Tadpole Development Research Articles

Predators and invasive plants affect performance of amphibian larvae

Exotic ecosystem engineers induce structural and qualitative habitat changes in invaded landscapes, yet studies rarely examine the effects of both of these changes on native taxa. We used a factorial experiment in natural, predator-containing environments to determine whether performance of amphibian larvae was affected by predators and/or changes in habitat structure or chemistry associated with the invasive shrub Lonicera maackii. Invertebrate predators significantly reduced survival of American toad Anaxyrus americanus larvae, whereas tadpole development was accelerated in pools inoculated with the chemical signature of L. maackii. The significant effect of L. maackii chemistry on A. americanus larvae suggests that invasive species may have non-intuitive effects even on native taxa with which they share no trophic connection, and may represent cryptic components of the multiple, interactive drivers of biodiversity change.

Toward a General Theory for How Climate Change Will Affect Infectious Disease

Toward a General Theory for How Climate Change Will Affect Infectious Disease

17β‐Estradiol Exposure Accelerates Skeletal Development in Xenopus laevis Tadpoles

Although it is well established that estrogen regulates skeletal growth and ossification in mammals, the effects of estrogen on skeletal development in amphibians are relatively uncharacterized. This study was conducted to characterize the impact of 17β-estradiol exposure on skeletal development in Xenopus laevis tadpoles. On day 48 postfertilization, tadpoles were placed in tanks containing 50% Holtfreter's Solution ±17β-estradiol at one of four concentrations (10(-11), 10(-10), 10(-9), and 10(-8) M). At 7-11 day intervals until day 91, 7-10 tadpoles per group were killed, fixed, measured, and staged. Specimens were then cleared and double-stained for cartilage and bone, and 34 skeletal elements were analyzed for ossification. Results from the study indicate that both low (10(-11) M) and high (10(-8) M) concentrations of 17β-estradiol have a significant stimulatory effect on tadpole development. Both the larval stage and ossification index of tadpoles exposed to 10(-11) or 10(-8) M 17β-estradiol were significantly greater than those observed in control animals by day 91 postfertilization. These results are consistent with the hypothesis that endogenous and exogenous estrogen could play a role in the regulation of bone ossification in amphibians.

Parasitism in a community context: trait‐mediated interactions with competition and predation

Predation and competition can induce important density- and trait-mediated effects on species, with implications for community stability. However, interactions of these factors with parasitism remain understudied. Here we investigate interactions among competition, predation and parasitism by crossing tadpole density (Bufo americanus), presence of a caged predator (Notophthalmus viridescens), and Echinostoma trivolvis trematodes, experimentally partitioning their effects on tadpole exposure and susceptibility to infection. Predation did not affect E. trivolvis infection but accelerated tadpole development and growth, and decreased activity. The presence of E. trivolvis caused the opposite effects on these three responses and reduced tadpole survival. High conspecific density reduced tadpole survival, growth, and development, and increased tadpole activity. Effects of predation and parasitism on activity were only evident at low tadpole density. High-density mesocosms also had twice the number of E. trivolvis infections as low-density mesocosms, despite a lack of evidence for stress-induced immunomodulation. Instead, this effect was explained by high density delaying tadpole development, which increased both the duration of exposure to cercariae and susceptibility to infection, because tadpoles spent more time in highly susceptible early stages. These results highlight the importance of accounting for trait-mediated effects, host plasticity, and exposure vs. susceptibility in parasite ecology.

Patterns of Development and Abnormalities among Tadpoles in a Constructed Wetland Receiving Treated Wastewater

Constructed wetlands are promoted for effectiveness at treating wastewater and potential value as wildlife habitat; however, wildlife performance studies in treated wastewater wetlands are limited. We used repeated surveys of larval amphibians along three wetland systems and four reference sites to test the hypothesis that bullfrog tadpoles exposed to direct inputs of treated wastewater develop slower, show a higher frequency of abnormalities, and are smaller at metamorphosis compared to tadpoles from reference ponds. Bullfrog tadpoles from wastewater wetlands were similar in size at metamorphosis compared to tadpoles from reference sites; however, they did show a much higher frequency of abnormalities including severe edema, scoliosis, and extreme calcinosis of soft tissues. Calcinosis was novel to the literature on amphibian abnormalities, the most frequent abnormality, and restricted exclusively to treatment wetlands. Within the constructed wetlands, tadpole development was slower and the frequency of scoliosis and calcinosis was higher in those cells receiving direct inputs of treated wastewater. Our results suggest that portions of constructed wetlands that directly receive treated wastewater may be poor amphibian habitat.

