Suspension feeding of anuran larvae at low concentrations of Chlorella algae (Amphibia, Anura)

Effects of a commonly used glyphosate-based herbicide formulation on early developmental stages of two anuran species

Environmental contamination, especially due to the increasing use of pesticides, is suggested to be one out of six main reasons for the global amphibian decline. Adverse effects of glyphosate-based herbicides on amphibians have been already discussed in several studies with different conclusions, especially regarding sublethal effects at environmentally relevant concentrations. Therefore, we studied the acute toxic effects (mortality, growth, and morphological changes) of the commonly used glyphosate-based herbicide formulation Roundup® UltraMax on early aquatic developmental stages of two anuran species with different larval types (obligate vs. facultative filtrating suspension feeders), the African clawed frog (Xenopus laevis) and the Mediterranean painted frog (Discoglossus pictus). While X. laevis is an established anuran model organism in amphibian toxicological studies, we aim to establish D. pictus as another model for species with facultative filtrating larvae. A special focus of the present study lies on malformations in X. laevis embryos, which were investigated using histological preparations. In general, embryos and larvae of X. laevis reacted more sensitive concerning lethal effects compared to early developmental stages of D. pictus. It was suggested, that especially the different morphology of their filter apparatus and the higher volume of water pumped through the buccopharynx of X. laevis larvae lead to higher exposure to the formulation. The test substance induced similar lethal effects in D. pictus larvae as it does in the teleost standard test organism used in pesticide approval, the rainbow trout (Oncorhynchus mykiss), whereas embryos of both species are apparently more tolerant and, conversely, X. laevis larvae about two times more sensitive. In both species, early larvae always reacted significantly more sensitive than embryos. Exposure to the test substance increased malformation rates in embryos of both species in a concentration-dependent manner, but not at environmentally relevant concentrations. However, the assumed field safety, based on calculated surface water concentrations of the active ingredient (glyphosate), should be validated with realistic field data and buffer strips have to be urgently regarded to any aquatic amphibian habitat.

Thyroid endocrine disruption of azocyclotin to Xenopus laevis during metamorphosis

Organotin compounds are ubiquitous contaminants that are frequently detected in the environment and in biota, which raises concern about their risk to wildlife and human health. In the present study, Nieuwkoop & Faber stage 51 Xenopus laevis tadpoles were exposed to different concentrations of azocyclotin (0, 0.02, 0.1 and 0.5μg/L) for 21 days, during which time the tadpoles underwent morphological development. Exposure to azocyclotin caused an inhibitory effect on the pre-metamorphic development of X. laevis (e.g., a shortened hind limb length). Azocyclotin induced an alteration of the triiodothyronine (T3) content, which indicated thyroid endocrine disruption. Real-time PCR was performed to examine the expression levels of the genes involved in the thyroid hormone (TH) signaling pathway. Significant down-regulation of the type 2 deiodinase gene was observed, which may be partially responsible for the decreased T3 concentrations. Furthermore, the expression of T3 responsive genes, including thyroid hormone receptor, basic transcription element binding protein, 2tromelysins-3 and matrix metalloproteinase 2, were down-regulated in tadpoles, suggesting that azocyclotin induced a decrease in the T3 contents and, in turn, affected the mRNA expression of downstream genes involved in multiple physiological responses. Chemical analysis showed that azocyclotin could accumulate in X. laevis after 21 days of exposure. In conclusion, the results of the present study showed that azocyclotin could alter the mRNA expression of genes involved in TH signaling as well as the thyroid hormone concentrations in X. laevis tadpoles, leading to endocrine disruption of thyroid system, and that azocyclotin had obvious inhibitory effects on X. laevis metamorphosis.

Acute Toxic Effects of the Herbicide Formulation Focus(®) Ultra on Embryos and Larvae of the Moroccan Painted Frog, Discoglossus scovazzi

