more than 400 million years. The overwhelming majority of studies of hemostasis in vertebrates have focused on mammals with very sparse attention paid to reptiles. There have been virtually no studies of the coagulation pathway in sea turtles whose ancestors date back to the Jurassic period. Sea turtles are often exposed to rapidly altered environmental conditions during diving periods. This may reduce their blood pH during prolonged hypoxic dives. This report demonstrates that five species of turtles possess only one branch of the mammalian coagulation pathway, the extrinsic pathway. Mixing studies of turtle plasmas with human factor-deficient plasmas indicate that the intrinsic pathway factors VIII and IX are present in turtle plasma. These two factors may play a significant role in supporting the extrinsic pathway by feedback loops. The intrinsic factors, XI and XII are not detected which would account for the inability of reagents to induce coagulation via the intrinsic pathway in vitro. The analysis of two turtle factors, factor II (prothrombin) and factor X, demonstrates that they are antigenically/functionally similar to the corresponding human factors. The turtle coagulation pathway responds differentially to both pH and temperature relative to each turtle species and relative to human samples. The coagulation time (prothrombin time) increases as the temperature decreases between 37 and 15 °C. The increased time follows a linear relationship, with similar slopes for loggerhead, Kemps ridley and hawksbill turtles as well as for human samples. Leatherback turtle samples show a dramatic nonlinear increased time below 23 °C, and green turtle sample responses were similar but less dramatic. All samples also showed increased prothrombin times as the pH decreased from 7.8 to 6.4, except for three turtle species. The prothrombin times decreased, to varying extents, in a linear fashion relative to reduced pH with the rate of change greatest in leatherbacks>greenloggerhead turtles. All studies were conducted with reagents developed for human samples which would impact on the quantitative results with the turtle samples, but are not likely to alter the qualitative results. These comparative studies of the coagulation pathway in sea turtles and humans could enhance our knowledge of structure/function relationships and evolution of coagulation factors.     

Aerobic metabolism of American alligators, Alligator mississippiensis, under standard conditions and during voluntary activity

We measured the rate of consumption of oxygen by alligators in a dry metabolic chamber and in a tank of water where they were free to dive and surface at will at 10-35 degrees C, a range spanning most of the body temperatures experienced by alligators in nature. Neither the standard metabolic rate nor the rate of oxygen consumption during one hour of sustained, voluntary activity varied with body mass, month of the year, duration of fasting before measurement, or experimental condition (terrestrial vs aquatic). Voluntary diving is not accompanied by any reduction in standard metabolic rate; these results and those of others suggest that the "diving reflex" of alligators is probably employed only in emergencies. Spontaneous activity for one hour is accompanied by a 1.9-4.4 fold rise in oxygen consumption; this factorial increase is less than that for other reptiles induced to maximal activity for brief intervals. Both standard and active oxygen consumption rise significantly with body temperature.     

环境决定爬行动物性别研究的进展

爬行动物的性别决定机制有两种,一种是由环境决定性别,另一种是异型性染色体决定性别。前者,在爬行动物中具有普遍性;未发现有异型性染色体的爬行动物,其性别由环境因子决定。剧烈的环境条件,可能压倒基因型性别决定。H-Y抗原,可检测未发现异型性染色体决定性别物种的遗传决定型。     

Temperature during embryonic and juvenile development influences growth in hatchling snapping turtles,Chelydra serpentina

Embryonic temperature influenced subsequent growth in juvenile snapping turtles, Chelydra serpentina: incubation temperatures of 24 and 26.5°C enhanced growth relative to a temperature of 29°C. Although embryonic temperature normally determines gonadal sex in this species, experimental manipulations revealed that temperature effects on growth were independent of sex. Ambient temperature also affected growth: juvenile turtles grew slowly in a cool (19°C) versus a warm (28°C) environment. In a parallel experiment, turtles from different embryonic temperatures displayed different patterns of temperature choice in response to nutritional status or time of day. We tentatively conclude that embryonic temperature has both direct and indirect (i.e., through temperature choice) effects on growth in snapping turtles.     