Echinostome-Induced Mortality Varies Across Amphibian Species in the Field

Echinostomes are receiving increased attention because of their emerging parasite status in landscapes associated with human development and their ability to infect and kill many North American larval amphibians. While laboratory experiments have shown that echinostomes can cause extensive mortality in their amphibian hosts, their effect on tadpoles in the field is less clear. I conducted a controlled-infection field-enclosure experiment in 4 ponds to compare the effects of echinostomes on green frog (Rana clamitans) and gray tree frog (Hyla versicolor) tadpoles in the field. I measured tadpole growth, development, mortality, and infection intensity. Echinostome infection resulted in high mortality in green frog tadpoles and less mortality in gray tree frogs. However, metacercariae encystment rates were higher in gray tree frog tadpoles than in green frog tadpoles. The effect of echinostomes on mortality varies across amphibian species, with the result that some species may experience more extensive echinostome-induced mortality than others. Mortality as a result of echinostome infection in green frog tadpoles was similar to mortality observed in predation experiments.

Evolutionary reorganizations of ontogenesis in related frog species of the family Myobatrachidae

Literary data on the comparative embryology of Australian endemic frogs of the family Myobatrachidae have been considered. Six main types of development have been recognized, the important criteria for which are egg size, the site of embryo and tadpole development (in water or on land), the character of larval nutrition (exotrophic or endotrophic), the occurrence of direct development, brooding, etc. An attention has been also paid to the character of the early cleavage process in the embryos of various species. The peculiarities of ontogeny in Myobatrachidae are compared with those in representatives of other families in the order Anura. The perspectives of some further embryological studies of this Australian frog family have been outlined.

Insight on the development of neuroventilatory GABA‐mediated inhibition in bullfrog tadpoles

GABA is a main inhibitory neurotransmitter in the adult vertebrate CNS. In mammals, activation of GABA‐A receptors transitions from depolarization to hyperpolarization early in development; GABA acting via GABA‐A receptors matures from an excitatory to an inhibitory neurotransmitter. In bullfrogs this transition was previously associated with late metamorphosis. To identify precisely the timing of this development, we quantified neuroventilatory responses to GABA (0.1 mM) in tadpoles across all metamorphic stages. Neuroventilatory responses to GABA were evaluated via whole‐nerve recordings from isolated brainstems. Throughout development GABA elicited a marked neuroventilatory depression. The results were confirmed at higher GABA concentrations (0.25 & 0.5 mM). Our data suggest that GABAergic inhibition develops earlier than previously reported for bullfrogs. Alternatively, the GABA‐A receptor‐mediated excitation associated with early development could be masked by signaling through GABA‐B receptors, which are consistently inhibitory and play a greater role in GABAergic signaling in early tadpole development. To resolve this issue, studies using receptor‐specific agonists are underway.

A developmental analysis of periodic albinism in the amphibian Xenopus laevis

The periodic albino of Xenopus laevis displays a transitory presence of black melanin pigment in the embryo but looses this during tadpole development. This mutation, involving a recessive allele, affects melanogenesis in dermal melanophore pigment cells. It has been suggested that the mutation is intrinsic to the melanophore cell itself or, alternatively, reflects malfunction in the neuroendocrine system that regulates melanophore cell function. This latter system, involving pituitary melanotrope cells which produces α-melanophore stimulating hormone (α-MSH), is responsible for stimulating the production and dispersion of melanin pigment in dermal melanophores. The purpose of the present study was to determine to which degree the albinism is intrinsic to the melanophore or involves neuroendocrine malfunction. Experiments involved transplantation of presumptive melanophores from wild-type to albino embryos, and vice versa, immunocytochemical analysis of the albino neuroendocrine system and the creation of wild-type/albino parabiotic animals to determine if the neuroendocrine system of the albino can support melanotrope cell function. We show that the albino has a functional neuroendocrine system and conclude that the defect in the albino primarily affects the melanophore cell, possibly rendering it incapable of responding to α-MSH. It is also apparent from our results that in later stages of development the cellular environment of the melanotrope cell does become important to its development, but the nature of the critical cellular factors involved remains to be determined.