For regulatory and scientific purposes, there is a need to understand the sensitivity of a wider variety of wild species of amphibians and the sensitivities within their life stages to chemicals of widespread use such as herbicides. We investigated the acute toxic effects of the herbicide formulation Focus Ultra [with the active ingredient (a.i.) cycloxydim plus solvent naphtha and sodium dioctylsulphosuccinate as added substances] on embryos and early stage larvae of the Moroccan painted frog (Discoglossus scovazzi). Different clinical signs (twitching, convulsion, and narcosis) occurred at 40 and 80 mg/L in embryos (4 and 8 mg a.i./L) and narcotic effects (total immobilization or irregular escape responses) at 10, 15, and 20 mg/L in larvae (1, 1.5, and 2 mg a.i./L). Growth inhibition (total length), starting at 20 mg/L in embryos and 2.5 mg/L in larvae (2 and 0.25 mg a.i./L, respectively) was understood as sign of toxicity (retardation) and not as sign of teratogenicity. However, the connection to teratogenesis remained unclear though total length reduction occurred at concentrations <20 % of the 96-h LC50 value and at a minimum concentration that inhibits growth of only 17 % of the 96-h LC50 value. Starting at 20 mg/L, mortality in embryos significantly increased and at 15 mg/L in early larvae (2 and 1.5 mg a.i./L, respectively). Mortality of larvae was enhanced during the first 24 h of exposure to 15 and 20 mg/L (1.5 and 2 mg a.i./L). Morphology of the embryos remained unobtrusive. In contrary, axial malformations significantly increased in the early larvae starting at 10 mg/L (1 mg a.i./L), a concentration free of lethal effects. In all considered end points, larvae were significantly more sensitive than embryos, probably because of developmental and physiological properties or different exposure and bioavailability of the compound. Focus Ultra induced comparable lethal and immobilization effects in D. scovazzi as it does to standard test organisms in pesticide approval. However, to validate the apparent safety in the field, which is based on calculated surface water concentrations of the a.i., more data on real contamination levels is necessary (e.g., peak concentrations, concentrations of added substances). Furthermore, sufficient buffer strips between the farmland and amphibian ponds must be considered, and the effects of the substance on terrestrial life stages have not been assessed yet.

Effects of an environmentally relevant temporal application scheme of low herbicide concentrations on larvae of two anuran species

Cultivation of herbicide-tolerant crops involves repeated applications of the complementary herbicide throughout the growing season, while in conventional corn production, herbicide application is restricted to the beginning of cultivation. Repeated application of herbicides increases both the likelihood an organism will be exposed to the herbicide and the concentration it may be exposed to. We examined effects of short and pulsed exposure of the cycloxydim-based herbicide formulation Focus® Ultra at doses close to the calculated LC5 (0.01 and 0.5 mg a.i. L(-1)) and LC10 values (0.05 and 1.0 mg a.i. L(-1)) on early premetamorphic and prometamorphice larvae of two anuran model organisms, Xenopus laevis and Discoglossus scovazzi. In addition, larvae were repeatedly exposed, i.e. at all considered developmental stages. The herbicide did not induce effects on body size at and time to metamorphosis or increase deformation rates in both species. Exposure to calculated LC5 values did not increase mortality or cause clinical signs in both species. At calculated LC10 values, narcotic effects were seen in all developmental stages. There was no clear evidence of developmental-specific mortality. Metamorphic success was independent of time point and duration of application in X. laevis. Only repeated exposure significantly increased mortality at metamorphosis in D. scovazzi. Narcosis may result in increased mortality under field conditions due to rise of predation risk. Different sensitivity of the test species to the compound was attributed to their physiological properties. Different filtering rates were understood as an accompanying factor influencing exposition.

Acute toxic effects of the herbicide formulation and the active ingredient used in cycloxydim-tolerant maize cultivation on embryos and larvae of the African clawed frog, Xenopus laevis

Most genetically engineered herbicide-tolerant crops are still awaiting approval in Europe. There is, however, a recent trend for the cultivation of cycloxydim-tolerant maize hybrids for use in maize production. We studied the acute toxic effects of the complementary herbicide Focus(®) Ultra and its active ingredient cycloxydim on embryos and early-stage larvae of the African clawed frog (Xenopus laevis). The results indicate that the herbicide formulation is significantly more toxic than the active ingredient alone. Therefore, it is suggested that the added substances either solely or in a synergistic action with the active ingredient are responsible for adverse effects. The formulation was found to be moderately toxic to embryos but highly toxic to early larvae. Based on calculated teratogenic indices, both cycloxydim and Focus(®) Ultra seem to be non-teratogenic and also the minimum Focus(®) Ultra concentration to inhibit growth in embryos and larvae was close to the LC50 values. The data suggest that tests with the rainbow trout are not in all cases appropriate to assess the risk in aquatically developing anurans. This is demonstrated by 96-h LC50 values, which are for rainbow trout more than 50- to 20-fold higher than for early X. laevis larvae. However, based on worst-case predicted environmental concentrations for surface waters, there is apparently a large safety margin in field use of Focus(®) Ultra if buffer strips between the farm land and the amphibian habitats are regarded.

