 the 0.4% sw treatment group (Table 1).
During development, several deformities were noticed in some individuals. Tail crimps or kinks [cf. 7] were noticed in individuals from the 0.4% sw (3/20), 10% sw (3/20) and 16% sw (1/20) treatments in L. tasmaniensis, and the 10% sw (1/20) and 16% sw (2/20) treatments in L. aurea However, all but one (1/10) of these individuals were still able to successfully complete metamorphosis.
Secondary salinisation has been suggested as an important factor contributing to anuran decline in Australia, but empirical data demonstrating negative fitness impacts remains limited. This study examined the effects of elevated salinity on growth, metamorphosis and survival of the tadpoles of three species of native Australian frog. Our results show the potential for elevated salinity to act as an agent of selection in native anuran species, with each species studied here showing a marked difference in survival under differing salinity treatments.
A previous study on L. aurea tadpoles from a coastal population (Kurnell NSW, Australia, 34°00′ S, 151°12′ E, 1998-9) reported that tadpoles tolerated salinities of up to 4% sw without apparent adverse effects, but showed a salinity threshold (above which significant mortality occurred) somewhere between 5.5% and 10% sw [8]. In contrast, our results showed a significantly higher survival in all salinities above 4% sw, and a lower survival in freshwater. There are several explanations for this contrast in findings including increased phenotypic plasticity, local adaptation or lineage adaptation in response to elevated levels of salinity in our experimental animals. Adaptation to salinity has previously been reported in other non-Australian amphibians. For example Gomez-Mestre et. al [23], [24] provided evidence for elevated salt tolerance of the embryonic and larval stages of populations of natterjack toads (Bufo calamita) inhabiting brackish ponds in southern Spain. However, in these cases adapted individuals transplanted to freshwater environments showed similar survival probabilities, length of larval period, and mass at metamorphosis compared to those native to freshwater. To our knowledge our study is the first report of anurans reared in freshwater having a lower survival than animals of the same population reared in brackish conditions. However, loss of performance of stress tolerant individuals in non-stressful environments has been demonstrated in other organisms such as bacteria [25], [26], and fish [27]. In order to identify the underlying mechanisms involved in the increased larval salinity tolerances shown here further studies comparing the effects of salinity on tadpoles of L. aurea from various populations along a salinity gradient will be required. Because differences in a mean phenotype among populations must have a genetic basis to be considered adaptive [28], conducting reciprocal transplant experiments would also be a valuable next step towards elucidating the adaptive response of L. aurea to challenging osmotic environments caused by secondary salinisation [29].
When compared to L. aurea, L. tasmaniensis and N. sudelli were less tolerant of high salinity, with both species showing a reduction in survival at salinities above 10% sw. Of the deaths that occurred in the 16% sw treatment groups, 85% of L. tasmaniensis and 45% of N. sudelli occurred within the first 30 days of treatment. In other species, newly hatched tadpoles have been demonstrated to be more vulnerable to increases in salinity compared to older cohorts (e.g. Fejervarya cancrivora [30], Litoria ewingii [7]). This vulnerability has been linked to differences in gill development [31] and kidney function [32] between cohorts.
Previous studies of a range of anurans (e.g. R. cancrivora, B. calamita, L. ewingii,) have reported that high salinity levels can retard tadpole development [7], [23], [31]. Our results indicate that when salinity rises above a tolerable concentration (in this case 10% sw), tadpoles of L. tasmaniensis accelerate metamorphosis instead of depending on weak osmoregulatory responses. L. aurea showed a similar pattern of accelerated development in the environment with the lowest prospective survival (in this case the 0.4% sw treatment group) when compared to the environment with intermediate survivals (4% sw). Our results suggest that these some species can invest energy in development when exposed to unfavourable salinities, be they too high or too low. Accelerating metamorphosis in unfavourable environments is expected to be costly, and in other anuran species studied rapid development typically results a reduction of body size at metamorphosis [7], [23]. While we observed a reduction in snout-vent length in L. aurea in freshwater, there were no other significant differences in weight or size in any other treatments or species. The tail crimps or kinks we observed in the 0.4% sw, 10% sw and 16% sw treatments in L. tasmaniensis and the 10% sw and 16% sw treatments in L. aurea may be indicators of an additional cost associated with increasing development. While not lethal in laboratory conditions, these tail crimps may affect a tadpole’s ability to effectively forage and evade predation [33].