Longevity and larval development among southern bell frogs (Litoria raniformis) in the Coleambally Irrigation Area - implications for conservation of an endangered frog

Context. With the flow of many of the world’s rivers regulated such that water can be diverted for agriculture and human consumption, basic ecological information on the current status of key biota in significant floodplain wetlands and their response following inundation is needed. The maintenance of natural habitat to ensure amphibian survival is gaining increasing recognition, given the ongoing decline of anuran populations. Information on longevity, time required to emerge from the water and to reach sexual maturity, all provide important information about the required timing, frequency and duration of environmental water allocations to ensure successful recruitment among populations of southern bell frogs (Litoria raniformis Keferstein, 1867). Aims. The aims of this research were to establish the longevity of southern bell frogs in the Coleambally Irrigation Area (CIA) in the Riverina region of New South Wales, Australia, and to evaluate the capacity for southern bell frog tadpoles to survive and successfully metamorphose following an extended overwintering period. Methods. Skeletochronology studies were carried out using toe-clips taken from adult and juvenile frogs captured in irrigation channels and rice fields over two rice-growing seasons. For the metamorphosis assay, southern bell frog tadpoles were held back in their development by low temperatures and low food allocation for 290 days, before temperatures and food allocation were increased adequately to allow metamorphosis to occur. Key results. The study indicated that skeletochronological examination of toe-bones was a useful technique for establishing the age structure of southern bell frogs in this region. The oldest animals in the population were found to be 4–5 years old, although the majority of frogs were typically 2–3 years old. Also, the metamorphosis assay indicated that successful metamorphosis was the exception rather than the rule if tadpole development was held back by low food ration and low temperatures. Conclusions. If southern bell frogs reach sexual maturity only after 2 years, and the oldest animals observed in the field are 4 or 5 years old, then there is a very narrow window of opportunity – two to three seasons – for each individual to successfully breed. Implications. The implications for environmental flow management are that habitats for key species identified for protection such as the endangered southern bell frog will need water every 1–2 years to enable each cohort to breed and maintain the wild populations. The extent of the environmental flows needs to be adequate to ensure that water persists long enough for critical biological events such as anuran metamorphosis to occur during the spring and summer months.

Growth, development and sex ratios of Spotted Marsh Frog ( Limnodynastes tasmaniensis) larvae exposed to atrazine and a herbicide mixture

Exposures of Limnodynastes tasmaniensis tadpoles to atrazine (0.1, 1, 3 and 30 μg L −1), metolachlor (0.1, 1 and 10 μg L −1) and thiobencarb (90, 180 and 360 μg L −1) from Gosner stage 28 to 42 under controlled laboratory conditions gave no significant effects on tadpole growth, development and sex ratios. A binary mixture of atrazine and thiobencarb as well as a ternary mixture of all three herbicides also had no significant effects on the developing larvae to show no evidence of interactive toxicity. Abnormal gonad morphology was observed on two occasions; both from 0.1 μg L −1 atrazine treatments with one tadpole observed with testicular ovarian follicles. The low frequencies of abnormal gonadal morphology and testicular ovarian follicles did not indicate a concentration associated response to herbicide exposure. No significantly unbalanced sex ratios were observed to suggest any evidence of chemically induced feminisation. These observations suggest that environmentally relevant concentrations of atrazine, metolachlor and thiobencarb do not present a significant threat to the normal development of L. tasmaniensis larvae in surface waters of irrigated agricultural areas.