Ontogeny, phylogeny, and morphology in anuran larvae: Morphometric analysis of cranial development and evolution inRana tadpoles (Anura: Ranidae)

Comparative studies of chondrocranial morphology in larval anurans are typically qualitative in nature, focusing primarily on discrete variation or gross differences in the size or shape of individual structures. Detailed data on chondrocranial allometry are currently limited to only two species, Rana sylvatica and Bufo americanus. This study uses geometric morphometric and multivariate statistical analyses to examine interspecific variation in both larval chondrocranial shape and patterns of ontogenetic allometry among six species of Rana. Variation is interpreted within the context of hypothesized phylogenetic relationships among these species. Canonical variates analyses of geometric morphometric datasets indicate that species can be clearly discriminated based on chondrocranial shape, even when whole ontogenies are included in the analysis. Ordinations and cluster analyses based on chondrocranial shape data indicate the presence of three primary groupings (R. sylvatica; R. catesbeiana + R. clamitans; and R. palustris + R. pipiens + R. sphenocephala), and patterns of similarity closely reflect phylogenetic relationships. Analysis of chondrocranial allometry reveals that some patterns are conserved across all species (e.g., most measurements scale with negative allometry, those associated with the posterior palatoquadrate tend to scale with isometry or positive allometry). Ontogenetic scaling along similar allometric trajectories, lateral transpositions of individual trajectories, and variable allometric relationships all contribute to shape differences among species. Overall patterns of similarity among ontogenetic trajectories also strongly reflect phylogenetic relationships. Thus, this study demonstrates a tight link between ontogeny, phylogeny, and morphology, and highlights the importance of including both ontogenetic and phylogenetic data in studies of chondrocranial evolution in larval anurans.

Functional morphology of feeding and gill irrigation in the anuran tadpole: electromyography and muscle function in larval Rana catesbeiana

This study provides the first data on muscle activity patterns during active feeding in a larval anuran. Data regarding muscle function during gill irrigation and hyperexpiration are also provided. Electromyographic and kinematic data were recorded from six mandibular and hyoid muscles in unanesthetized, unrestrained larvae of Rana catesbeiana. Only three (hyoangularis, orbitohyoideus, anterior interhyoideus) of the six muscles examined are active during gill irrigation. Feeding cycles are characterized by the recruitment of three additional muscles: intermandibularis, suspensorioangularis, and levator mandibulae longus superficialis. The latter two contribute, respectively, to wide opening and forceful closing of the mouth during feeding. Hyperexpiration is characterized by a reversal of water flow anteriorly out of the mouth. This hydrodynamic change occurs due to modulation of the timing of firing of the anterior interhyoideus, as well as recruitment of the posterior interhyoideus, which is only active during hyperexpiration. Both regions of the interhyoideus, which are responsible for evacuation of the buccal cavity, are active during the opening phase of hyperexpiration. Kinematically, transitioning from gill irrigation to feeding involves both an overall shortening of the gape cycle and a shift in the relative length of opening phase vs. closing phase. Our results corroborate many of the findings of Gradwell ([1972] Can J Zool 50:501-521) regarding muscle function during gill irrigation and hyperexpiration. Furthermore, we demonstrate that in larval anurans the transition from gill irrigation to feeding involves modulation of gape cycle kinematics, changes in the level of activity of muscles, and recruitment of muscles that are not active during irrigation. In light of new data presented here, a review of muscle function in tadpoles is also provided.

Chondrocranial development in larval Rana sylvatica (Anura: Ranidae): morphometric analysis of cranial allometry and ontogenetic shape change

This study provides baseline quantitative data on the morphological development of the chondrocranium in a larval anuran. Both linear and geometric morphometric methods are used to quantitatively analyze size-related shape change in a complete developmental series of larvae of the wood frog, Rana sylvatica. The null hypothesis of isometry was rejected in all geometric morphometric and most linear morphometric analyses. Reduced major axis regressions of 11 linear chondrocranial measurements on size indicate a mixture of allometric and isometric scaling. Measurements in the otic and oral regions tend to scale with negative allometry and those associated with the palatoquadrate and muscular process scale with isometry or positive allometry. Geometric morphometric analyses, based on a set of 11 chondrocranial landmarks, include linear regression of relative warp scores and multivariate regression of partial warp scores and uniform components on log centroid size. Body size explains about one-quarter to one-third of the total shape variation found in the sample. Areas of regional shape transformation (e.g., palatoquadrate, otic region, trabecular horns) are identified by thin-plate spline deformation grids and are concordant with linear morphometric results. Thus, the anuran chondrocranium is not a static structure during premetamorphic stages and allometric patterns generally follow scaling predictions for tetrapod cranial development. Potential implications regarding larval functional morphology, cranial development, and chondrocranial evolution in anurans are discussed.