Salinity increases growth and pathogenicity of water mold to cause mortality and early hatching in Rana sylvatica embryos

Amphibian embryos often suffer increased mortality and altered hatching when exposed to road deicing salt runoff or pathogens such as water molds. However, the combined effects of such contaminants on embryos remains understudied. To test how pond salinization interacts with water mold (Saprolegnia sp.) to influence hatching timing and survival, we first measured pond water conductivity and temperature and quantified the prevalence and abundance of water mold in nature. Second, we experimentally placed wood frog (Rana sylvatica) embryos in the presence or absence of water mold, crossed with environmentally realistic salt concentrations (100 μS, 300 μS, or 600 μS). Lastly, we quantified growth and colonization of water mold in this range of salinities. Our results demonstrate that salt had synergistic effects with water mold exposure that affected hatching time, though water mold had less of an effect at higher salinities. Water mold significantly reduced egg survival whereas salt did not. Higher salinities also increased water mold growth and colonization on new substrates. These results indicate that road salt runoff may enhance colonization of amphibian eggs by water molds increasing mortality and premature hatching of surviving embryos, which may in turn have detrimental effects on amphibian communities.

Evaluation of Two Formalin Concentrations During Yellow Perch Egg Incubation

Daily 15-minute treatments of formalin at 1667 mg/L are typically used to prevent water mold infestations in coolwater fish eggs. This study evaluated the use of a reduced concentration of 834 mg/L during yellow perch Perca flavescens egg incubation until the eyed stage of egg development. No water mold was observed on the perch eggs using either formalin treatment regime. Median survival to the eyed stage of egg development was not significantly different between the treatments and approached 90%. There was also no significant difference in egg survival to hatch. In addition, fry length at hatching, although relatively short overall, was not significantly different between the formalin treatment concentrations. The results of this study indicate that reducing daily 15-minute formalin concentration from 1667 to 834 mg/L during yellow perch egg incubation will not affect survival through hatch or larval length at hatching.

Singly laid mole salamander (Ambystoma talpoideum) eggs resist mortality from water mold infection

Parasites and pathogens are noteworthy forces in evolutionary and population biology. Water molds are known to infect and kill amphibian eggs, and masses placed in clusters sometimes suffer greater losses. Thus, selection may favor single oviposition behavior in which females scatter individual eggs to avoid contact with dead eggs, thereby lowering the chances of hyphal spread. I tested the hypothesis that eggs in physical contact with dead eggs are more susceptible to death via pathogen infection using a singly laying urodele, the mole salamander, whose congeners mostly lay eggs in masses. Focal eggs were placed in replicated containers with variable egg contact (physically touching or separated) by treatment eggs that varied in status (dead or alive). The effect of egg arrangement was significant, with focal eggs surviving better to hatching when not contacting treatment eggs. Although treatment egg status (dead vs. alive) was not influential on its own, the interaction between egg contact and status was an important determinant of focal egg survival. Only when eggs were physically touching was it important whether they were dead or alive. Moreover, when eggs were touching there was a significant positive association between focal egg survival and the number of surviving treatment eggs. Water mold infected most dead eggs in all treatment groups. This study shows that mole salamander eggs are more likely to die when touching other eggs that have perished, most likely because of a greater chance of water mold infection. Such mortality may have contributed to diversification of egg-laying behavior in amphibians and shows a potentially compounding fitness cost of diminished fertilization success and embryo viability.