Growth and development of tadpoles (Xenopus laevis) exposed to selective serotonin reuptake inhibitors, fluoxetine and sertraline, throughout metamorphosis

Selective serotonin reuptake inhibitors (SSRIs) are widely prescribed drugs that are present in sewage effluents and surface waters. The objective of the present study was to determine whether low environmentally relevant concentrations of the SSRIs fluoxetine and sertraline could impair growth and development in tadpoles of the African clawed frog (Xenopus laevis) and to evaluate if such effects may be caused by a disruption of the neuroendocrine system. Tadpoles were exposed to SSRIs at concentrations of 0.1, 1, and 10 microg/L for 70 d throughout metamorphosis. No effects on deformities were observed. Tadpoles exposed to fluoxetine (10 microg/L) and sertraline (0.1, 1, and 10 microg/L) exhibited reduced growth at metamorphosis. Tadpoles exposed to sertraline (0.1 and 1 microg/L) exhibited an acceleration of development as indicated by an increase in the time to tail resorption. The effects of SSRIs on growth and development in tadpoles were likely driven by reduced food intake. Reduced feeding rates were observed in SSRI-exposed tadpoles, and nutritional status can influence growth and development in amphibians via effects on the neuroendocrine system. Only sertraline was capable of causing developmental toxicity in tadpoles at environmentally relevant concentrations. These data warrant additional research to characterize the risks to human health and wildlife from pharmaceutical exposures.

Discrimination of offspring by indirect recognition in an egg-feeding dendrobatid frog, Oophaga pumilio

Offspring discrimination, the differential treatment of offspring and unrelated young, functions in numerous animal taxa to ensure that vital and costly parental care behaviours are appropriately directed. Discrimination can be facilitated either by direct (phenotypic) recognition of offspring or by indirect (nonphenotypic) recognition of offspring location. Offspring discrimination and recognition mechanisms have not been identified in an amphibian. In the strawberry poison frog, Oophaga pumilio, a dendrobatid frog with obligatory maternal provisioning behaviour, I tested whether mothers discriminate between offspring and unrelated young, whether they use direct or indirect recognition cues, and whether prior parental investment plays a contextual role in the differential treatment of young. Mother frogs utilized tadpole-rearing cups attached to tree trunks in wet tropical forest. After manipulating the identity, location and/or age of tadpoles in cups, I determined whether maternal provisioning behaviour was maintained by measuring tadpole growth and development. Mothers provisioned young regardless of tadpole identity, but were sensitive to location and did not provision tadpoles that were moved 2 cm to an adjacent cup. When given a choice between related and unrelated tadpoles in originally chosen or adjacent cups, mothers discriminated by location, but not by relatedness. Maternal provisioning behaviour persisted when a tadpole provisioned for 10 days was replaced with either an age-matched or newly hatched unrelated tadpole, so direct offspring recognition does not appear to be dependent on prior parental investment. Together, these results provide strong evidence that mother O. pumilio use indirect recognition cues to discriminate between offspring and unrelated offspring.

Effects of temperature and volume of water on the growth and development of tadpoles of Pleurodema diplolister and Rhinella granulosa (Amphibia: Anura)

The capacity of tadpoles to reduce their metamorphosis time and body size in response to fluctuations in environmental variables of temporary ponds has been recorded in field and laboratory studies. The main alterations in this habitat are related to the decrease of the water level and increase in temperature. However, few studies tried to dissociate the effect of these two variables. The aim of the present study was to analyze simultaneously the effects of water volume reduction and temperature on the development and growth in tadpoles of Pleurodema diplolister (Peters, 1870) and Rhinella granulosa (Spix, 1824) - species that use temporary ponds for reproduction. The tadpoles of these two species were subject to four treatments: (1) constant volume of water of 2000 ml and constant temperature of 26oC; (2) gradually decreasing water volume from 2000 ml to 200 ml or 150 ml and constant temperature of 26 oC; (3) Constant water volume as in (1) and constant temperature of 30 oC or 33 oC and (4) Decreasing water volume (as above) and constant temperature as in the treatment (3). There was no interaction between both tested variables on the growth and development of tadpoles of both species. Tadpoles of P. diplolister and R. granulosa responded to high temperature by decreasing development time. Tadpoles responded to decreasing volume of water by metamorphosing into smaller size. Tadpoles of P. diplolister maintained at 30oC showed growth reduction. Tadpoles of R. granulosa increased their body size when subject to the treatment at 30oC.

