Objective 2. Evaluate habitat use and tolerance to extreme habitat conditions in the two species (i.e. aridity and salinity).

Tolerance to salinity

Though they usually inhabit freshwater environments, anuran amphibians have adapted differently to living along various salinity gradients. A recent overview on salinity tolerance in amphibians shows that different taxa have different degrees of adaptability to osmotic stress (Alexander et al., 2012). The tolerance thresholds to saline environments seem highly variable within amphibians and are probably due to the diversity of life history strategies exhibited by members of this group. Moreover, the physiological mechanisms that underlie osmoregulation differ between larval and post-metamorphic stages (Gordon & Tucker, 1965; Seiter et al., 1978; Degani & Nevo, 1986, Gomez-Mestre & Tejedo, 2005). Above the tolerance thresholds, the survival rate and normal development of larvae are considerably reduced, regardless the species, while various salt concentrations below may further affect the individual fitness of the metamorphs.

Extensive field studies already confirmed the negative impact of increased salinity in amphibian spawning habitats and amphibian assemblages’ composition (Karraker, 2008; Collins & Russell, 2009) and shown that water salinity is an important factor that may cause and predict population declines, distribution and diversity patterns in amphibians, since it influences their survival, development, and fitness (Gomez-Mestre et al., 2004; Smith et al., 2007; Haramura, 2008; Rios-López, 2008; Alexander et al., 2012).

Tadpole development to metamorphosis

In addition to natural threats such as desiccation, predatorism and parasitism, tadpole development and survival may be also threatened by anthropogenic activities. Spawning sites are frequently subject to pollution as a result of chemicals use in agriculture, de-icing salts for road maintenance, farming, industry and other human activities.   

There are only few studies related to osmotic stress in genus Pelobates.

We test the salinity tolerance threshold during larval development within spadefoot toads populations to identify both the impact of increased salinity on tadpole metamorphosis and the threshold that prevents tadpoles of the two species to metamorphose. We are looking for differences in (i) time to metamorphosis, (ii) size at metamorphosis, and (iii) survival across various salinity gradients.

We use experimental designs with various salinity treatments compared to a control (deionized water). We obtain the desired salt concentration by mixing deionized water and Ocean Fish marine salt for ornamental aquariums. Tadpoles are kept in plastic bowls and fed ad libitum rabbit pellets and TetraMin fish flakes. Water parameters (i.e. salinity, temperature) are checked with multiparameter tools. Photoperiod exposure is the same for all the individuals and respects the natural cycle. The metamorphs are later released at the site of capture.

Hypotheses tested:

  • Tolerance to salinity is higher in P. syriacus tadpoles.
  • Increased salinity will trigger earlier metamorphosis of tadpoles (and smaller sizes).

Behavioural effects of exposure to salinity

Any given species often requires specific environmental conditions for development and metamorphosis of embryos and larvae. Thus, the distribution of adult anurans depends largely on the location of suitable oviposition sites and on the ability of embryos and larvae to survive, develop, and metamorphose at those sites. Aquatic habitats in coastal areas can experience highly variable incursions of saline water from tides, land subsidence, receding coastlines, and storms, as well as from land clearing, river regulation, agriculture, de-icing, and construction of levees and canals. Salinity tolerance in sensitive embryonic and larval stages is therefore particularly important to the survival and distribution of anurans in coastal areas.

In the case of closely related syntopic species living in this environment any difference in the tolerance to salinity that one species would exhibit over the other would constitute a huge adaptive advantage. We choose to investigate if long term exposure to salinity would affect the behavior of the tadpoles, considering that behavioral endpoints have proved their usefulness in evidencing impacts of chemicals at environmental concentrations that do not necessarily cause mortality.

The tadpoles are raised up to metamorphosis in one of 3 treatments - control (deionized water) and 2 salt concentration (the salinity levels are within the limits of what we encountered in the field, where both species are present). Every 2 weeks their behaviour is monitored for a 30-minute period and the distance they cover and their average speed is recorded.

We expect that contaminated tadpoles would travel shorter distances and swim less often at a lower mean speed. Also, we suspect that P. syriacus tadpoles are less affected by salinity, which would explain their larger population in the area compared to P. fuscus.

Tolerance to aridity

Adaptive phenotypic plasticity in timing of metamorphosis of the tadpoles

Amphibians exhibit extreme plasticity in the timing of metamorphosis, and many species respond to pond drying by accelerating their metamorphosis. In this study we investigate the adaptive phenotypic plasticity of the developmental response to water volume reduction in the spadefoot toads, Pelobates fuscus and P. syriacus.

The response of tadpoles to the simulated drying conditions is evaluated by gradually reducing the water level in the experimental containers under controlled laboratory conditions. We test if tadpoles can accelerate development in a drying aquatic habitat and if the accelerated development causes a reduced body size at metamorphosis. We used various experimental designs, with water levels usually being: constant high (control), decrease and a constant low water level for each species.

Our results indicate that the P. syriacus larvae were able to respond to pond drying by speeding up their metamorphosis faster then P. fuscus. The first metamorphs were those from the fast decrease water level treatments in both species. The accelerated development caused by decreasing water level resulted in smaller body size at metamorphosis.


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