Growth rates and body condition factors of Alligator mississippiensis in coastal Louisiana wetlands: A comparison of wild and farm-released juveniles

• 1.1. Growth rates and body condition factors for native wild and captive-raised juvenile alligators (Alligator mississippiensis) that had been released to the wild were studied using tag-recapture methods for 274 alligators over a 4-year period. Alligators were grouped by sex, size class, source (farm-released vs native wild) and as to whether they had overwintered or not.
• 2.2. In most groups, the farm-released alligators grew significantly better than wild alligators matched for sex and size; in the remaining groups the post-release alligators grew as well as their counterparts, though not better.
• 3.3. Overwintering tended to slow growth rates in both groups, but farm-released alligators still demonstrated superior growth over native wild alligators even after overwintering.
• 4.4. Males tended to grow faster than females, though this trend was not always significantly greater. In no matched group did females grow faster than males.
• 5.5. Growth rates diminished with increasing size in native wild alligators (smaller alligators grew faster), but growth rates of farm-released alligators remained accelerated even at the larger size classes.
• 6.6. Growth curves were constructed using known recapture data with three growth models (von Bertlanffy, Gompertz and logistic); the calculated maximum attainable length and growth parameters were significantly larger (P < 0.01) for farm-released alligators than wild using all three models.
• 7.7. Body condition factors were not different in captive-raised post-released alligators than native wild alligators.

     

Circadian organization of an animal lacking a pineal organ; the young American alligator,Alligator mississippiensis

Summary Circadian rhythms were examined in young American alligators,Alligator mississippiensis, an animal that naturally lacks a pineal organ. Individual alligators displayed persistent, stable, free-running circadian activity rhythms under both constant darkness and constant illumination. Free-running activity was entrained by 24 h light-dark cycles. The circadian period length was also temperature compensated, with Q₁₀'s of 1.0–1.40 obtained for a 22–32 °C interval. The presence of this quite normal circadian rhythmicity in alligators indicates that the pineal organ is not always essential for the expression of circadian rhythms in lower vertebrates.This research was supported by grant NS 12257 for N.I.H. to C.L. Ralph. Alligators were obtained through the courtesy of C.R. Tracy and T. Joanen of the Rockefeller Wildlife Refuge, Grand Chenier, Louisiana.     

Replacement of the axial histidine ligand with imidazole in cytochrome c peroxidase. 1. Effects on structure

Replacement of the axial histidine ligand with exogenous imidazole has been accomplished in a number of heme protein mutants, where it often serves to complement the functional properties of the protein. In this paper, we describe the effects of pH and buffer ion on the crystal structure of the H175G mutant of cytochrome c peroxidase, in which the histidine tether between the heme and the protein backbone is replaced by bound imidazole. The structures show that imidazole can occupy the proximal H175G cavity under a number of experimental conditions, but that the details of the interaction with the protein and the coordination to the heme are markedly dependent on conditions. Replacement of the tethered histidine ligand with imidazole permits the heme to shift slightly in its pocket, allowing it to adopt either a planar or distally domed conformation. H175G crystallized from both high phosphate and imidazole concentrations exists as a novel, 5-coordinate phosphate bound state, in which the proximal imidazole is dissociated and the distal phosphate is coordinated to the iron. To accommodate this bound phosphate, the side chains of His-52 and Asn-82 alter their positions and a significant conformational change in the surrounding protein backbone occurs. In the absence of phosphate, imidazole binds to the proximal H175G cavity in a pH-dependent fashion. At pH 7, imidazole is directly coordinated to the heme (d(Fe--Im) = 2.0 A) with a nearby distal water (d(Fe--HOH) = 2.4 A). This is similar to the structure of WT CCP except that the iron lies closer in the heme plane, and the hydrogen bond between imidazole and Asp-235 (d(Im--Asp) = 3.1 A) is longer than for WT CCP (d(His--Asp) = 2.9 A). As the pH is dropped to 5, imidazole dissociates from the heme (d(Fe--Im) = 2.9 A), but remains in the proximal cavity where it is strongly hydrogen bonded to Asp-235 (d(Im--Asp) = 2.8 A). In addition, the heme is significantly domed toward the distal pocket where it may coordinate a water molecule. Finally, the structure of H175G/Im, pH 6, at low temperature (100 K) is very similar to that at room temperature, except that the water above the distal heme face is not present. This study concludes that steric restrictions imposed by the covalently tethered histidine restrain the heme and its ligand coordination from distortions that would arise in the absence of the restricted tether. Coupled with the functional and spectroscopic properties described in the following paper in this issue, these structures help to illustrate how the delicate and critical interactions between protein, ligand, and metal modulate the function of heme enzymes.     