Our study shows that elevated salinity negatively impacts on survival of N. sudelli and negatively on the survival, growth and development of L. tasmaniensis tadpoles, indicating that the salinisation of natural freshwater breeding habitats could contribute to the decline of local populations. In contrast, L. aurea tadpoles displayed a higher salinity tolerance, and this was higher than previously reported for this species, indicating this species may have the capacity to adapt to elevated environmental salinity. However, because survival of L. aurea was low in freshwater treatments, adaptation to elevated salinity may be associated with a loss of function under low saline conditions. Testing salinity tolerance of tadpoles from populations experiencing a range of salt conditions, running transplant experiments, and examining changes in osmoregulatory function will be needed to advance our understanding of the physiological and evolutionary responses of Australian anurans to secondary salinisation. This knowledge will help to determine how amphibians adapt to threatening environmental conditions, and will potentially assist with the management of the world’s declining frog populations.
We thank B. Lees for her technical support and A. de Kretser and M. Dragon for field assistance.
Conceived and designed the experiments: BDK PGB RDR. Performed the experiments: BDK. Analyzed the data: BDK. Contributed reagents/materials/analysis tools: BDK PGB RDR. Wrote the paper: BDK.
2. Smekens MJ, van Tienderen PH (2001) Genetic variation and plasticity of Plantago coronopus under saline conditions. Acta Oecologica-International Journal of Ecology 22: 187–200.
3. Hazell D (2003) Frog ecology in modified Australian landscapes: a review. Wildlife research 30: 193–205.
4. Blaustein AR, Wake DB, Sousa WP (1994) Amphibian declines: Judging stability, persistence, and susceptibility of populations to local and global extinctions. Conservation Biology 8: 60–71.
5. Australian Government Department of Environment and Heritage (2006) Threat abatement plan: infection of amphibians with chytrid fungus resulting in chytridiomycosis. Canberra: Department of Environment and Heritage,.
6. Smith MJ, Schreiber ESG, Scroggie MP, Kohout M, Ough K, et al. (2007) Associations between anuran tadpoles and salinity in a landscape mosaic of wetlands impacted by secondary salinisation. Freshwater Biology 52: 75–84.
7. Chinathamby K, Reina RD, Bailey PCE, Lees BK (2006) Effects of salinity on the survival, growth and development of tadpoles of the brown tree frog, Litoria ewingii. Australian Journal of Zoology 54: 97–105.
8. Christy MT, Dickman CR (2002) Effects of salinity on tadpoles of the green and golden bell frog (Litoria aurea). Amphibia-Reptilia 23: 1–11.
9. Nielsen DL, Brock MA, Rees GN, Baldwin DS (2003) Effects of increasing salinity on freshwater ecosystems in Australia. Australian Journal of Botany 51: 555–565.
10. Hart BT, Bailey P, Edwards R, Hortle K, James K, et al. (1991) A review of the salt sensitivity of the Australian fresh-water biota. Hydrobiologia 210: 105–144.
11. Williams WD (2001) Anthropogenic salinisation of inland waters. Hydrobiologia 466: 329–337.
12. Boutilier RG, Stiffler DF, Toews DP (1992) Exchange of respiratory gases, ions and water in amphibious and aquatic amphibians. In: Feder ME, Burggren WW, editors. Environmental Physiology of the Amphibians. Chicago: The University of Chicago Press. 81–124.
13. Viertel B (1999) Salt tolerance of Rana temporaria: spawning site selection and survival during embryonic development (Amphibia, Anura). Amphib