Antipredator response of Eurycea nana to a nocturnal and a diurnal predator: avoidance is not affected by circadian cycles of predators

Both predators and prey exhibit cyclic shifts in activity throughout the day, which should cause the threat posed by predators to change in a recurrent pattern. If the threat posed by a predator is dependent on their circadian foraging cycle, prey may respond more or less intensely to predators at different times of day, thereby maximizing the effectiveness and efficiency of avoidance behaviors. We examined whether predator-naïveEurycea nana, a federally threatened neotenic salamander, exhibits a different antipredator response to chemical cues of a diurnal predator, the green sunfish (Lepomis cyanellus), and a nocturnal predator, the red swamp crayfish (Procambarus clarkii). We predicted thatE. nanawould show more intense antipredator responses (reduced activity) to a diurnal predator during the day and to a nocturnal predator at night. We found that, although there was significant antipredator behavior ofE. nanatoward sunfish, there was no detectable response to crayfish and no effect of time of day on responses to either predator, suggesting that eitherE. nanadoes not innately exhibit circadian patterns in avoidance of these species or that those patterns were undetectable in this study. Future studies should examine whether experience with predators may cause these salamanders to be more sensitive to the diel variation in threat, as has been found with some other amphibians and fish. Due to the threatened nature of this species, understanding the factors that influence antipredator behavior are crucial for management.

Batrachochytrium dendrobatidis in natural and farmed Louisiana crayfish populations: prevalence and implications

The pathogenic chytrid fungus Batrachochytrium dendrobatidis (Bd) has been linked to global declines and extinctions of amphibians, making it one of the most devastating wildlife pathogens known. Understanding the factors that affect disease dynamics in this system is critical for mitigating infection and protecting threatened species. Crayfish are hosts of this pathogen and can transmit Bd to amphibians. Because they co-occur with susceptible amphibian communities, crayfish may be important alternative hosts for Bd. Understanding the prevalence and seasonal dynamics of crayfish infections is of agricultural and ecological interest in areas where crayfish are farmed and traded for human consumption. We conducted a survey of Bd in farmed and natural crayfish (Procambarus spp.) populations in Louisiana, USA. We found that Bd prevalence and infection intensity was low in both farmed and native populations and that prevalence varied seasonally in wild Louisiana crayfish. This seasonal pattern mirrors that seen in local amphibians. As crayfish are an important globally traded freshwater taxon, even with low prevalence, they could be an important vector in the spread of Bd.

Influence of ammonium nitrate on larval anti-predatory responses of two amphibian species

Sublethal effects of toxicants can upset normal behavioural responses to predators, leading to increased predation. For example, sensory capabilities can be impaired by toxicants, leading to difficulty in detecting predators or other threats. Alteration of locomotor abilities by pollutants can also explain the difficulty of tadpoles to escape from predators. Here we assess the effects of a nitrogenous fertilizer on the response to predators shown by anuran tadpoles. In a first experiment, we chronically exposed Iberian painted frog (Discoglossus galganoi) and spadefoot toad (Pelobates cultripes) tadpoles to environmentally relevant concentrations of ammonium nitrate. After the exposure, we tested tadpoles' ability to avoid predation by the red crayfish (Procambarus clarkii). In a second experiment, we analysed the escape behaviour of P. cultripes tadpoles as a function of ammonium nitrate exposure and presence of predatory crayfishes. Tadpoles of both species that were exposed to ammonium nitrate were consumed by crayfishes faster than controls (mean time of predation: Dg controls=18.03 h, 90.3 mg N-NO(3)NH(4)/L=7.48 h; Pc controls=16.12h, 90.3 mg N-NO(3)NH(4)/L=9.46 h). Control larval P. cultripes showed specific anti-predator escape responses, whereas those exposed to the fertilizer did not. We demonstrate, for the first time in amphibians, how nitrogenous fertilizers can affect larval defensive behaviours, and thereby increase the risk of predation. Our results emphasize the importance of considering environmental stresses on the ecotoxicological studies with amphibians.

Developmental windows and origins of the chemical cues mediating hatching responses to injured conspecific eggs in the common frog (Rana temporaria)

In amphibians, embryonic exposure to chemical cues resulting from a predation event on conspecific eggs can influence hatching traits. However, there is no information on the precise origin of the active substances, or on the critical period of embryonic development mediating such a process. In this context, common frog ( Rana temporaria L., 1758) eggs were exposed at Gosner stage 2, 16, or 20 to chemical cues simulating predation on whole eggs, jelly envelopes, or embryos. Embryonic movement rate, hatching time, and developmental stage at hatching appeared unaffected by the nature of the treatment. In contrast, the embryonic treatments strongly affected the morphology of hatchlings, with the groups exposed to crushed whole eggs and jelly envelopes showing longer (exposures at stages 16 and 20) and deeper (exposure at stage 20) tails than their unexposed counterparts. In addition, exposure at stage 20 to crushed embryos also produced hatchlings with longer tails than the controls. Thus, morphological plasticity at hatching can result from a relatively short period of embryonic exposure to conspecific chemical cues. This critical period occurs at the completion of neurulation (stage 16), with the most marked effects resulting from an exposure at the last stage of embryonic development (stage 20).