Venous blood gases and lactates of wild loggerhead sea turtles (Caretta caretta) following two capture techniques

During summer of 2001, venous blood gases were determined in loggerhead sea turtles (Caretta caretta) captured by trawl (n = 16) in coastal waters of South Carolina and Georgia (USA) as part of a sea turtle census program and captured in pound nets (n = 6) in coastal North Carolina (USA) during a study of sea turtle population biology. Trawls were towed for 30 min, so turtles captured were forcibly submerged for < or = 30 min. Pound nets are passive gear in which fish and sea turtles are funneled into a concentrated area and removed periodically. Sea turtles in pound nets are free to surface and to feed at will. Blood was obtained from the dorsal cervical sinus as quickly as possible after landing on the boat (range 2-10 min trawl, 1-2 min pound net) and at 30 min after landing just prior to release. Blood gases including pH, partial pressures of O2 and CO2 (pO2, pCO2), and lactate were measured within 10 min. Instrument measurements for pH, pO2, and pCO2 made at 37 C were corrected to cloacal temperature and HCO3- was calculated from temperature-corrected pH and pCO2. Venous blood pH and bicarbonate were higher, and pO2 and lactate were lower from pound net-captured turtles compared to trawl captured turtles at the initial sampling time. In pound net turtles, pH and bicarbonate declined and lactate increased during 30 min on deck. In trawled sea turtles, venous blood pH increased and pCO2 and pO2 decreased during the 30 min on deck. Both capture systems caused perturbations in blood gas, acid-base, and lactate status, though alterations were greater in trawl captured turtles.     

Interaction of the unique competitive inhibitor imidazole and related compounds with the active site metal of carbonic anhydrase: linkage between pH effects on the inhibitor binding affinity and pH effects on the visible spectra of inhibitor complexes with the cobalt-substituted enzyme

Previous studies on the interaction of carbonic anhydrase (CA) with the unique CO2 competitive inhibitor imidazole and related compounds were all interpreted as showing that an ionizable water ligand on the metal of this zinc metalloenzyme is not displaced by inhibitor binding. Internal inconsistencies in the pH dependence of binding and the pH dependence of the visible spectra of complexes with cobalt-substituted enzyme prompted us to reinvestigate this binding. Visible spectroscopy was used to measure the binding of imidazole and 1,2,4-triazole to Co(II)-substituted human CA I and active site carboxymethylated human CA I (CmCA I) and the binding of 1,2,4-triazole to bovine CoIICA II. The limiting visible spectra for these enzyme-inhibitor adducts were also computed and examined for pH dependence. It was shown that the pKa of visible spectral changes can be independently predicted from studies on the pH dependence of binding. After consideration of possible contributions from effects of His-200 ionization in CA I and CmCA I, and His-64 in CA II, the pH effects on binding affinity and spectra were found to be of the correct magnitude to establish linkage between binding and an ionization. It was also shown, however, that pH effects on binding and spectra cannot distinguish whether neutral imidazole binds to both ionization forms of the enzyme (Zn-OH2 and Zn-OH) or whether neutral imidazole and its anion both bind to only the acid form of the enzyme, presumably after displacing the water. These findings have implications to the crystallographic interpretations on the imidazole-enzyme complex and to the catalytic mechanism of CO2 hydration.     

Embryonic temperature affects metabolic compensation and thyroid hormones in hatchling snapping turtles.

