Landscape Composition: Effects on the Diet of Tadpole
The concentration of ions in water is usually expressed as conductivity, an important factor in the distribution of biota (Cañedo Arguelles, 2013). In this sense, streams water can be classified as freshwater, with salt concentrations lower than 0.5 g/L, according to the Venice System (1959). Mining activity is one of the most common sources of salt addition to mountain rivers (Cañedo Arguelles, 2013). However, the low concentration of salts registered in this study indicates low anthropic activity in the basin. Thus, in absence of major anthropic influences, the main source of major ions in the stream is the weathering of the basin (Weinstein and Davison, 2004). Therefore, given that the level of fluoride in Los Cerros Negros (HF-CN) stream is above the limits (0.01–0.3 mg F/L) of unpolluted waters, the integrity of aquatic community could be affected (Camargo, 2003).
There is information available on fluoride toxicity in higher plants and algae, but laboratory data are contradictory and the environmental effects of this compound are not known (Camargo, 2003; Davison and Weinstein, 2006). To our knowledge, no published research have addressed in situ the response of algal community to high natural fluoride content in freshwater. Therefore, our research represents the first study to evaluate the influence of natural fluoride in lotic freshwater ecosystems on diet and morphology of tadpoles.
Algae are used to assess the quality of the environment (Sabater et al., 2007), especially diatoms due to their sensitivity to many environmental factors (Gómez and Licursi, 2001; Zampella et al., 2006; Seeligmann et al., 2008; Dunck et al., 2015). Fluoride has unfavorable effects for algae because it interacts negatively on photosynthesis, respiration, pigment synthesis, among other biological systems (Bhatnagar, 1997), causing chlorosis, necrosis, and morphological abnormalities in algae and aquatic plants (McPherson et al., 2014). Some experimental studies have shown that fluoride may suppress or intensify population growth of algae depending on its concentration, time of exposure, and algal species. For example, concentrations between 123 and 190 mg F/L showed 82% inhibition of growth of Chlorella sp. and Selenastrum sp. (Chlorophyta). However, other freshwater genera such as Oscillatoria sp. (Cyanobacteria), Scenedesmus sp. (Chlorophyta), Cyclotella sp. (Bacillariophyta) exposed to concentrations ?50 mg F?/L is not affected (Camargo, 2003). Joy and Balakrishnan (1990) determined in laboratory that at concentrations of 10–100 mg F/L Nitzschia palea (Bacillariophyta) improved its growth, which could be due to a fluoride requirement for optimal growth. Nevertheless, Ali (2004) determined that with 4 mg F/L, it caused an inhibition in the growth of N. palea at low pH, given that F- become more toxic when crosses the membrane.
Our study of gut analysis showed that the two streams are different, according to the NMDS plot. The analysis of composition and structure of tadpole diets also allowed to differentiated the sites. Diversity, richness and evenness were lower in the diets of tadpoles collected from stream HF-CN. This suggested that a natural concentration of fluoride higher than 2 mg F/L could be deleterious for some algal species. A possible explanation could be that fluoride crosses the cell membrane, lodges in the cytoplasm and interacts with most of the cellular components, altering the general functioning of cell (Barbier et al., 2010).
Fluoride and Morphological Measures in Tadpoles
Feeding of anuran tadpoles is generalist; therefore, their diets are indicative of the quality and abundance of the nutritional resource in their environment (Heyer, 1974; Lajmanovich, 2000; Rossa Feres et al., 2004; Bionda et al., 2011). However, in our study some more abundant algae were not consumed by the tadpoles. This could be due to the architecture of periphyton, making some algae easily consumed than others (Pollo et al., 2015). Food quality and ecological conditions may influence the age and size at metamorphosis of tadpoles (Carey and Bryant, 1995; Altig et al., 2007), thus affecting biological interactions such as competition and predation (Kupferberg, 1997).
In amphibians the body size can be strongly influenced by age and/or ambient conditions such as food supply, temperature, pollutants (Stepanyan et al., 2011; Cabrera-Guzmán et al., 2013; Babini et al., 2015). Our results showed that the total length of tadpoles of B. cordobae was not influenced by the larval stage. Therefore, the shortest total length recorded in larvae of the HF-CN stream could be due to high concentrations of fluoride, due to is a sensitive parameter a the effects of fluoride (Goh and Neff, 2003; Zhao et al., 2013; Chai et al., 2017; Pollo et al., 2016; Zhang et al., 2018). For example, Chai et al. (2017) found that total length of embryo of Rana chensinensis (Chordata, Ranidae) decreases with increasing levels of fluoride (?0.7 mg F/L). The specific mechanism of inhibition may be the accumulation of fluoride in the body, which causes an imbalance in bone deposition and the remodeling activities that lead to skeletal fluorosis (Zhang et al., 2018). In addition, the metamorphosis of the tadpoles is delayed in the presence of high concentrations of fluoride (Chai et al., 2017). This effect is due to the fact that this element changes the histomorphology of the thyroid gland (Zhao et al., 2013) or alters thyroid hormone (Chen et al., 2016). In nature, the effect on total length of the tadpoles may result in an increased risk of predation and higher mortality. In addition, it can compromise persistence of the population because final size of the individuals also influences their reproduction and recruitment (Wilbur, 1980; Semlitsch et al., 1988; Gray and Smith, 2005).
However, when we analyzed biometric measurements together (total length and body weight) as an index of body condition, our results showed no differences in tadpoles between sites. Body condition is an indicator of the physiological state of the organism (Bagenal and Tesch, 1978; Jakob et al., 1996) directly related to diet, which is affected by the quality and quantity of food. At the same time, body weight could increase and decrease rapidly (Reading and Clarke, 1995). This could be explained by the nutritional quality of the algal community towards more or less inedible species. The diet of tadpoles of both streams showed higher abundance of chain-forming diatoms and large species, which can contribute to their body weight. In tadpoles from LF-LV stream this algae were represented by filamentous cyanobacteria and large diatoms, while in tadpoles from HF-CN large algae were long chains of diatoms Melosira sp. and Eunotia sp. In addition, there is a higher proportion of colonial and high profile algae in HF-CN site, which could have been more easily consumed (Cibils Martina et al., 2014). Another possible explanation could be that fluoride concentrations between 0.5 and 5 mg F/L increase gastrointestinal microbes, which contributes greatly to the health and digestive efficiency of tadpoles (Wang et al., 2019).