Obligate crayfish burrow use and core habitat requirements of crawfish frogs

Effects of fine-scale habitat quality on activity, dormancy, habitat use, and survival after reproduction in Rana dybowskii (Chordata, Amphibia)

Amphibians are facing population declines and extinctions, and protecting and supplementing refuges can help species survive. However, the microhabitat requirements of most species are unknown, and artificial shelters or burrows have not been well tested for amphibians. Some amphibians exhibit complex behaviour during the transition from post-reproductive dormancy to activity. However, little is known about the ecology, post-reproductive dormancy, and terrestrial activity of amphibians. Here, habitat site selection in experimental enclosures and the effects of shelters (stones, soil) and shade (with and without shade netting) on the activity, exposed body percentage, burrow depth, body-soil contact percentage, and survival of Rana dybowskii were investigated during post-reproductive dormancy and post-dormant activity. The results showed that R. dybowskii live individually under leaves, soil, stones or tree roots. Furthermore, although the dormant sites of frogs are significantly different, the distribution of male and female frogs in these sites is similar. Shading and shelter significantly affected the exposed body percentage, burrow depth and body-soil contact percentage of frogs compared with soil. In the stone group, soil and stone form the frog's refuge/burrow, whereas in the soil group, the refuge/burrow is composed entirely of soil. Even though the soil group has a deeper burrow and a larger area of soil contact with the body, it still has a higher exposure rate than the stone group. Frog activity frequency was affected by shelter and shade; the interaction of shelter and time and the interaction of shading and time were significant. The soil group had a higher activity frequency than the stone group, and the no-shade group had a higher activity frequency than the shade group. Shelter and shading differences do not significantly affect frog survival; however, the death rate during post-reproductive dormancy is lower than that during the active period.

Batrachochytrium dendrobatidis (Bd) exposure damages gill tissue and inhibits crayfish respiration

Batrachochytrium dendrobatidis (Bd) is a pathogenic fungus known to infect amphibians and crayfish. In crayfish, Bd causes gill tissue damage, and in some cases, mortality. Most research has focused on the amphibian-Bd system, so to date, little is known about the effects of Bd on the crayfish host. Here, we studied the effects of sublethal exposure to Bd and the metabolites produced by Bd on crayfish Procambarus alleni survival, gill damage, and oxygen consumption (as a proxy for mass-specific metabolic rate). Oxygen consumption increased 24 h post-exposure to live Bd, indicative of a stress response, followed by a decrease in oxygen consumption over time (χ21 = 6.39, p = 0.012). There was no difference in response when comparing the crayfish exposure to live Bd and Bd-metabolites alone (χ21 = 2.70, p = 0.101), indicating that the metabolites may have been the causative agent responsible for the response. Additionally, oxygen consumption decreased with gill damage (tissue recession) in Bd-exposed individuals. We found that high doses of Bd cause outright mortality in crayfish, and we show here that sublethal Bd-induced inhibition of oxygen consumption could negatively impact crayfish in the field, possibly reducing their overall fitness. More research is needed to understand this understudied host-parasite system. It is essential that we incorporate the disease dynamics associated with Bd and crayfish in conservation disease models, as this is the only way to develop comprehensive community-based models.

Drought reduces chytrid fungus (Batrachochytrium dendrobatidis) infection intensity and mortality but not prevalence in adult crawfish frogs (Lithobates areolatus)

To fully understand the impacts of the chytrid fungus Batrachochytrium dendrobatidis (Bd) on amphibians it is necessary to examine the interactions between populations and their environment. Ecologic variables can exacerbate or ameliorate Bd prevalence and infection intensity, factors that are positively related when Bd is acting on naive amphibian populations as an epidemic disease. In crawfish frogs (Lithobates areolatus), a North American species with a complex life history, we have shown that Bd acts as an endemic disease with impacts that vary seasonally; the highest infection prevalences and intensities and highest frog mortality occurred during late spring in postbreeding individuals. In this study, conducted between 28 February and 23 August 2011 in southwestern Indiana on the same population, we report an uncoupling of the previously observed relationship between Bd prevalence and intensity following an extreme drought. Specifically, there was a postdrought reduction in Bd infection intensity and mortality, but not in infection prevalence. This result suggests that the relationship between prevalence and intensity observed in Bd epidemics can be uncoupled in populations harboring endemic infections. Further, constant prevalence rates suggest either that crawfish frogs are being exposed to Bd sources independent of ambient moisture or that low-level infections below detection thresholds persist from year to year. Drought has several ecologically beneficial effects for amphibians with complex life histories, including eliminating fish and invertebrate populations that feed on larvae. To these ecologic benefits we suggest another, that drought can reduce the incidence of the severe skin disease (chytridiomycosis) due to Bd infection.