Moderate Echinostoma trivolvis Infection Has No Effects on Physiology and Fitness-Related Traits of Larval Pickerel Frogs (Rana palustris)

High intensity infections of Echinostoma trivolvis metacercariae decrease survival and growth of young amphibian larvae. However, in nature, parasites are highly aggregated, which results in a large proportion of the amphibian population being only moderately infected. Survival and growth responses at these more-common, low-infection levels remain poorly studied. Thus, we investigated the effects of moderate Echinostoma trivolvis metacercariae infection (following exposure to 0, 10, 30, or 90 cercariae) on the growth and development of pickerel frog (Rana palustris) tadpoles. We measured metabolism to determine whether increased energy expenditure is a potential physiological mechanism underlying previously documented reduced growth. Furthermore, we quantified tadpole intestine size, which can exhibit plasticity in response to changing metabolic demands, and we characterized metacercariae distribution in tadpole kidneys. Metacercariae encysted in the pronephros significantly more than in the mesonephros, but tended to occur equally in right and left kidneys. Two mo post-infection (PI), there were no changes in tadpole survival, development, intestine size, or growth related to metacercariae infection. Similarly, metacercariae did not significantly increase metabolic rates during encystment or at 1 mo PI. Our study demonstrated that modest E. trivolvis infections, representative of a large proportion of the host population, had no detectable effects on fitness-related traits in laboratory isolation from other ecological variables.

Effects of Salinity on the Survival, Growth, Development, and Metamorphosis ofFejervarya limnocharisTadpoles Living in Brackish Water

We studied salinity tolerance and the effects of salinity on growth, development, and metamorphosis in Fejervarya limnocharis tadpoles living in brackish water. Specifically, we examined whether tadpoles exhibit adaptive plasticity in development when exposed to different salinities. Tadpoles collected on Green and Orchid Islands off Southeastern Taiwan were assigned to salinities of 0, 3, 5, 7, 9, 11, and 13 parts per thousand (ppt). The daily survival, weekly growth, and development of tadpoles were recorded until metamorphosis. More than 50% of tadpoles survived in 9 ppt for over a month, and a few individuals survived in 11 ppt for 20 days, suggesting that F. limnocharis tadpoles tolerate salinity better than the tadpoles of most species studied to date. Tadpoles at 9 ppt had lower survivorship, and retarded growth and development (from Gosner stage 26 to 35) compared to the other treatments. Tadpoles metamorphosed early at a smaller size as salinity increased, suggesting the existence of adaptive developmental plasticity in F. limnocharis in response to osmotic stress. Phenotypic plasticity in the age and size at metamorphosis in response to salinity may provide a means for tadpoles to adapt to the unpredictable salinity variation in coastal rock pools.

Multisensory Integration in Mesencephalic Trigeminal Neurons in Xenopus Tadpoles

Mesencephalic trigeminal (M-V) neurons are primary somatosensory neurons with somata located within the CNS, instead of in peripheral sensory ganglia. In amphibians, these unipolar cells are found within the optic tectum and have a single axon that runs along the mandibular branch of the trigeminal nerve. The axon has collaterals in the brain stem and is believed to make synaptic contact with neurons in the trigeminal motor nucleus, forming part of a sensorimotor loop. The number of M-V neurons is known to increase until metamorphosis and then decrease, suggesting that at least some M-V neurons may play a transient role during tadpole development. It is not known whether their location in the optic tectum allows them to process both visual and somatosensory information. Here we compare the anatomical and electrophysiological properties of M-V neurons in the Xenopus tadpole to principal tectal neurons. We find that, unlike principal tectal cells, M-V neurons can sustain repetitive spiking when depolarized and express a significant H-type current. M-V neurons could also be driven synaptically by visual input both in vitro and in vivo, but visual responses were smaller and longer-lasting than those seen in principal tectal neurons. We also found that the axon of M-V neurons appears to directly innervate a tentacle found in the corner of the mouth of premetamorphic tadpoles. Electrical stimulation of this transient sensory organ results in antidromic spiking in M-V neurons in the tectum. Thus M-V neurons may play an integrative multisensory role during tadpole development.

