Altered thyroid hormone levels affect the capacity for temperature-induced developmental plasticity in larvae of Rana temporaria and Xenopus laevis.

Anuran larvae show phenotypic plasticity in age and size at metamorphosis as a response to temperature variation. The capacity for temperature-induced developmental plasticity is determined by the thermal adaptation of a population. Multiple factors such as physiological responses to changing environmental conditions, however, might influence this capacity as well. In anuran larvae, thyroid hormone (TH) levels control growth and developmental rate and changes in TH status are a well-known stress response to sub-optimal environmental conditions. We investigated how chemically altered TH levels affect the capacity to exhibit temperature-induced developmental plasticity in larvae of the African clawed frog (Xenopus laevis) and the common frog (Rana temporaria). In both species, TH level influenced growth and developmental rate and modified the capacity for temperature-induced developmental plasticity. High TH levels reduced thermal sensitivity of metamorphic traits up to 57% (R. temporaria) and 36% (X. laevis). Rates of growth and development were more plastic in response to temperature in X. laevis (+30%) than in R. temporaria (+6%). Plasticity in rates of growth and development is beneficial to larvae in heterogeneous habitats as it allows a more rapid transition into the juvenile stage where rates of mortality are lower. Therefore, environmental stressors that increase endogenous TH levels and reduce temperature-dependent plasticity may increase risks and the vulnerability of anuran larvae. As TH status also influences metabolism, future studies should investigate whether reductions in physiological plasticity also increases the vulnerability of tadpoles to global change.

Altered thyroid hormone levels affect body condition at metamorphosis in larvae of Xenopus laevis.

Chemical, physical and biological environmental stressors may affect the endocrine system, such as the thyroid hormone (TH) axis in larval amphibians with consequences for energy partitioning among development, growth and metabolism. We studied the effects of two TH level affecting compounds, exogenous l-thyroxine (T4 ) and sodium perchlorate (SP), on various measures of development and body condition in larvae of the African clawed frog (Xenopus laevis). We calculated the scaled mass index, hepatosomatic index and relative tail muscle mass as body condition indices to estimate fitness. Altered TH levels significantly altered the growth, development, survival and body condition in metamorphic larvae in different directions. While exogeno us T4 reduced growth and accelerated development, SP treatment increased growth but slowed down development. Altered TH levels improved body conditions in both treatments and particularly in larvae of the SP treatment but to the detriment of lower survival rates in both TH level altering treatments. The hepatosomatic index was negatively affected by exogenous T4 , but not by SP treatment indicating a lower lipid reserve in the liver in larvae of T4 treatment. These altered TH levels as caused by several environmental stressors may have an influence on individual fitness across life, as body condition at the onset of metamorphosis determines metamorphic and juvenile survival. Further research is needed to determine synergetic effects of environmental stressors on TH levels and its effects on physiological traits such as metabolic rate.

Thyroid hormone levels and temperature during development alter thermal tolerance and energetics of Xenopus laevis larvae.

Environmental variation induced by natural and anthropogenic processes including climate change may threaten species by causing environmental stress. Anuran larvae experiencing environmental stress may display altered thyroid hormone (TH) status with potential implications for physiological traits. Therefore, any capacity to adapt to environmental changes through plastic responses provides a key to determining species vulnerability to environmental variation. We investigated whether developmental temperature (T dev), altered TH levels and whether the interactive effect of both affect standard metabolic rate (SMR), body condition (BC), survival and thermal tolerance in larvae of the African clawed frog (Xenopus laevis) reared at five temperatures with experimentally altered TH levels. At metamorphosis, SMR, BC and survival were significantly affected by T dev, TH status and their interaction with the latter often intensified impacts. Larvae developing at warmer temperatures exhibited significantly higher SMRs and BC was reduced at warm T dev and high TH levels suggesting decreased ability to acclimate to variation in temperature. Accordingly, tadpoles that developed at warm temperatures had higher maximum thermal limits but more narrow thermal tolerance windows. High and low TH levels decreased and increased upper thermal limits, respectively. Thus, when experiencing both warmer temperatures and environmental stress, larvae may be less able to compensate for changes in T dev. Our results demonstrate that physiological traits in larvae of X. laevis are strongly affected by increased TH levels and warmer temperatures. Altered TH levels and increasing T dev due to global change may result in a reduced capacity for physiological plasticity. This has far reaching consequences since the energetic requirement at the onset of metamorphosis is known to determine metamorphic success and thus, is indirectly linked to individual fitness in later life stages.