Sodium Chloride Inhibits the Growth and Infective Capacity of the Amphibian Chytrid Fungus and Increases Host Survival Rates

The amphibian chytrid fungus Batrachochytrium dendrobatidis is a recently emerged pathogen that causes the infectious disease chytridiomycosis and has been implicated as a contributing factor in the global amphibian decline. Since its discovery, research has been focused on developing various methods of mitigating the impact of chytridiomycosis on amphibian hosts but little attention has been given to the role of antifungal agents that could be added to the host's environment. Sodium chloride is a known antifungal agent used routinely in the aquaculture industry and this study investigates its potential for use as a disease management tool in amphibian conservation. The effect of 0–5 ppt NaCl on the growth, motility and survival of the chytrid fungus when grown in culture media and its effect on the growth, infection load and survivorship of infected Peron's tree frogs (Litoria peronii) in captivity, was investigated. The results reveal that these concentrations do not negatively affect the survival of the host or the pathogen. However, concentrations greater than 3 ppt significantly reduced the growth and motility of the chytrid fungus compared to 0 ppt. Concentrations of 1–4 ppt NaCl were also associated with significantly lower host infection loads while infected hosts exposed to 3 and 4 ppt NaCl were found to have significantly higher survival rates. These results support the potential for NaCl to be used as an environmentally distributed antifungal agent for the prevention of chytridiomycosis in susceptible amphibian hosts. However, further research is required to identify any negative effects of salt exposure on both target and non-target organisms prior to implementation

Effect of NaCl on developing zoosporangia.

<p>The mean (±SE) number of developing <i>Batrachochytrium dendrobatidis</i> zoosporangia counted within an ocular frame at 400× magnification when grown for 11 days in liquid media with varying salt concentrations. Different letters above bars indicate significant differences were found between groups.</p

Effect of NaCl on infection load.

<p>The number of <i>Batrachochytrium dendrobatidis</i> genomic equivalents detected on epidermal swabs from juvenile <i>Litoria peronii</i> housed in tanks with water bodies at varying salt concentrations. Different letters indicate significant differences were found between groups.</p

Effect of NaCl on chytrid zoospores.

<p>The mean (±SE) number of <i>Batrachochytrium dendrobatidis</i> zoospores that were counted in or moving through an ocular frame at 400× magnification after 11 days growth in liquid media with varying salt concentrations. Different letters above bars indicate significant differences were found between groups.</p

Effect of NaCl on mature zoosporangia.

<p>The mean (±SE) number of mature <i>Batrachochytrium dendrobatidis</i> zoosporangia counted within an ocular frame at 400× magnification when grown for 11 days in liquid media with varying salt concentrations. Different letters above bars indicate significant differences were found between groups.</p

Effect of NaCl on survival rates.

<p>Survival curves for juvenile <i>Litoria peronii</i> inhabiting tanks with waterbodies of varying salt concentrations (0–5 ppt) following inoculation with <i>Batrachochytrium dendrobatidis</i>. Death was assumed following the expression of lethargy and slow righting reflex typical of the terminal stages of chytridiomycosis. Different letters above lines indicate significant differences were found between groups.</p

Investigating behaviour for conservation goals:Conspecific call playback can be used to alter amphibian distributions within ponds

Conspecific attraction can prevent occupancy of restored or created habitats by limiting dispersal to unoccupied areas. This may cause problems for threatened taxa where habitat restoration and creation programmes are implemented as part of species recovery plans. Studies on birds have found that the introduction of artificial communication cues such as calling can increase occupancy of restored habitat. The endangered green and golden bell frog (<i>Litoria aurea</i>) has a number of behavioural traits which suggest conspecific attraction occurs via a vocal mechanism, including a loud conspicuous call and large chorusing aggregations. To date, attempts to repopulate restored and created habitat through natural immigration and active translocation of tadpoles and juveniles have been met with limited success for this species. We used <i>L. aurea</i> to determine if distribution could be manipulated via conspecific attraction using artificial communication cues. We placed speaker systems in uninhabited areas of five inhabited ponds across two locations and broadcast calls of <i>L. aurea</i> to see if we could manipulate distribution into previously unoccupied pond areas. Surveys undertaken before and after broadcast indicate that we successfully manipulated <i>L. aurea</i> distribution for adults increasing both occupancy and calling around the speaker locations. This occurred in four of five replicate ponds over three months of experimental treatment, but controls remained low in abundance. We suggest that manipulation of distribution via conspecific attraction mechanisms could be a useful conservation tool for endangered amphibian habitat restoration and creation programmes, resulting in increased occupancy and programme success

Six-year demographic study reveals threat of stochastic extinction for remnant populations of a threatened amphibian

Sustained demographic studies are essential for early detection of species decline in time for effective management response. A paucity of such background data hindered the potential for successful conservation during the global amphibian decline and remains problematic today. The current study analysed 6 years of mark-recapture data to determine the vital demographic rates in three habitat precincts of the threatened frog, Litoria aurea (Hylidae) and to understand the underlying causes of variability in population size. Variability in population size of L.aurea was similar to many pond-breeding species; however this level of fluctuation is rare among threatened amphibians. Highly variable populations are at greater risk of local extinction and the low level of connectivity between L.aurea populations means they are at a greater risk of further decline due to stochastic extinction events and incapacity to recolonize distant habitat. We recommend that management of this species should encourage recolonization through creation of habitat corridors and reintroduction of L.aurea to areas where stochastic extinction events are suspected

Six-year demographic study reveals threat of stochastic extinction for remnant populations of a threatened amphibian

Sustained demographic studies are essential for early detection of species decline in time for effective management response. A paucity of such background data hindered the potential for successful conservation during the global amphibian decline and remains problematic today. The current study analysed 6 years of mark-recapture data to determine the vital demographic rates in three habitat precincts of the threatened frog, Litoria aurea (Hylidae) and to understand the underlying causes of variability in population size. Variability in population size of L. aurea was similar to many pond-breeding species; however this level of fluctuation is rare among threatened amphibians. Highly variable populations are at greater risk of local extinction and the low level of connectivity between L. aurea populations means they are at a greater risk of further decline due to stochastic extinction events and incapacity to recolonize distant habitat. We recommend that management of this species should encourage recolonization through creation of habitat corridors and reintroduction of L. aurea to areas where stochastic extinction events are suspected