Timing and duration of developmental nicotine exposure contribute to attenuation of the tadpole hypercapnic neuroventilatory response

The ability for air-breathing vertebrates to adjust ventilation in response to increased CO(2) (hypercapnia) is fundamental to maintaining pH homeostasis. Developmental nicotine exposure has been shown to impair tadpole neuroventilatory responses to hypercapnia following 8-12 weeks of exposure. It is not clear, however, to what extent the timing of exposure during development and/or the duration over which the exposure takes place contribute to this impairment. Here, tadpoles were exposed to 30 microg/L of nicotine for 3- or 10-week durations, either early or late in tadpole development. Correlates of tadpole lung neuroventilation were monitored during normocapnic (1.5% CO(2)) and hypercapnic (5% CO(2)) conditions of isolated brainstems. Preparations derived from early metamorphic tadpoles failed to increase lung neuroventilation in response to hypercapnia whether they had been exposed to nicotine for 3 or 10 weeks. Preparations derived from late metamorphic tadpoles failed to respond to hypercapnia after being exposed to nicotine for 10 weeks. These results suggest that both the developmental timing and duration of exposure are important when considering nicotine's effect on the hypercapnic neuroventilatory response.

Protein Expression Profiling in the African Clawed Frog Xenopus laevis Tadpoles Exposed to the Polychlorinated Biphenyl Mixture Aroclor 1254

Exposure to environmental pollutants such as polychlorinated biphenyls (PCBs) is now taken into account to partly explain the worldwide decline of amphibians. PCBs induce deleterious effects on developing amphibians including deformities and delays in metamorphosis. However, the molecular mechanisms by which they express their toxicity during the development of tadpoles are still largely unknown. A proteomics analysis was performed on developing Xenopus laevis tadpoles exposed from 2 to 5 days postfertilization to either 0.1 or 1 ppm Aroclor 1254, a PCB mixture. Two-dimensional DIGE with a minimal labeling method coupled to nanoflow liquid chromatography-tandem mass spectrometry was used to detect and identify proteins differentially expressed under PCBs conditions. Results showed that 59 spots from the 0.1 ppm Aroclor 1254 condition and 57 spots from the 1 ppm Aroclor 1254 condition displayed a significant increase or decrease of abundance compared with the control. In total, 28 proteins were identified. The results suggest that PCBs induce mechanisms against oxidative stress (peroxiredoxins 1 and 2), adaptative changes in the energetic metabolism (enolase 1, glycerol-3-phosphate dehydrogenase, and creatine kinase muscle and brain types), and the implication of the unfolded protein response system (glucose-regulated protein, 58 kDa). They also affect, at least at the highest concentration tested, the synthesis of proteins involved in normal cytogenesis (alpha-tropomyosin, myosin heavy chain, and alpha-actin). For the first time, proteins such as aldehyde dehydrogenase 7A1, CArG binding factor-A, prolyl 4-hydroxylase beta, and nuclear matrix protein 200 were also shown to be up-regulated by PCBs in developing amphibians. These data argue that protein expression reorganization should be taken into account while estimating the toxicological hazard of wild amphibian populations exposed to PCBs.

Effects of Density on Metamorphosis of Bullfrogs in a Single Season

Although the number of temporary wetlands used by many amphibian species has declined nationally, permanent wetlands have increased on the landscape in many regions. Species like Bullfrogs (Rana catesbeiana) may benefit from permanent wetlands and increase their density on the landscape, making them more likely to encounter wetlands used by amphibians breeding in temporary ponds. Although Bullfrogs are typically viewed as inhabitants of permanent wetlands because of their long larval periods, they are known to use temporary ponds. This study examined how larval density influenced proportion of Bullfrogs metamorphosing, time to metamorphosis, mass at metamorphosis, and total Bullfrog survival in mesocosm ponds. Proportion of Bullfrogs metamorphosing and tadpole development were greatest in mesocosms with low tadpole density with up to 25% of tadpoles reaching metamorphosis. Our study indicates Bullfrogs can metamorphose in more northern climates within a single season across a range of densities, and highlights the potential for Bullfrogs to successfully use temporary pond environments.