Influence of prey diet quality on predator-induced traits in wood frog tadpoles (Lithobates sylvaticus)

Diet quality and predation are two critical factors in determining the growth and development of organisms. Various anurans are susceptible to phenotypic changes influenced by these factors. Yet, few studies examined prey diet quality as potential influence over predator-induced traits. Using wood frog tadpoles (Lithobates sylvaticus) as a model species, we investigated the effects of three diet compositions (plant-based, animal-based, omnivorous) crossed with presence or absence of chemical cues from predatory dragonfly larvae (Aeshnidae). After 35 days, we recorded 11 morphological measurements, Gosner stage, and intestinal length of tadpoles to assess phenotypic changes under the six different experimental conditions. Our results showed the additive effects of both diet quality and predator chemical cue without detection of interactions between the two. Tadpoles receiving the omnivorous diet grew and developed faster with wider denticle rows than those receiving the plant or animal diets. The growth and development of tadpoles receiving only the animal diet were significantly hindered. These results emphasize the importance of diet quality in the growth and development of larval wood frogs. Chemical cues from predators significantly reduced tadpole body size but, in contrast to previous findings, did not affect tail size. Our experimental procedure of providing water containing predator and injured conspecific chemical cues on a weekly basis likely provided relatively weak predation risk perceived by tadpoles compared to previous studies using caged predators. The predator environment in our experiment, however, represents one ecologically relevant scenario in which predation risk is not urgent.

Reconstituted Mining Effluent Reduces Neuronal Proliferation in the Developing Brain and Slows Growth of Body and Facial Features in Wild-Caught Wood Frog Tadpoles

Mining, whether current or inactive, generally increases salt concentrations in catchment watersheds due to precipitation on and through exposed rock surfaces. Practices like mountaintop removal mining have exacerbated this issue, with measurements of salt concentrations in nearby catchment systems well above normal levels. Nevertheless, the impact of the ionic composition of mining effluent on aquatic animal health is not well understood. This is a particularly important issue in Appalachia because it is home to an enormous diversity of organisms, including a huge array of amphibians that live in streams that receive mining effluent from operating and abandoned mines. To investigate this issue, we examined the effects of reconstituted mining effluent on the development of wild-caught wood frog (Lithobates sylvaticus) tadpoles. We collected day-old fertilized eggs from a creek near Blacksburg, VA in early March, 2018 and raised them to hatch. Tadpoles were then assigned to either sulfate or chloride-based reconstituted mining effluent diluted to six different conductivities (100 μS/cm - 2,400 μS/cm). After 7 or 14 days of treatment, tadpoles were euthanized and fixed in paraformaldehyde. We imaged the heads and bodies of tadpoles for morphometric analysis before dissecting out brains and immunostaining them for phospho-histone H3, which labels dividing progenitor cells in the brain. We found that sulfate-based reconstituted mining effluent significantly lowered progenitor cell division at 1200 μS/cm at Day 7 and at 600 μS/cm at Day 14 relative to control. Chloride-based reconstituted mining effluent was less impactful, with no significant differences observed at Day 7 and significantly lowered progenitor cell division at 2400 μS/cm at Day 14. In addition, both treatments slowed growth of some head morphological features, including head size and interocular distance. Chloride treatment slowed growth of body length at Day 14 at 600 μS/cm, whereas sulfate-based reconstituted mining effluent had no effect on body length. These data show that sulfate-based mining effluent has a substantial impact on aspects of neural development, whereas chloride-based reconstituted mining effluent had less effect. In contrast, chloride-based reconstituted mining effluent had a much greater impact than sulfate on body morphology and growth. These experiments demonstrate that the chemical composition of salts in mining effluent can have divergent effects on the development of amphibians.