Observations on the intraspecific variation in tadpole morphology in natural ponds
Intraspecific morphological variation of anuran tadpoles occurs in response to several factors. Causes and consequences of this variation have been largely studied hitherto in controlled environments, but data from natural habitats is clearly less abundant. Here, we present a series of observations on the morphology – mainly tail depth – of three tadpole species from NE Iberian Peninsula across different pond typologies. According to experimental data on tadpole morphology and selective pressures along the pond permanency gradient, we should expect that tadpoles inhabiting ponds with a short hydroperiod – mainly facing desiccation risk – have shallower tail fins than tadpoles from ponds with longer hydroperiod – mainly facing predation risk. Thus, we expected that the link between these complementary selective pressures – predation risk, desiccation risk – and hydroperiod could make possible to detect intraspecific variation in tadpole morphology among different typologies of natural ponds. Morphological differences were found in all studied species, and variation, when present, agreed with theory: tadpoles had deeper fin tails as they were collected in ponds with a longer hydroperiod. Interestingly, in most cases these morphological differences were more marked as tadpoles were larger in size. Although distances among the studied ponds were generally short – posing phenotypic plasticity as the most plausible proximate mechanism – specifically designed studies would be needed to disentangle the relative role of other processes like local adaptation.