Temperature acclimation of adult vertebrates typically induces changes in metabolic physiology. During early development, such metabolic compensation might have profound consequences, yet acclimation of metabolism is little studied in early life stages. We measured the effect of egg incubation temperature on resting metabolic rate (RMR) and blood thyroid hormone levels of hatchling snapping turtles (Chelydra serpentina). Like many reptiles, snapping turtles have temperature-dependent sex determination (TSD), in which embryonic temperature determines sex. Therefore, we designed the experiments to separately measure effects of temperature and of sex on the response variables. We incubated eggs in the laboratory at 21. 5 degrees, 24.5 degrees, 27.5 degrees, and 30.5 degrees C, producing both sexes, all males, both sexes, and all females, respectively. Hatchling RMR, when measured at a common temperature (either 25 degrees or 31 degrees C), was negatively correlated with egg temperature in both males and females, such that RMR of turtles from 21.5 degrees C-incubated eggs averaged 160% that of turtles from 30.5 degrees C-incubated eggs. These results indicate that egg temperatures induced positive metabolic compensation in both sexes. Thyroid hormone levels of hatchlings showed similar correlations with egg temperature; thyroxine level of turtles from 21.5 degrees C-incubated eggs averaged 220% that of turtles from 30.5 degrees C-incubated eggs. To examine the possibility that thyroid hormones contribute to positive metabolic compensation, we added triiodothyronine to eggs during mid-incubation. RMR of hatchlings from these treated eggs averaged 131% that of controls, consistent with the previous possibility. Moreover, the effects of embryonic temperature on metabolic physiology, in combination with effects on sex, can result in differences in RMR and thyroid hormone levels between male and female hatchling turtles. Such differences may be important to the ecology and evolution of TSD.     

A study of the mechanism of ferritin formation. The effect of pH, ionic strength and temperature, inhibition by imidazole and kinetic analysis

The rate of ferritin formation in the buffers 4-morpholinepropanesulphonic acid (Mops), 4-morpholineethanesulphonic acid (Mes) and imidazole at pH values from 5.0 to 6.5 is quite similar. However, the rate of iron deposition is much greater in Mops and Mes at pH values above 6.5 than in imidazole. Increasing the concentration of imidazole inhibits ferritin formation and also leads to a transformation in the shape of the kinetic curves observed. This inhibiton is also observed at constant ionic strength but is not found for non-complexing buffers such as Mops. An inhibition of ferritin formation in imidazole and in Mops buffers is also observed with increasing ionic strength. We conclude that the unprotonated form of imidazole inhibits iron deposition, possibly by binding to the active site of the apoferritin molecule. The temperature dependence of iron deposition was examined. An optimum temperature of 50 degrees C was found but the Arrhenius plots were non-linear. On the basis of these and previous results, a kinetic model is developed which accounts well for ferritin formation at pH values below 6.5 and above 7.0 in non-complexing buffers. The model does not account for the kinetics observed at pH values close to neutrality.     

Microsatellite DNA analyses support an east-west phylogeographic split of American alligator populations

We examined the population genetic structure of American alligators (Alligator mississippiensis) sampled from 12 localities across the southeastern United States. The primary goal of this study was to determine the extent of population differentiation among alligators from four Florida lakes using eight microsatellite loci and compare the results to additional sites located at varying distances from them. Analyses of population structure revealed little differentiation (F(ST)=0.039; Rho=0.012) among the four Florida lakes, Apopka, Griffin, Orange and Woodruff, which are all located in the St. John&'s River watershed in north-central Florida. Further, there was little differentiation among these samples and samples collected from the Everglades National Park (F(ST)=0.044; Rho=0.009) and south Georgia (F(ST)=0.045; Rho=0.032). Therefore, these six samples were pooled together as a "FL/sGA group." Similarly, samples collected in the western extent of the range, Anahuac National Wildlife Refuge in Texas and Salvador Wildlife Management Area, Marsh Island Wildlife Refuge and Rockefeller Wildlife Refuge in Louisiana, also lacked population structure (F(ST)=0.024; R(ST)=0.040). These four populations were pooled into the "TX/LA group." Comparisons of these two groups with samples taken from the Santee Coastal Reserve in South Carolina and Mobile, Alabama yielded three to four times more differentiation among groups (F(ST)=0.131; Rho=0.187). These and other analyses support the hypothesis of an east-west phylogeographic split in American alligator populations and are consistent with studies of many freshwater fish and aquatic and terrestrial turtles distributed throughout this same geographic region.     

Parasitism by Placobdella multilineata (Hirudinoidea: Glossiphoniidae) and its first record from Asia

Placobdella multilineata (Hirudinoidea: Glossiphoniidae) a temporary, bloodsucking ectoparasite, was previously recorded only from the southern United States, with turtles and the American alligator (Alligator mississipiensis) as hosts. This is the first record of P. multilineata from Asia (Beijing, People's Republic of China) and also a new host record for the estuarine crocodile (Crocodylus porosus). The introduction of P. multilineata to Asia occurred as a result of the transfer of American alligators. All 34 specimens of P. multilineata collected in Beijing were non-clitellate but blood in their guts indicated that feeding had occurred on the host.